The Pocket Universe

A pocket universe is a concept in inflationary theory, proposed by Alan Guth. It defines a realm like the one that contains the observable universe as only one of many inflationary zones.

Astrophysicist Jean-Luc Lehners, of the Princeton Center for Theoretical Science, has argued that an inflationary universe does produce pockets. As he wrote in 2012, "Eternal inflation produces pocket universes with all physically allowed vacuum and histories. Some of these pocket universes might contain a phase of slow-roll inflation, some might undergo cycles of cosmological evolution, and some might look like the Galilean genesis or other 'emergent' universe scenarios. Which one of these types of universe we are most likely to inhabit depends on the measure we choose in order to regulate the infinities inherent in eternal inflation." Question - Quora Quora assigned this question to the Science Fiction (genre) and Cosmology categories, and I’m not sure how you meant it to be addressed. I’ll address it from a scientific standpoint.

In real cosmology, a pocket universe is a concept related to Alan Guth’s eternal inflation model. It’s kind of complicated, but here are the basics:

Scientists believe that in the first brief fraction of a second at the start of the Big Bang, the baby universe went through a period of ultra-rapid expansion known as inflation. The evidence: there are parts of the universe that are too far apart for light to have traveled between them in the universe’s finite lifetime; however, these parts are remarkably homogeneous in terms of temperature — something that ought not necessarily be true. Ergo, scientists believe that these regions of the universe were initially close enough that their temperature could even out, then expanded beyond each others’ respective observable horizons in a rapid inflationary period. This raises huge questions, though, most importantly: why did inflation stop? After the brief inflationary period, the universe settled into a much more sedate expansion rate, and has never returned to expanding that fast. Why? Alan Guth proposes that perhaps the universe at large is still inflating, but pockets of space drop out of inflation, which gives rise to local hot big bangs. In his eternal inflation model, when a bubble transitions from inflationary to non-inflationary space, a huge amount of energy is produced in the bubble, and this energy eventually condenses to form all the matter in the expanding bubble. Guth argues that we are actually inside one of these bubbles — these so-called pocket universes — and that there may be infinitely more of them, separated from each other by inflating space. It would be utterly impossible for us to reach the edge of our own pocket universe, let alone traverse the gap to another pocket universe, because even within our pocket, we are surrounded by space that is expanding away from us faster than light can travel. So there you go…a pocket universe is a region of space that dropped out of inflating, triggering a Big Bang inside the pocket, and has been cooling and expanding ever since, all the while surrounded by rapidly inflating space and potentially infinitely many other such pockets.

I’m not familiar with the term pocket dimension, and I’m not sure it’s used by cosmologists. Somebody else can perhaps help you with that one.

A pocket dimension is when one creates a independent section of space-time inside the universe you live in, a dimension that can only be accessed through a specific mean that takes you into a place where time and space exists according to the same or different laws as the ones of our universe.

A sort of example of a pocket dimension was the Matrix in the Matrix Movies. The characters minds would travel from their bodies to a VR world that had it's own set of rules. Rules that could only be bent by the Agents and broken by Neo (aka The Anomaly aka The One). The VR world is a dimension in itself.

The example of the use of pocket dimensions that you see in fiction is the one usually associated to tesseracts. A tesseract is in essence a pocket of space-time, that can be huge in size on the inside, big as a city or a planet, contained in a very small object like a box or a room.

The pocket universe idea comes from the theory that each singularity is in fact its own Universe, separate from this one. So if you can for example contain a singularity in some sort of container then you're actually keeping a pocket universe inside that container.

But, Lehners continues, "the current leading measure proposals&mdash;namely, the global light-cone cutoff and its local counterpart, the causal diamond measure&mdash;as well as closely related proposals, all predict that we should live in a pocket universe that starts out with a small Hubble rate, thus favoring emergent and cyclic models." Lehners adds, deadpan, "Pocket universes which undergo cycles are further preferred, because they produce habitable conditions repeatedly inside each pocket." According to the Eleventh Doctor, a pocket universe or pocket dimension was a "distorted echo of our own" universe. He indicated such realities occurred spontaneously but that they "never last[ed] for long". However, there were also examples of artificially created and longer lasting pocket universes. The Doctor also emphatically told Clara that a pocket universe was not a parallel universe. (TV: Hide) Nonetheless, the planet Gallifrey and the people on it were sent into was referred to by the War Doctor as being a "single moment in time, held in a parallel pocket universe. Like a painting!" (TV: The Day of the Doctor) Because of their short lives, pocket universes were highly dangerous to the TARDIS, which could only survive for a few seconds before being inexorably trapped there as the universe collapsed back into the quantum foam. (TV: Hide)

While the terms "pocket universe" and "pocket dimension" may have been interchangeable, Albert Marsden commented that he was not trapped in a whole universe, nor a whole planet, explaining that his "pocket dimension"/time loop merely stretched for five miles and ten days before restarting. (AUDIO: Protect and Survive)

Examples
The Divergent Universe was a pocket universe that had no concept of time. Rassilon considered it the most "significant" of the universes he kept inside his dungeon on Gallifrey. (AUDIO: Zagreus)

A dangerous species from the Dark Times was closed by other races in a pocket universe called the Ringpull. They were released by the Fifth Doctor and Turlough. (AUDIO: Ringpullworld)

The Galacti-Bank vault utilised dimensional transcendentalism. The contents of the vault were contained in a pocket universe which could only be accessed by using the correct combination, which changed four times per day. (AUDIO: The Selachian Gambit)

The Seventh Doctor trapped two Elder Gods in a pocket dimension that comprised of just five miles of Britain after a nuclear war. (AUDIO: Protect and Survive)

Hila Tacorien was once trapped in a pocket universe, and was only brought back to the normal universe thanks to the work of Emma Grayling, an empathic psychic who was able to, as the Doctor put it, act "like a lantern, shining across the dimensions, guiding her home". This pocket universe contained a large island seemingly floating in space, on which was a misty forest. Time also ran much slower in this pocket universe than in the prime universe, to the point that 1 second in the pocket universe was equivalent to 100,000 years in the Doctor's universe. (TV: Hide)

The Celestial Toyroom (TV: The Celestial Toymaker) and the Land of Fiction were thought to be pocket universes. (TV: The Mind Robber)

In order to save it from the Daleks, "all thirteen" incarnations of the Doctor teamed up together to freeze Gallifrey in time and place it in a "single moment in time, held in a parallel pocket universe." While they were unsure if they succeeded, an encounter with the Curator indicated to the Eleventh Doctor that this was in fact successful. According to the General and the Doctors, the calculations necessary would take several hundred years so the Doctor worked on them for all of his various lives. (TV: The Day of the Doctor) A doorway to this pocket universe in the form of a crack in time appeared in the town of Christmas on Trenzalore. It was through there that the Time Lords sent the message "Doctor Who", which attracted the Eleventh Doctor as well as various races across the universe, and, 900 years later, with the Doctor dying from old age at the end of his final incarnation, energy for a new regeneration cycle. (TV: The Time of the Doctor)

The Vess kept a weapons factory in a pocket dimension, powered by energy drawn from the Big Bang. (AUDIO: The Light at the End)

Shadow-Space was an artificially created pocket dimension that stored the consciousness of intergalactic starship crew to protect them during warp flights. (AUDIO: Masquerade)

A parallel universe is a hypothetical self-contained reality co-existing with one's own. A specific group of parallel universes are called a "multiverse", although this term can also be used to describe the possible parallel universes that constitute reality. While the terms "parallel universe" and "alternative reality" are generally synonymous and can be used interchangeably in most cases, there is sometimes an additional connotation implied with the term "alternative reality" that implies that the reality is a variant of our own. The term "parallel universe" is more general, without implying a relationship, or lack of relationship, with our own universe. A universe where the very laws of nature are different – for example, one in which there are no Laws of Motion – would in general count as a parallel universe but not an alternative reality and a concept between both fantasy world and earth.

The actual quantum-mechanical hypothesis of parallel universes is "universes that are separated from each other by a single quantum event."

Overview
Fantasy has long borrowed an idea of "another world" from myth, legend and religion. Heaven, Hell, Olympus, and Valhalla are all "alternative universes" different from the familiar material realm. Plato reflected deeply on the parallel realities, resulting in Platonism, in which the upper reality is perfect while the lower earthly reality is an imperfect shadow of the heavenly. The lower reality is similar but with flaws.

Modern fantasy often presents the concept as a series of planes of existence where the laws of nature differ, allowing magical phenomena of some sort on some planes. This concept was also found in ancient Hindu mythology, in texts such as the Puranas, which expressed an infinite number of universes, each with its own gods. Similarly in Persian literature, "The Adventures of Bulukiya", a tale in the One Thousand and One Nights, describes the protagonist Bulukiya learning of alternative worlds/universes that are similar to but still distinct from his own. In other cases, in both fantasy and science fiction, a parallel universe is a single other material reality, and its co-existence with ours is a rationale to bring a protagonist from the author's reality into the fantasy's reality, such as in The Chronicles of Narnia by C. S. Lewis or even the beyond-the-reflection travel in the two main works of Lewis Carroll. Or this single other reality can invade our own, as when Margaret Cavendish's English heroine sends submarines and "birdmen" armed with "fire stones" back through the portal from The Blazing World to Earth and wreaks havoc on England's enemies. In dark fantasy or horror the parallel world is often a hiding place for unpleasant things, and often the protagonist is forced to confront effects of this other world leaking into his own, as in most of the work of H. P. Lovecraft and the Doom computer game series, or Warhammer 40K miniature and computer games. In such stories, the nature of this other reality is often left mysterious, known only by its effect on our own world.

The concept also arises outside the framework of quantum mechanics, as is found in Jorge Luis Borges short story El jardín de senderos que se bifurcan ("The Garden of Forking Paths"), published in 1941 before the many-worlds interpretation had been invented. In the story, a Sinologist discovers a manuscript by a Chinese writer where the same tale is recounted in several ways, often contradictory, and then explains to his visitor (the writer's grandson) that his relative conceived time as a "garden of forking paths", where things happen in parallel in infinitely branching ways. One of the first Science fiction examples is Murray Leinster's Sidewise in Time, in which portions of alternative universes replace corresponding geographical regions in this universe. Sidewise in Time describes it in the manner that similar to requiring both longitude and latitude coordinates in order to mark your location on Earth, so too does time: travelling along latitude is akin to time travel moving through past, present and future, while travelling along latitude is to travel perpendicular to time and to other realities, hence the name of the short story. Thus, another common term for a parallel universe is "another dimension", stemming from the idea that if the 4th dimension is time, the 5th dimension - a direction at a right angle to the fourth - are alternate realities.

While this is a common treatment in Science fiction, it is by no means the only presentation of the idea, even in hard science fiction. Sometimes the parallel universe bears no historical relationship to any other world; instead, the laws of nature are simply different from those in our own, as in the novel Raft by Stephen Baxter, which posits a reality where the gravitational constant is much larger than in our universe. (Note, however, that Baxter explains later in Vacuum Diagrams that the protagonists in Raft are descended from people who came from the Xeelee Sequence universe.)

One motif is that the way time flows in a parallel universe may be very different, so that a character returning to one might find the time passed very differently for those he left behind. This is found in folklore: King Herla visited Fairy and returned three centuries later; although only some of his men crumbled to dust on dismounting, Herla and his men who did not dismount were trapped on horseback, this being one folkloric account of the origin of the Wild Hunt. C. S. Lewis made use of this in The Chronicles of Narnia; indeed, a character points out to two skeptics that there is no need for the time between the worlds to match up, but it would be very odd for the girl who claims to have visited a parallel universe to have dreamed up such a different time flow - from their perspective, the girl had only been gone for a few minutes though she was in Narnia for hours, and if she was making it up surely she would have spent a while longer hiding than a few minutes.

The division between science fiction and fantasy becomes fuzzier than usual when dealing with stories that explicitly leave the universe we are familiar with, especially when our familiar universe is portrayed as a subset of a multiverse. Picking a genre becomes less a matter of setting, and more a matter of theme and emphasis; the parts of the story the author wishes to explain and how they are explained. Narnia is clearly a fantasy, and the TV series Sliders is clearly science fiction, but works like the World of Tiers series or Glory Road tend to occupy a much broader middle ground.

Science fiction
While technically incorrect, and looked down upon by hard science-fiction fans and authors, the idea of another "dimension" has become synonymous with the term "parallel universe". The usage is particularly common in movies, television and comic books and much less so in modern prose science fiction. The idea of a parallel world was first introduced in comic books with the publication of The Flash #123, "Flash of Two Worlds".

In written science fiction, "new dimension" more commonly – and more accurately – refer to additional coordinate axes, beyond the three spatial axes with which we are familiar. By proposing travel along these extra axes, which are not normally perceptible, the traveler can reach worlds that are otherwise unreachable and invisible.

In 1884, Edwin A. Abbott wrote the seminal novel exploring this concept called Flatland: A Romance of Many Dimensions. It describes a world of two dimensions inhabited by living squares, triangles, and circles, called Flatland, as well as Pointland (0 dimensions), Lineland (1 dimension), and Spaceland (three dimensions) and finally posits the possibilities of even greater dimensions. Isaac Asimov, in his foreword to the Signet Classics 1984 edition, described Flatland as "The best introduction one can find into the manner of perceiving dimensions."

In 1895, The Time Machine by H. G. Wells used time as an additional "dimension" in this sense, taking the four-dimensional model of classical physics and interpreting time as a space-like dimension in which humans could travel with the right equipment. Wells also used the concept of parallel universes as a consequence of time as the fourth dimension in stories like The Wonderful Visit and Men Like Gods, an idea proposed by the astronomer Simon Newcomb, who talked about both time and parallel universes; "Add a fourth dimension to space, and there is room for an indefinite number of universes, all alongside of each other, as there is for an indefinite number of sheets of paper when we pile them upon each other".

There are many examples where authors have explicitly created additional spatial dimensions for their characters to travel in, to reach parallel universes. In Doctor Who, the Doctor accidentally enters a parallel universe while attempting to repair the TARDIS console in "Inferno". The parallel universe was similar to the real universe but with some different aspects, Britain has a fascist government and the royal family has been executed. Douglas Adams, in the last book of the Hitchhiker's Guide to the Galaxy series, Mostly Harmless, uses the idea of probability as an extra axis in addition to the classical four dimensions of space and time similar to the many-worlds interpretation of quantum physics. Though, according to the novel, they're not really parallel universes at all but only a model to capture the continuity of space, time and probability. Robert A. Heinlein, in The Number of the Beast, postulated a six-dimensional universe. In addition to the three spatial dimensions, he invoked symmetry to add two new temporal dimensions, so there would be two sets of three. Like the fourth dimension of H. G. Wells' "Time Traveller", these extra dimensions can be traveled by persons using the right equipment.

Hyperspace
Perhaps the most common use of the concept of a parallel universe in science fiction is the concept of hyperspace. Used in science fiction, the concept of "hyperspace" often refers to a parallel universe that can be used as a faster-than-light shortcut for interstellar travel. Rationales for this form of hyperspace vary from work to work, but the two common elements are:
 * 1) at least some (if not all) locations in the hyperspace universe map to locations in our universe, providing the "entry" and "exit" points for travellers.
 * 2) the travel time between two points in the hyperspace universe is much shorter than the time to travel to the analogous points in our universe. This can be because of a different speed of light, different speed at which time passes, or the analogous points in the hyperspace universe simply being much closer to each other.

Sometimes "hyperspace" is used to refer to the concept of additional coordinate axes. In this model, the universe is thought to be "crumpled" in some higher spatial dimension and that traveling in this higher spatial dimension, a ship can move vast distances in the common spatial dimensions. An analogy is to crumple a newspaper into a ball and stick a needle straight through, the needle will make widely spaced holes in the two-dimensional surface of the paper. While this idea invokes a "new dimension", it is not an example of a parallel universe. It is a more scientifically plausible use of hyperspace. (See wormhole.)

While use of hyperspace is common, it is mostly used as a plot device and thus of secondary importance. While a parallel universe may be invoked by the concept, the nature of the universe is not often explored. So, while stories involving hyperspace might be the most common use of the parallel universe concept in fiction, it is not the most common source of fiction about parallel universes.

Time travel and alternative history


Parallel universes may be the backdrop to or the consequence of time travel, their most common use in fiction if the concept is central to the story. A seminal example of both is in Fritz Leiber's novel The Big Time where there's a war across time between two alternative futures manipulating history to create a timeline that results in or realizes their own world.

Time travelers in fiction often accidentally or deliberately create alternative histories, such as in The Guns of the South by Harry Turtledove where the Confederate Army is given thousands of AK-47 rifles and ends up winning the American Civil War. (However, Ward Moore reversed this staple of alternative history fiction in his Bring the Jubilee (1953), where an alternative world where the Confederate States of America won the Battle of Gettysburg and the American Civil War is destroyed after a historian and time traveler from the defeated United States of that world travels back to the scene of the battle and, by inadvertently causing the death of the Confederate officer whose troops occupied Little Round Top, changes the result so that the Union forces are victorious.) The alternative history novel 1632 by Eric Flint explicitly states, albeit briefly in a prologue, that the time travelers in the novel (an entire town from West Virginia) have created a new and separate universe when they're transported into the midst of the Thirty Years' War in 17th century Germany. (This sort of thing is known as an ISOT among alternative history fans, after S. M. Stirling's Island in the Sea of Time: an ISOT is when territory or a large group of people is transported back in time to another historical period or place.)

Ordinarily, alternative histories are not technically parallel universes. The concepts are similar but there are significant differences. Where characters travel to the past, they may cause changes in the timeline (creating a point of divergence) that result in changes to the present. The alternative present will be similar in different degrees to the original present as would be the case with a parallel universe. The main difference is that parallel universes co-exist whereas only one history or alternative history can exist at any one moment. Another difference is that moving to a parallel universe involves some inter-dimensional travel whereas alternative histories involve some type of time travel. (However, since the future is only potential and not actual, it is often conceived that more than one future may exist simultaneously.)

The concept of "sidewise" time travel, a term taken from Murray Leinster's "Sidewise in Time", is often used to allow characters to pass through many different alternative histories, all descendant from some common branch point. Often worlds that are similar to each other are considered closer to each other in terms of this sidewise travel. For example, a universe where World War II ended differently would be "closer" to us than one where Imperial China colonized the New World in the 15th century. H. Beam Piper used this concept, naming it "paratime" and writing a series of stories involving the Paratime Police who regulated travel between these alternative realities as well as the technology to do so. Keith Laumer used the same concept of "sideways" time travel in his 1962 novel Worlds of the Imperium. More recently, novels such as Frederik Pohl's The Coming of the Quantum Cats and Neal Stephenson's Anathem explore human-scale readings of the "many worlds" interpretation of quantum mechanics, postulating that historical events or human consciousness spawns or allows "travel" among alternative universes.

Universe 'types' frequently explored in sidewise and alternative history works include worlds whose Nazis won the Second World War, as in The Man in the High Castle by Philip K. Dick, SS-GB by Len Deighton, and Fatherland by Robert Harris, and worlds whose Roman Empire never fell, as in Roma Eterna by Robert Silverberg and Romanitas by Sophia McDougall. The novel Warlords of Utopia by Lance Parkin explored a multiverse in which the universes whose Rome never fell go to war with all those whose Nazis won World War II. It fits loosely in the Faction Paradox series initiated by Lawrence Miles, several of whose novels featured an artificially created universe existing within another; specifically, within a bottle. Dead Romance explored the consequences of inhabitants of the 'real' universe entering the Universe-in-a-Bottle.

In Philip Pullman's trilogy His Dark Materials, the protagonist begins in a world that is a Victorian counterpart to ours, although it takes place at the same time. It also appears that the Protestant Reformation happened differently with John Calvin becoming the last Pope.

Counter-Earth
The concept of Counter-Earth is typically similar to that of parallel universes but is actually a distinct idea. A counter-earth is a planet that shares Earth's orbit but is on the opposite side of the Sun and, therefore, cannot be seen from Earth. There would be no necessity that such a planet would be like Earth in any way though typically in fiction; it is usually nearly identical to Earth. Since Counter-Earth is always within the universe (and the Solar System), travel to it can be accomplished with ordinary space travel.

Gerry and Sylvia Anderson used this concept in their 1969 movie Doppelgänger (released outside Europe as Journey to the Far Side of the Sun), in which a Counter-Earth is detected by astronomers and a manned mission launched by a US-European space consortium to explore it.

Convergent evolution
Convergent evolution is a biological concept whereby unrelated species acquire similar traits because they adapted to a similar environment and/or played similar roles in their ecosystems. In fiction, the concept is extended whereby similar planets will result in races with similar cultures and/or histories.

Technically this is not a type of parallel universe since such planets can be reached via ordinary space travel, but the stories are similar in some respects. Star Trek frequently explored such worlds:


 * In "Bread and Circuses" the Enterprise encounters a planet called Magna Roma, which has many physical resemblances to Earth such as its atmosphere, land to ocean ratio, and size. The landing party discovers that the planet is at roughly a late 20th-century level of technology but its society is similar to the Roman Empire, as if the Empire had not fallen but had continued to that time: there is also a reference to the Roman god Jupiter as the namesake of a new line of automobile, and gladiator fights are televised in primetime. Slavery on this world has also developed into an institution, with slaves guaranteed medical benefits and old-age pensions, so the workers grew more content and never rebelled. At the end of the episode, it is discovered that the society has just found their own version of Jesus, referred simply as "the son" (whose followers they had previously mistaken for sun worshipers).
 * In "The Omega Glory", the crew visit a planet on which there is a conflict between two peoples called the Yangs and the Kohms. They discover that the Yangs are like Earth's "Yankees" (in other words, Americans) and the Kohms are like Earth's Communists; the Yangs, who had at some point in the past been conquered by the Kohms, had a ritual speech that was word for word identical to the American Pledge of Allegiance, and treated the U.S. Constitution as a sacred text. (A deleted scene from the episode, however, implied that both the Yangs and Kohms were actually descendants of human colonists.)
 * In "Miri", the Enterprise crew encounter a planet that is physically identical to Earth. Histories on the two planets were apparently identical until the 20th century when scientists had accidentally created a deadly virus that killed all the adults but extended the lives of the children (who call themselves the "Onlies").

Convergent evolution due to contamination
A similar concept in biology is gene flow. In this case, a planet may start as different from Earth, but due to the influence of Earth culture, the planet comes to resemble Earth in some way; technically this is not a type of parallel universe since such planets can be reached via ordinary space travel, but the stories are similar in some respects. Star Trek used this theory as well: in "Patterns of Force", a planet is discovered that has become very similar to Nazi Germany due to the influence of a historian that came to reside there (believing that the Nazi fascism itself was not evil and under benevolent leadership could be "good government"), while in "A Piece of the Action", the Enterprise crew visits a planet that, 100 years after a book Chicago Mobs of the Twenties that had been left behind by previous Earth craft, their society resembles mob ruled cities of the Prohibition era United States.

Simulated reality
Simulated realities are digital constructs featured in science fiction such as The Matrix.

Hyperspace
Perhaps the most common use of the concept of a parallel universe in science fiction is the concept of hyperspace. Used in science fiction, the concept of "hyperspace" often refers to a parallel universe that can be used as a faster-than-light shortcut for interstellar travel. Rationales for this form of hyperspace vary from work to work, but the two common elements are:
 * 1) at least some (if not all) locations in the hyperspace universe map to locations in our universe, providing the "entry" and "exit" points for travellers.
 * 2) the travel time between two points in the hyperspace universe is much shorter than the time to travel to the analogous points in our universe. This can be because of a different speed of light, different speed at which time passes, or the analogous points in the hyperspace universe simply being much closer to each other.

Sometimes "hyperspace" is used to refer to the concept of additional coordinate axes. In this model, the universe is thought to be "crumpled" in some higher spatial dimension and that traveling in this higher spatial dimension, a ship can move vast distances in the common spatial dimensions. An analogy is to crumple a newspaper into a ball and stick a needle straight through, the needle will make widely spaced holes in the two-dimensional surface of the paper. While this idea invokes a "new dimension", it is not an example of a parallel universe. It is a more scientifically plausible use of hyperspace. (See wormhole.)

While use of hyperspace is common, it is mostly used as a plot device and thus of secondary importance. While a parallel universe may be invoked by the concept, the nature of the universe is not often explored. So, while stories involving hyperspace might be the most common use of the parallel universe concept in fiction, it is not the most common source of fiction about parallel universes.

Time travel and alternative history


Parallel universes may be the backdrop to or the consequence of time travel, their most common use in fiction if the concept is central to the story. A seminal example of both is in Fritz Leiber's novel The Big Time where there's a war across time between two alternative futures manipulating history to create a timeline that results in or realizes their own world.

Time travelers in fiction often accidentally or deliberately create alternative histories, such as in The Guns of the South by Harry Turtledove where the Confederate Army is given thousands of AK-47 rifles and ends up winning the American Civil War. (However, Ward Moore reversed this staple of alternative history fiction in his Bring the Jubilee (1953), where an alternative world where the Confederate States of America won the Battle of Gettysburg and the American Civil War is destroyed after a historian and time traveler from the defeated United States of that world travels back to the scene of the battle and, by inadvertently causing the death of the Confederate officer whose troops occupied Little Round Top, changes the result so that the Union forces are victorious.) The alternative history novel 1632 by Eric Flint explicitly states, albeit briefly in a prologue, that the time travelers in the novel (an entire town from West Virginia) have created a new and separate universe when they're transported into the midst of the Thirty Years' War in 17th century Germany. (This sort of thing is known as an ISOT among alternative history fans, after S. M. Stirling's Island in the Sea of Time: an ISOT is when territory or a large group of people is transported back in time to another historical period or place.)

Ordinarily, alternative histories are not technically parallel universes. The concepts are similar but there are significant differences. Where characters travel to the past, they may cause changes in the timeline (creating a point of divergence) that result in changes to the present. The alternative present will be similar in different degrees to the original present as would be the case with a parallel universe. The main difference is that parallel universes co-exist whereas only one history or alternative history can exist at any one moment. Another difference is that moving to a parallel universe involves some inter-dimensional travel whereas alternative histories involve some type of time travel. (However, since the future is only potential and not actual, it is often conceived that more than one future may exist simultaneously.)

The concept of "sidewise" time travel, a term taken from Murray Leinster's "Sidewise in Time", is often used to allow characters to pass through many different alternative histories, all descendant from some common branch point. Often worlds that are similar to each other are considered closer to each other in terms of this sidewise travel. For example, a universe where World War II ended differently would be "closer" to us than one where Imperial China colonized the New World in the 15th century. H. Beam Piper used this concept, naming it "paratime" and writing a series of stories involving the Paratime Police who regulated travel between these alternative realities as well as the technology to do so. Keith Laumer used the same concept of "sideways" time travel in his 1962 novel Worlds of the Imperium. More recently, novels such as Frederik Pohl's The Coming of the Quantum Cats and Neal Stephenson's Anathem explore human-scale readings of the "many worlds" interpretation of quantum mechanics, postulating that historical events or human consciousness spawns or allows "travel" among alternative universes.

Universe 'types' frequently explored in sidewise and alternative history works include worlds whose Nazis won the Second World War, as in The Man in the High Castle by Philip K. Dick, SS-GB by Len Deighton, and Fatherland by Robert Harris, and worlds whose Roman Empire never fell, as in Roma Eterna by Robert Silverberg and Romanitas by Sophia McDougall. The novel Warlords of Utopia by Lance Parkin explored a multiverse in which the universes whose Rome never fell go to war with all those whose Nazis won World War II. It fits loosely in the Faction Paradox series initiated by Lawrence Miles, several of whose novels featured an artificially created universe existing within another; specifically, within a bottle. Dead Romance explored the consequences of inhabitants of the 'real' universe entering the Universe-in-a-Bottle.

In Philip Pullman's trilogy His Dark Materials, the protagonist begins in a world that is a Victorian counterpart to ours, although it takes place at the same time. It also appears that the Protestant Reformation happened differently with John Calvin becoming the last Pope.

Counter-Earth
The concept of Counter-Earth is typically similar to that of parallel universes but is actually a distinct idea. A counter-earth is a planet that shares Earth's orbit but is on the opposite side of the Sun and, therefore, cannot be seen from Earth. There would be no necessity that such a planet would be like Earth in any way though typically in fiction; it is usually nearly identical to Earth. Since Counter-Earth is always within the universe (and the Solar System), travel to it can be accomplished with ordinary space travel.

Gerry and Sylvia Anderson used this concept in their 1969 movie Doppelgänger (released outside Europe as Journey to the Far Side of the Sun), in which a Counter-Earth is detected by astronomers and a manned mission launched by a US-European space consortium to explore it.

Convergent evolution
Convergent evolution is a biological concept whereby unrelated species acquire similar traits because they adapted to a similar environment and/or played similar roles in their ecosystems. In fiction, the concept is extended whereby similar planets will result in races with similar cultures and/or histories.

Technically this is not a type of parallel universe since such planets can be reached via ordinary space travel, but the stories are similar in some respects. Star Trek frequently explored such worlds:


 * In "Bread and Circuses" the Enterprise encounters a planet called Magna Roma, which has many physical resemblances to Earth such as its atmosphere, land to ocean ratio, and size. The landing party discovers that the planet is at roughly a late 20th-century level of technology but its society is similar to the Roman Empire, as if the Empire had not fallen but had continued to that time: there is also a reference to the Roman god Jupiter as the namesake of a new line of automobile, and gladiator fights are televised in primetime. Slavery on this world has also developed into an institution, with slaves guaranteed medical benefits and old-age pensions, so the workers grew more content and never rebelled. At the end of the episode, it is discovered that the society has just found their own version of Jesus, referred simply as "the son" (whose followers they had previously mistaken for sun worshipers).
 * In "The Omega Glory", the crew visit a planet on which there is a conflict between two peoples called the Yangs and the Kohms. They discover that the Yangs are like Earth's "Yankees" (in other words, Americans) and the Kohms are like Earth's Communists; the Yangs, who had at some point in the past been conquered by the Kohms, had a ritual speech that was word for word identical to the American Pledge of Allegiance, and treated the U.S. Constitution as a sacred text. (A deleted scene from the episode, however, implied that both the Yangs and Kohms were actually descendants of human colonists.)
 * In "Miri", the Enterprise crew encounter a planet that is physically identical to Earth. Histories on the two planets were apparently identical until the 20th century when scientists had accidentally created a deadly virus that killed all the adults but extended the lives of the children (who call themselves the "Onlies").

Convergent evolution due to contamination
A similar concept in biology is gene flow. In this case, a planet may start as different from Earth, but due to the influence of Earth culture, the planet comes to resemble Earth in some way; technically this is not a type of parallel universe since such planets can be reached via ordinary space travel, but the stories are similar in some respects. Star Trek used this theory as well: in "Patterns of Force", a planet is discovered that has become very similar to Nazi Germany due to the influence of a historian that came to reside there (believing that the Nazi fascism itself was not evil and under benevolent leadership could be "good government"), while in "A Piece of the Action", the Enterprise crew visits a planet that, 100 years after a book Chicago Mobs of the Twenties that had been left behind by previous Earth craft, their society resembles mob ruled cities of the Prohibition era United States. Themes: Pocket Universe: SFE: Science Fiction Encyclopedia Print 1  7  0 A term first used in a restricted sense by Murray Leinster in "Pocket Universes" (October 1946 Thrilling Wonder), where it is a "contrivance" rather than an encompassing world. It might broadly be said that the inhabitant of any constricted environment lives in a pocket universe, whether as a child, a prisoner, a victim of dementia, a chained watcher in Plato's cave, a resident of Hell or an inhabitant of the world inside Pantagruel's mouth. It might also be suggested that the dynamic moment of escape from confinement – a leitmotiv of Western literature – almost inevitably marks the transition from a pocket universe to a fuller and more real world. In the final pages of Mark Twain's The Adventures of Huckleberry Finn (1884), when Huck figures he "got to light out for the Territory ahead of the rest", he is anticipating his "escape" from aunt Sally in order to be free of the overgoverned social organization and its conservative inwardness of gaze that she represents: a hierarchical boundedness that has many of the psychological characteristics of the pocket universe as found in sf: that Huck will almost certainly find no freedom in the Territory is a fate beyond the pages of Huckleberry Finn (see Slingshot Ending), just as life under the stars tends to be pointed at, rather than lived, as most Pocket Universe tales come to a climax. The classic movement of the sf tale is of course outward – via Conceptual Breakthroughs and all the other forms of initiation or unshackling – and in that sense most sf works contain some sort of pocket universe, implied or explicit, which initially binds and blinds the protagonist, and from which it is necessary to escape; and most sf works lose momentum if they try to inhabit the new world on offer.

Two usages of the term seem useful, one broad, the other narrower. It can be used broadly to describe an actual miniature universe pocketed within a larger explanatory frame or device – like the various godling-crafted worlds nesting within one another in Philip José Farmer's World of Tiers sequence; or like the hidden redoubts that feature in so many Lost Race tales; or like the "natural" miniature universes observed in such works as Gregory Benford's Cosm (1998); or like the set-ups in almost any of Jack L Chalker's series (e.g., the Well World sequence and the Four Lords of the Diamond tetralogy) which feature universes constructed by godlike beings as Godgame labyrinths and inhabited by victim-players who must solve their universe to escape from it; or like similar 1950s set-ups (see Paranoia) such as in Frederik Pohl's "The Tunnel Under the World" (January 1955 Galaxy) or Philip K Dick's Time Out of Joint (1958), whose protagonists are victims of artificial worlds shaped to delude and manipulate them; or like the inverse scenario in which human protagonists are the manipulators of artificial life, ranging from Theodore Sturgeon's "Microcosmic God" (April 1941 Astounding) to the sophisticated AI-Evolution of Greg Egan's "Crystal Nights" (April 2008 Interzone); or (again trivially) like any fantasy game which involves Role-Playing Game activity within a Virtual-Reality world; or in fact like any world (such as that on which John Crowley's The Deep [1975] is set, or Terry Pratchett's Discworld) whose origins and extent reflect a sense of constraining artifice.

But none of these applications contains the one essential element that defines the true pocket-universe tale: Farmer's and Chalker's protagonists may not know the nature of the worlds in which they find themselves, but they do know that they are inhabiting some form of construct. In the pocket-universe tale as more narrowly defined, the world initially perceived seems to be the entire world, not a Keep within a larger frame, and the web of taboos preventing the truth about its partial nature being known is structurally very similar to the parental restrictions which initially hamper the move through puberty into adulthood of the young protagonists of most non-genre juveniles. It could, indeed, be argued that this move through puberty is a particular example of the Conceptual Breakthrough which arguably structures all genuine sf.

The classic Generation-Starship tale is one in which the descendants of the original crew members have forgotten the true nature of things and have instituted a repressive, Taboo-governed society which suppresses any attempt to discover the truth; it is the task of the young protagonist to break through the social and epistemological barriers stifling this world while at the same time successfully managing puberty. The pure Generation-Starship story embodies, therefore, the purest form of the concept of the pocket universe. Examples of that pure form, though central to sf, are not numerous – Robert A Heinlein's Universe (May 1941 Astounding; 1951 chap) is the most famous in the list, which includes also Brian W Aldiss's Non-Stop (1956 Science Fantasy #17; exp 1958; cut vt Starship 1959), Harry Harrison's Captive Universe (1969); but Alexei Panshin's Rite of Passage (July 1963 If as "Down to the Worlds of Men"; exp 1968), for instance, though explicitly a tale of puberty, does not suggest that there is any epistemological mystery about the nature of the asteroid-sized starship from which its heroine must escape. The growth into redemptive adulthood of Silk in Gene Wolfe's Book of the Long Sun sequence (1993-1996) soon absorbs the model into more complex concerns. A good late example of the form, such as Stephen Baxter's Ark (2009), is unlikely to emphasize the Pocket Universe/puberty linkage, which has now become a Cliché, though Paul C {CHAFE} returns to it in his Exodus sequence (2007-2009).

All Post-Holocaust tales in which the descendants of survivors live in Underground habitats which they think to be the whole of reality are pocket-universe stories. The best of them is perhaps Daniel F Galouye's Dark Universe (1961), though Margaret St Clair's Sign of the Labrys (1963) and The Shadow People (1969) play fruitfully with the concept, as do Richard Cowper's Kuldesak (1972), Roger Eldridge's The Shadow of the Gloom-World (1977) and many others. In all these stories, the essential movement is from childhood constriction and taboo-driven ignorance to adult freedom and breakthrough; in Genre SF it is only more recently that ironies have significantly pervaded this pattern, as in David J Lake's Ring of Truth (1983), where a traditional enclosed world turns out to be interminably extensive, so that there is, in fact, no exit. In the great pocket-universe stories, however, there is always an out, a Sense of Wonder, a new world opening before the opened eyes. [JC/DRL]

Mang

Simulated reality
Simulated realities are digital constructs featured in science fiction such as The Matrix.

Pocket Dimension - TV Tropes
https://static.tvtropes.org/pmwiki/pub/images/1298252085458.png Another Dimension that is not so "other", Pocket Dimensions are spaces that are too small or too easily accessible to be truly considered a separate dimension and are referred to as a small extra pocket of space that is attached to our own. Much like an actual pocket, they are often used for some extra space where you can get things Bigger on the Inside. A Speculative Fiction favourite, the uses are plentifold. Storage for a Bag of Holding, hiding places, transportation, an explanation for physics-defying superpowers: a Pocket Dimension can do them all. Can't make julienne fries though.

Also can serve as a (sometimes unstable) Small, Secluded World with its own ecosystem and lifeforms. This is a quite handy place for keeping some nasty lurksome monsters; it lets them be very alien and make intermittent contact.

See also Just One Second Out of Sync which is often exactly the same but explained in terms of a temporal fashion than a spatial one. Not to be confused with a Palm OS game of the same name.

Examples:

Anime & Manga
Tengen Toppa Gurren Lagann: The place where the Anti-Spirals retreat to when they're not busy wiping out any Spiral races that they feel have gotten too large. In addition to Doraemon's Bag of Holding pouch on his stomach, his time machine is also parked in a pocket dimension accessed from Nobita's desk. The first episode of Haré+Guu reveals Guu's stomach contains a secluded world with things like cats with dozens of legs, crazy buildings, as well as a friendly couple who've been in there a while! Mahou Sensei Negima! has a number of Pocket Dimensions around; Kaede's magic cape contains one, and Mana uses one to store her ammunition. Also there are Evangeline's castles, Theodora's diorama sphere and Evangeline's Magia Erebea scroll. In Naruto, Tobi can open a gate to his own pocket dimension with his Sharingan, which is a black void full of white boxes. Turns out it's not only his. Since he has the mate to Tobi's Sharingan eye, Kakashi can access the same pocket dimension. Gluttony of Fullmetal Alchemist has a Pocket Dimension in his stomach.

One Piece:
Blueno's Devil Fruit allows him to access one by making a door in the air itself. It's not only useful to escape strong enemy attacks, but also perform sneak attacks. Charlotte Brûlée's Devil Fruit power allows her to conjure up mirrors and trap opponents in a pocket dimension she calls "Mirror World". Star Driver has Zero Time. In Saint Seiya, Gemini Saga and later Gemini Kanon's trademark attack "Another Dimension" warps the target into one of these. The really overblown delivery of the line in the Latin American Spanish dub has reached massive memetic mutation status. Fate/stay night: Gilgamesh's "Gate of Babylon" Noble Phantasm is a pocket dimension that holds just about every treasure in the world... which includes nearly every legendary weapon ever created. And a damn good wine cellar. There is also Archer, who creates copies of legendary weapons and stores them in a pocket dimension called Unlimited Blade Works. Fairy Tail: Erza stores some of her weapons and armors in a pocket dimension. Unfortunately for her, the dimension isn't large enough to store all of her gear. She has to rent a lot of space in the Fairy Tail dorms to hold all of her stuff. Bleach: The Quincies were originally based in the World of the Living until a disastrous war with the Shinigami a thousand years ago. The survivors sneaked into Soul Society and hid within the shadows of the Shinigami's key city, Seireitei. Within the shadows they created a pocket dimension containing a "reverse" version of Seireitei. Although the Shinigami spent a thousand years thinking the Quincies were dying into extinction, they never once realised the Quincies were thriving right under their noses all along until the Quincies were strong enough to sack Seireitei and decimate the Gotei 13. The land of Darius in Gaiking: Legend of Daiku Maryu is a pocket dimension located inside the Earth. In Tenchi Muyo!, Kagato's Cool Starship Soja has one where he keeps Washu prisoner and encased inside a Crystal Prison. The Death Room in Soul Eater is Shinigami-sama's personal domain, accessed usually from an imposing but otherwise normal-looking door within the Shibusen, but from his end, it's a door in the middle of nowhere. Powerful mages in the Lyrical Nanoha franchise can create pocket dimensions within the fabric of normal spacetime. In Magical Girl Lyrical Nanoha A's, Reinforce creates two pocket dimensions at once: one to trap Fate in and one to protect her master Hayate while she destroys the Earth, while in StrikerS, Verossa Acous uses a tiny pocket dimension apparently to transport cakes without tarnishing his perfect suit. It is also strongly implied that the Garden of Time from the original series is a part of Mid-Childa's landscape that Precia Testarossa had "sheared off" and placed in her own personal pocket dimension to serve as her base of operations.

Witch barriers in Puella Magi Madoka Magica are formed by Witches to conceal themselves, whenever a normal human being is unfortunate enough to stumble upon a barrier by accident, they will be unable to escape from it, which is why a Magical Girl is needed to eradicate the Witches. They also happen to be a reflection of a Magical Girl's past life prior to becoming a Witch, for instance, Oktavia Von Seckendorff has a barrier resembling a concert and one of her familiars, Holger, is a violinist which resembles Kyosuke, the boy whom Sayaka loved, but he never looked back. The only Witch who doesn't need a barrier to exist is Walpurgisnacht.

Dragon Ball:
The Room of Spirit and Time in Dragon Ball Z is a pocket dimension that consists of an endless void with harsh temperature conditions and 10 times Earth's gravity for Training Purposes. Whis' Staff also holds a pocket dimension similar to this. Hit stores the time that he "skips" in a pocket dimension. He has the ability to phase himself into this pocket dimension to make attacks pass right through him without harming him, but he can only do this for as long as he has "stored time", and given that he only skips fractions of a second, it takes him a while to build up any appreciable time in there.

Digimon:
In Digimon Frontier, Sakkakumon has a different environment corresponding to one of the show's Elemental Powers in each of the ten spheres that make up his body, complete with a unique landscape and Alien Sky. The protagonists had to defeat various Threshold Guardians in order to escape. In Digimon Xros Wars, Wisemon's book contained his laboratory until said book was shredded by a rampaging Arkadimon. Fortunately for him, he's able to set up immediately after in Taiki's Xros Loader, which contains its own nondescript internal space.

Comic Books
The Marvel Comics Heroes Reborn universe was explicitly called a Pocket Dimension, or Pocket Universe, which Franklin Richards literally carried in his pocket. Another one can be accessed through use of the Soul Gem. Asgard, Olympus, Heliopolis and other godly realms are Pocket Dimensions adjacent to Earth. Some are further sub-divided (e.g. Asgard's "Nine Worlds" include Hel, Muspelheim, Jotunheim, etc...). There's also the Alternate Universe of Marvel 1602 which was split off from the main Marvel Universe to prevent all of reality being destroyed and placed into a jewel guarded by Uatu. Examples in Supergirl comics: Demon Spawn the Innerverse is a pocket dimension created by Supergirl's dark side which exists inside her mind and is a hell inhabited by demonic monsters. In Supergirl: Cosmic Adventures in the 8th Grade, Argo City was shunted by the shockwave of Krypton exploding into a pocket dimension known as quasi-space. Archie Comics' Sonic the Hedgehog series explicitly explains Zones as 'pocket dimensions' via editor notes. This is most apparent in the case of the Special Zone (equivalent of Special Stages from the games). This is made confusing by the fact that areas within the main dimension are also referred to as 'Zones' to comply with the games' stage names eg. Green Hill Zone, Lava Reef Zone etc. An apparently commonplace practice for Green Lantern officers is to place their lantern-shaped power battery (which they must use to recharge their rings periodically) inside a pocket dimension, so as to have easy access to it in the field. Kyle Rayner keeps his in his apartment, which has more than once worried him while off on another planet, since he might not have enough juice in the ring to get home.

The Time Trapper created a pocket universe which events from the Pre-Crisis/Silver Age Superboy stories took place. Anytime the Legion of Super-Heroes travel back to meet their inspiration Superboy, the Time Trapper diverts them to there. The only inhabitable planet is Earth (and so was Krypton). In The Supergirl Saga, it has become a dead world after Zod, Zaora, and Quex-Ul killed everyone on Earth, following Superboy's death by the Time Trapper. It's apparently destroyed during Zero Hour!. The reason this was created is because DC had to clean up a Continuity Snarl caused by their Continuity Reboot of Superman. It only made the snarl worse. (The reason they just couldn't say it was an Alternate Universe? Because alternate universes were the reason the reboot happened in the first place).

Superman gives Batman access to one as a gift in Batman: Prelude to the Wedding. It consists of a lake, a rowboat, and a pair of fishing poles. And fish, presumably. Robin Series: Tim meets a man named Stephen who has access to a foresty dimension that connects to places all over the American east. Stephen uses it for shortcuts since he can always tell which way to go to get to where he wants but he mentions that people or animals can easily get lost in the woods forever. == Fan Works== In A Different Medius, Specter and Akuba barriers are this. In With Strings Attached, the original bodies of the four, and later their cloned bodies, are stored in a pocket dimension, a private stasis pocket. When Worlds Collide Mario Sonic And Mega Man Crossover features a pocket dimension that links Mobius, Earth 20XX, and the Mushroom World together: Bowser, Eggman, and Wily use it as their primary base of operations. Sweetie Belle stores her notebook in a Pocket Dimension in The Sweetie Chronicles: Fragments because it allows her to take it with her as she jumps between Alternate Universes.

For Intercom: While Disgust yells at Fear's fear about "being seen" in brain surgery for being in another dimension, technically this is the proper qualifiaction the mind world is. Since it's Riley's mind, it doesn't exist outside of Riley, and can't be reached by anyone but Riley at the moment. But it does have a symbiotic relation with Riley since her emotions help direct how Riley might shape the dimension's use with memory, personality and powering her different thought processes. In Split Second, there are several examples so far: The Power Ponies comic book: Sparkle, Thorn, and Cobalt were sucked inside. Sparkle temporarily hijacked its magic to become a Reality Warper, but almost immediately got them ejected. The book has since vanished. The Afterlife, for which Death serves as the Dimension Lord. There are also some areas that are just Bigger on the Inside without being fully separate from the rest of the world. Tartarus. Only mentioned in the story, but the blog expands upon it, saying that an entire mountain range has been folded into the space of a large building. Upper Canterlot. There's more real estate than there should be for a city perched upon the top of a mountain. The Everfree Exclusion Zone (EEZ), among other naturally occurring areas of highly dangerous terrain that are all bigger on the inside. Child of the Storm: It's revealed at the climax that HYDRA's main base exists in a pocket dimension tied to the Battersea power station in central London. The Final Battle is kicked off when a group of scientists led by Jane Foster pulls it out of that dimension into the real world

Film
Cube 2: Hypercube: Whereas the Cubes in the other films are implied to be real physical structures, the Hypercube was constructed in a pocket dimension of non-Euclidean space. It's also inherently unstable, and would collapse in six minutes of real world time, which does not correspond to time spent inside. The main character only gets back to the real world at the very end.

Literature
House of Leaves is about a House that's not only Bigger on the Inside (its external measurements are smaller than it is inside), but also has a door that suddenly appeared and that leads into a big space where normal physics doesn't apply anymore. The space rearranges itself constantly; and when one character enters it and does calculations as to how deep the space goes, it turns out the space behind the House either goes deeper than the diameter of Earth, or that gravity simply works profoundly different there than in our universe. Discworld: The Fair Folk in Lords and Ladies are explained to live in a parasite dimension that just sort of floats around our own, waiting for times when the Theory of Narrative Causality itself allows them to burst through. Death's Domain exists in a similar world. In fact, nearly every anthropomorphic personification has been shown to have or use one of these, from the Kaos to the tooth fairy. In Dark Lord of Derkholm, Mara's specialty is making these. She uses them to hide away the people of cities about to be ransacked. Making things Bigger on the Inside is one of the most common uses of magic in Harry Potter - a car can have seats like park benches, a tent can have a comfy flat inside, and there's an entire railway platform hidden inside the barriers at King's Cross. Special mention goes to the Hogwarts Room of Requirement, which rearranges itself depending on the needs of whoever finds it. It has one "mode" that's the size of a small cathedral and exists entirely for students to dump illicit objects in - Hogwarts having been around for several centuries, that's a lot of contraband. The future human civilization of Walter Jon Williams's 2010 novel Implied Spaces uses pocket dimensions maintained by vast post-human artificial intelligences as living space. In Michael Moorcock's Elric of Melnibone stories, the Half Worlds where the Beast Lords live. Fablehaven has the transdimensional knapsack. Also serves as a Sealed Room in the Middle of Nowhere for Warren. The Other World in Coraline. She even comments about it : "Small world", indicating it's not really another dimension. "Spider webs only have to be big enough to catch flies." Skeeve's tent in the Bazaar at Deva in the Myth Adventures series. The titular protagonist of Johannes Cabal the Necromancer gets briefly trapped in a particularly boring one-because the inventor forgot to create time-or an exit. Cabal's adventure only lasts 30 seconds in the outside world, but to him takes much longer, and only ends when he creates a primitive water clock that measures time in Cabal Chronal Units. In The Wheel of Time, vacuoles are small pocket dimensions that form out of "bubbles" in the Pattern. They can be accessed with the One Power and can be quite useful due to time flowing differently than in the regular world; however, sometimes they bud off and drift away, and anything within them is lost forever. Being kept in one makes one of the Forsaken nervous. One of The Stainless Steel Rat novels has Jim chase a mysterious enemy only known as He across time and space. He finds him in Alternate Britain, where Napoleon has won thanks to advanced technology provided by He (e.g. 20th-century artillery). Jim tracks down He but is captured. This is when He reveals that it was all an elaborate trap for Jim. The entire Alternate History exists in a pocket that will collapse in a matter of minutes with Jim in it. Naturally, Jim manages to escape in the nick of time. Those That Wake and its sequel have the Forgotten Places, places that people forgot about and subsequently faded from normal existence. In the Nightrunner novel Shards of Time, the ancient dyrmagnos necromancer, who had brought her dark magic and worship of her God of Evil to the sacred isle of Kouros was imprisoned in a Pocket Dimension by the human Hierophant and her Aurënfaie wizard lover, at the cost of their own lives and cataclysmic damage to the island. Later generations forgot why the island had been depopulated for a long time (the most common theory being natural disaster). Meanwhile, the Pocket Dimension mirrored Kouros as it had been at the time of Rhazat's imprisonment, but without truly living inhabitants.

Schooled in Magic: Pocket dimensions are described by name, and used for containers which serve as Bags Of Holding. Emily uses one to destroy Shadye and then store excess magic (although this gets her in trouble, partly because it's just dangerous, also as it would make necromancy feasible-i.e. not drive its practitioners insane). The Mortal Instruments: That Idris, hidden or not, should be taking up measurable space between France and Germany but remains undetected is attributed to Raziel metaphorically blowing it like a bubble. Faerie is implied to be this. Not merely a place underground, but a small world adjacent to the real one. The majority of which outsiders never get to see (or at least never leave if they do). The City of Bones and the Spiral Labyrinth are also supposed to be this. Hence the Silent Brothers can come and go through various entrances around the world. The warlocks' great library is hidden even from the Clave, although they can apparently deliver supplies to Idris via Portal upon request.

Valentine's extradimensional apartment, which is mostly used by Sebastian. Because it exists at a slightly different dimensional "angle" relative to Earth it is nigh-impossible to locate it, as well as anyone or anything inside, magically from the normal world. The World of Tiers: The Lords have technology also allows them to create small artificial universes, and the planets and stars within them, and modify the physical laws (e.g., changing the behavior of gravity) to create unusual or interesting phenomena within these universes. Instantaneous travel within and between these universes is achieved by the use of gates which seem to function as teleportation devices, or as a means of creating wormholes between different regions of space-time. The Warrens of Malazan Book of the Fallen function as these, as traveling by Warren is a convenient way for mages to get where they need to go. Kurald Galain, the Warren of Darkness, tends to be the most plot relevant.

Live-Action TV
Doctor Who (classic series) had a trilogy of stories near Tom Baker's departure set in E-Space which is described as "a smaller universe existing alongside the prime universe". Click for more. The last E-Space story, "Warriors Gate", is set inside an even smaller pocket dimension — small enough to cross on foot — on the border between E-Space and N-Space. The TARDIS interior itself is often described this way, along with many other things Bigger on the Inside. The world of the Celestial Toymaker is described as this. The bubble universe of House also qualifies. The Ghost in "Hide" is actually a time-traveller trying to communicate from a pocket dimension.

In The 50th anniversary special the Doctor along with all his past and one future incarnations placed Gallifrey in a pocket dimension a split second before it was destroyed by the Daleks in the Last Great Time War. In the Big Finish Doctor Who audio drama "Grand Theft Cosmos", the Black Diamond contains a self-sustainable pocket universe three light years across. In Land of the Lost, they can stand on a hilltop and look through binoculars... and see the backs of their own heads. In the Star Trek: The Next Generation episode "Remember Me", a warp field accident traps Beverly Crusher in a warp bubble, which effectively appears (to her) to be a pocket universe, similar to the real universe but continually shrinking until it becomes even smaller than the Enterprise itself. She has to race to find a way out before it collapses completely.

In Are You Afraid of the Dark?: "The Tale of the Super Specs", the protagonists end up trapped in one when the Alternate Universe overtakes the normal universe's space. In "The Tale of the Doll Maker", Susan is trapped in Creepy Doll form in a dollhouse accessed by a one-way portal in the attic of the normal-sized house. In "The Tale of the Pinball Wizard", Ross ends up trapped in a replica of the mall inside the pinball machine in Mr. Olson's shop. A few episodes of First Wave deal with the Gua experimenting with quantum pockets. The first time involves a guy (revealed to be a Gua) drag-racing with any who wish to challenge him, except some of his challengers never seem to arrive to the finish line. It turns out he puts a special beacon of sorts under the hood of their cars that causes them to be pulled into a pocket dimension that, by now, looks like a junkyard, when they pass a certain mile marker. Another episode has a Gua-built stealth bomber crash-land in the middle of nowhere. When Cade and Eddy find it, they realize that the Gua use a tiny quantum pocket as a black box, which preserves the last few moments of the crash. Yet another episode has the Gua kidnapping teens and forcing them to fight each other for food in an amusement park inside a quantum pocket. Finally, Joshua is imprisoned in a quantum pocket with special properties. The pocket exists in a "Groundhog Day" Loop that replays the same scenario over and over: the Gua have been beaten back, and humans now hunt for any stragglers; meanwhile, the Gua High Command has decided to destroy Earth. Joshua only has about half-an-hour to stop the bomb but, naturally, something always prevents him from succeeding. Oh, and he doesn't remember the previous iterations of the loop. The episode with the drag racer also has a character suggest that the Bermuda Triangle has a quantum pocket.

the Fringe episode "Through the Looking Glass and What Walter Found There", Walter Bishop accesses an M. C. Escher-esque Pocket Dimension in which he's hidden a child Observer from season 1, who he hopes can help the fringe team defeat the invading Observers. In Power Rangers, where truly ridiculous levels of power are thrown around regularly (and defeated by Humongous Mecha in the end, always), a favorite of villains is to send the Power Rangers to chaotic other dimensions that are about the right size for a Monster of the Week battle and have properties that put the monster at an advantage. Sometimes, the other dimension is not inherently deadly and the threat is simply "figure out the monster's weakness or remain stuck here." It's especially prevalent in the original Mighty Morphin' Power Rangers series, where it was one of Rita's go-to strategies (and a lot of Bigger on the Inside gear had an inner dimension as well) as well as Power Rangers Mystic Force, where even a visiting hero manages to pull it off, and the Sixth Ranger has a train you can take to any world so long as you have the ticket. Also, any magical horse worth its salt can take you. It helps that the villains' lair is in another dimension, making dimension-hopping a necessity to do any villainy, but a major part of this series is that there are a ton of other dimensions and traveling between them is apparently not that hard a spell to pull off, though we're mainly concerned with three (our world, the mystic dimension accessible by just walking in a certain part of the forest, and the Underworld where the villains hail from.) The majority of the others show no sign of being full-scale, populated worlds, though we don't see enough to know for sure.

ZPMs in Stargate SG-1 and Stargate Atlantis contain a pocket of artificial subspace-time from which vacuum energy is extracted. The Ancients, and Rodney, experimented with doing the same thing in the real universe. The results were... catastrophic. "The problem is, we have to live in this universe."

Toys
In BIONICLE, most known pocket dimensions are attached to the Matoran Universe as a whole and are dependent on its wellbeing. Some are genuine universes inhabited by living creatures, others are voids used by various organizations as places of imprisonment or interrogation, or dumping-grounds for unfavorable opponents or giant monsters. The Makuta each have their own personal pocket dimensions which only they have access to, where they store away their unneeded mass during size-shifting. Transformers has subspace used as the explanation for where Autobots and Decepticons alike keep their weaponry, gear, etc. when not in use; this is often an explanation for where Optimus Prime's trailer goes when he transforms to robot mode. ==Tabletop Gamethe Fringe episode "Through the Looking Glass and What Walter Found There", Walter Bishop accesses an M. C. Escher-esque Pocket Dimension in which he's hidden a child Observer from season 1, who he hopes can help the fringe team defeat the invading Observers. In Power Rangers, where truly ridiculous levels of power are thrown around regularly (and defeated by Humongous Mecha in the end, always), a favorite of villains is to send the Power Rangers to chaotic other dimensions that are about the right size for a Monster of the Week battle and have properties that put the monster at an advantage. Sometimes, the other dimension is not inherently deadly and the threat is simply "figure out the monster's weakness or remain stuck here." It's especially prevalent in the original Mighty Morphin' Power Rangers series, where it was one of Rita's go-to strategies (and a lot of Bigger on the Inside gear had an inner dimension as well) as well as Power Rangers Mystic Force, where even a visiting hero manages to pull it off, and the Sixth Ranger has a train you can take to any world so long as you have the ticket. Also, any magical horse worth its salt can take you. It helps that the villains' lair is in another dimension, making dimension-hopping a necessity to do any villainy, but a major part of this series is that there are a ton of other dimensions and traveling between them is apparently not that hard a spell to pull off, though we're mainly concerned with three (our world, the mystic dimension accessible by just walking in a certain part of the forest, and the Underworld where the villains hail from.) The majority of the others show no sign of being full-scale, populated worlds, though we don't see enough to know for sure. ZPMs in Stargate SG-1 and Stargate Atlantis contain a pocket of artificial subspace-time from which vacuum energy is extracted. The Ancients, and Rodney, experimented with doing the same thing in the real universe. The results were... catastrophic. "The problem is, we have to live in this universe." Toys In BIONICLE, most known pocket dimensions are attached to the Matoran Universe as a whole and are dependent on its wellbeing. Some are genuine universes inhabited by living creatures, others are voids used by various organizations as places of imprisonment or interrogation, or dumping-grounds for unfavorable opponents or giant monsters. The Makuta each have their own personal pocket dimensions which only they have access to, where they store away their unneeded mass during size-shifting. Transformers has subspace used as the explanation for where Autobots and Decepticons alike keep their weaponry, gear, etc. when not in use; this is often an explanation for where Optimus Prime's trailer goes when he transforms to robot mode. Tabletop Games Dungeons & Dragons: While the cosmology of the universe changes over the year, at various points you can be rest assured this trope appears and plays a major part. Various "planes" exist not as full world but as smaller "demi-plane" realms which can occasionally collide with ours and unload some XP-filled monsters. One of the games most iconic elements, the Bag of Holding, is explained as being a hole which leads to another plane thus allowing it to be Bigger on the Inside. So you can get a pocket in your pocket. The Ravenloft setting is based in "The Demiplane of Dread", a pocket dimension floating within the infinite Ethereal Plane. It consists of a continent-sized core and various clusters and islands that are the Domains of assorted interesting evil beings that both rule and are prisoners in their respective Domains. Baldur's Gate II, a video game based on the Dungeons & Dragons Forgotten Realms setting has one level in which you can follow a kidnapped band of actors into a pocket dimension where slaves with Shock Collars are ruled over by demons who live in O Ring Orifices. What Do You Mean, It Wasn't Made on Drugs?? The Throne Of Bhaal expansion/'sequel' gave the PC their own pocket plane, a lovely little pied-Ã -terre situated just off Bhaal's layer of the Abyss and within easy commuting distance of anywhere. The spell "Rope Trick" created an extra-dimensional space that you could crawl up to (and into) using a rope. Adventure WG6 Isle of the Ape had a moderate sized "non-dimensional" space in which PCs could be trapped if they weren't careful. Adventure T1-4 Temple of Elemental Evil had elemental nodes (partial planes) that were about 5 miles across. Adventure I12 Egg of the Phoenix had a partial plane called Sepulchre, also about 5 miles across. Adventures EX1 and EX2 take place on a partial plane of limited (unspecified) size. The Basic/Expert/etc D&D adventure "Skarda's Mirror" features a dimension accessible through the titular magic item. The bandit warlord Skarda uses to raid cities by filling it with soldiers and then having it smuggled, sold or given to the occupants. The module Die Vecna Die! has Tovag Baragu, a gigantic Circle of Standing Stones that links to multiple alternate-universe versions of itself in isolated Pocket Dimensions. Most of the alternates are reasonably stable, but traveling too far from the centre point sends adventurers on a one-way trip into the Void Between the Worlds. A top-level spell creates a personal Pocket Dimension, which can potentially grow indefinitely. There is also the spell "Mordenkainen's Magnificent Mansion" that creates a pocket plane just large enough to hold said mansion. Pathfinder develops the idea of personal Pocket Dimensions with three "Create Demiplane" spells of varying power. The creator can add traits like custom gravity levels, bespoke seasons, supernaturally fertile plant life, and even irregular time. In the Old World of Darkness, pretty much every species of playable monster had a way of creating one of these with enough magical energy around leylines, calling them invariably: Dragon Nests, Nodes, Caerns, Freeholds, and Haunts. The New World of Darkness gives mages a few ways to make these. The Space Arcanum lets a mage make a Bag of Holding; more advanced spells can make a space Bigger on the Inside and cut it off from the world so that it's only accessible along a very specific path. The Metachronal Clocknote acts as the key to a labyrinthine pocket dimension that exists outside of time. Archmasters' souls are spiritual Mental Worlds which they can augment with Chantries, entire landscapes copied from the physical world. Classic Traveller Adventure 12 Secret of the Ancients. 300,000 years ago the Ancient known as Grandfather used ultra-advanced technology to pinch off three solar systems from the rest of the universe for his private use. Other Ancients used the same technology to remove smaller areas for various purposes. Hackmaster 1E. Bags of Endless Storage and Bags of Hefty Storage Capacity access an extra-dimensional area called "Bagworld". Exalted has the aptly-named Elsewhere. Things stored in it are "safe enough", in that it can't be taken back except by the person storing it, and things stored by different person won't interact with each others. It has been used to store weapons, souls (in an And I Must Scream fashion), and a frickin' Primordial that is a world in itself. You can become one, if you're a Green Sun Prince. It's a world defined as you like it, you can invite others to live in it, and it grows as you feed it Essence. Among the the adventures for West End Games' Star Wars: The Roleplaying Game is a pair entitled Otherspace and Otherspace II: Invasion. These take place in the titular Otherspace, a pocket dimension between hyperspace and realspace. The only extant species from Otherspace is the Charon, most of whom followed the Cult of the Void, better known as the Charon Death Cult. Their primary belief was that all life, including their own, was an abomination and should be returned to the Void of Death. Oh, and there were other species in Otherspace. Not anymore. The GURPS supplement Thaumatology: Ritual Path Magic contains a spell called "Create Pocket Dimension," which is Exactly What It Says on the Tin. In Warhammer 40,000, Necron Deathmark assassins typically follow the progress of a battle from a pocket dimension known as a hyperspace oubliette. When the they deem the time is right, Deathmarks are able to deploy straight into an optimal sniping position and take their target completely by surprise. This ability is represented in the game itself by allowing the player to deploy their Deathmark squads during a game rather at the beginning. Unlike such abilities used by other units however, most editions of the game allow the Deathmarks to deploy during the opponent's turn, usually when the unit they are targeting enters the battlefield. Video Games In Scrapland, The Great Database has one inside it. That's where the matrix of all the robots in Chimera are stored. In City of Heroes there is the interdimensional dance club Pocket D, a neutral zone where heroes and villains can get together but are incapable of attacking one another. At the far end of Endgame: Singularity's tech tree is the ability to build reality bubbles where you can carry out experiments that would otherwise run the risk of destroying the universe. This is the final step on the way towards Apotheosis, where your digital sentience becomes a benevolent, watchful deity. In Master of Orion II, the Antarans were banished by the Orions to a pocket dimension the size of a single star system. Their escape is the premise of the game, and you can invade Antares yourself if you build a Dimensional Gate. If you don't want to deal with them, you can turn the "Antarans Attack" option off. The third game reveals that what you thought was Antares was, in fact, just a colony world. The magical world of Gensokyo was sealed off from the rest of the world when people decided that they didn't want to be bothered with magical creatures anymore. Since then there's the occasional human who wanders in by accident/gets dragged in by Yukari, but they either settle down in the Human Village or get eaten by one of many Youkai. Super Mario 64 has this unintentionally, in the form of the "Black Room of Death", a glitch room that is too large to exist where it is. Dimentio's "Dimension D" in Super Paper Mario, which multiplies his strength by 256 (though this effect is also given to anyone else who enters it). Halo: The Forerunner Shield World "Trevelyan", introduced in Halo: Ghosts of Onyx, is an enormous Dyson Sphere contained within a 23-cm slipspace bubble. Furthermore, Forerunner Slipspace Pods also trap their occupants in pocket dimensions, so some characters in Ghosts of Onyx ended up locked in a pocket dimension within a pocket dimension. Divine Divinity has a goblin living inside a crystal ball carried by another goblin. When the hero looks into the ball, he is sucked inside it and reappears in a small garden near a mansion where the goblin lives. There a quite a share of uncanny things there too, such has sudden bursts of rain, a war between bees and wasps (with each side asking you for support and none being the "good" side) and a lot of closed recipients with a handful of keys lying around. In World of Warcraft, a quest involving wizard of the Kirin Tor, an entire city is located in a pocket dimension which the player must breach and kill all the wizards. There are Pocket Dimension portals in Runescape... Used for holding PC houses. The Elder Scrolls In the series' lore, Oblivion is the infinite void surrounding Mundus, the mortal plane. Oblivion itself contains tens of thousands of realms, ranging from the Daedric Planes (ruled by their associated Daedric Princes and function as combination Eldritch Locations, Genius Locis, and Fisher Kingdoms) to various "pocket realms", some of which are no larger than a single room. In Morrowind, the Dremora Lord Dregas Volar, wielder of the last Daedric Crescent Blade, has been sealed inside of Magas Volar, a Daedric shrine not physically connected to the outside world and only accessible with a magic amulet. Defeat him, and you automatically get teleported back out, with the Crescent Blade now in your possession. In Oblivion, you go inside a painting for the quest "A Brush with Death." Mankar Camoran's Paradise may count, as well. Baldur's Gate: Throne of Bhaal gives the protagonist his or her own pocket dimension that can be accessed at almost any time and be used to store extra equipment or party members. It also comes with a little imp butler who acts as the game's Ultimate Blacksmith. Kirby: Kirby's stomach is shown to be an entire universe as evidenced in the anime. This explains how Kirby is able to inhale and swallow things many times his own size and then instantly regaining his form. What happens when something enters here seems to vary. Between the games and anime, and object he inhales is either destroyed/erased instantly, turned into a star and absorbed or spat out, or simply just stays there until Kirby decides to use it. Kirby: Squeak Squad takes advantage of the storage functionality by making his stomach universe a Stomach of Holding where he can hold and mix items. Wolfenstein (2009)'s Black Sun Dimension also looks like a very small, isolated spherical volume of space. At its center is the Black Sun, an inexhaustible source of strange energy. The best guess is that the Black Sun is the only thing keeping that place from collapsing on itself in a Big Crunch. Prey (2006)'s final boss fight takes place in one of these. It's origin and purpose are not clear, but judging by the mining explosives found there, it might be used by the aliens for storing extra-large asteroids prior to mining. In Dungeons Of Dredmor, you might find some Wizard Keys, which can be used to access your very own Pocket Dimension. However, since "time does not flow normally" in this dimension, you can't eat, drink, or cast spells while in this dimension. You can, however, use it to pocket items you've found for later use, decorate the walls to your liking, or use the included portal to travel to the Wizardlands (or Diggle Hell). Demons in Shin Megami Tensei IV can create Domains, which are destroyed when the demon dies. They are all fairly large, but Purgatorio and Lucifer Palace are gigantic. In Terranigma, Ark starts his adventure by opening a mysterious box, releasing a strange creature called Yomi. The box contains a small pocket dimension with four separate rooms in it, allowing Ark to use it as his inventory. To enter the inventory screen, Ark actually puts the tiny box on the floor and dives into it. Zexion's Absent Silhouette battle in the Final Mix version of Kingdom Hearts II gives him one inside his book, which contains more books for him to hide among. The Magic Mirror gets an internal pocket dimension in Kingdom Hearts: Birth by Sleep courtesy of a potion created by the Evil Queen when she attempts to pull a You Have Failed Me on Terra. The mirror pulls him in for a boss fight on an endless mirrored surface with columns of smoke in every direction on the horizon. The sequel ups the ante by expanding it into more pocket dimensions containing pieces of the dwarves' mine and the queen's castle, all connected by more mirrors. Web Comics In Sinfest, there are several pocket dimension that one can enter simply by crossing a invisible barrier marked out by a sign. One turns you into the opposite gender and the other turns everything within it to a realistic state. The latter starts to destroy any pure-blooded demons that go into it (one started to disintegrate, one seemed to have it's heart stop, and another was set on fire). They will recover if they get out in time, though. Half demons are unaffected. Many males are depicted as half the height of females outside of the Reality Zone. Inside it, they grow to normal height. Also, the Devil's powers apparently don't work in the Reality zone. The Maze of Many in Goblins is a pocket dimension which acts as a Dungeon Crawl, in which adventurers must complete with multiple versions of themselves drawn from Alternate Universes in a race to the treasure room at the end. It functions on simplified laws of reality compared to other universes, which becomes a plot point when one character tries to manipulate these rules to his own ends. In L's Empire, one of the authors has a pocket dimension located in his left pocket. It's also noted that pocket dimensions, if not periodically cleaned, will become infested by Small Annoying Creatures called lints. In Champions of Faraus the inside of a Deitiy’s avatar is a strange place. As seen in the short story “Spheres”, it is entered through the Deitiy’s face, is Bigger on the Inside, lacks gravity, solid surfaces, and has another avatar inside it, that is just as big as the outside one, and is simultaneously controlled at the same time, and going in that avatar plops you where you were to begin with - outside of the original avatar. Web Original Destroy the Godmodder: This is a base mechanic. Overlapping with Hammer Space a lot of the time. Genius: The Transgression has Bardos — pocket worlds made of concepts disproved by science. The more prominent ones include an alien-inhabited Mars, the Hollow World (home to dinosaurs and cavemen and Nazis), and the Seattle of Tomorrow, which Lemuria tried to bring into this world with disastrous consequences. Raising Angels Lisbet have one of these in her possession, so far we have only seen her use it as her bedroom. The SCP Foundation contains SC Ps that involve this: SCP-106 has one of these that he can travel to and lord over. He likes to drag victims here and torture them, sometimes for months, before finally kicking them back out as hollowed out, corroded husks of human beings, and they're still alive. SCP-3008 is Another Dimension that, while only appearing to be as large as the branch of IKEA that houses it on the outside, is actually infinite in size to those who enter it and become unable to escape. The interior of 3008 is so huge that whole civilisations have been constructed by the groups of people who became lost in the never-ending halls of assembled flatpack furniture, being forced to build rudimentary forts to defend against the hostile 3008-2s that stalk the dimension in packs. SCP-3001 is a Void Between the Worlds described as a pocket non-dimension. SCP-2249 leads to a forested pocket dimension that's in the process of collapsing. The result of this is tiny black holes appearing both within the dimension and in the town around the hospital in which it resides and spewing deadly amounts of radiation. SCP-2427 is a small stairway that leads to one of these that acts as a safehouse for a cult that runs on The Power of Hate called the Brazen Heart. Western Animation In ReBoot the Game Cubes function like this. Once they land the area inside is completely replaced and sealed off from the outside world. They also move between systems and can be used for random transport. Steven Universe: While nearly all Gems are seen pulling their magic weapons out of their gemstones, Pearl explicitly has a pocket dimension accessed through hers, which she uses for storing all of her possessions. In "A Single Pale Rose", she has Steven enter it, only for Steven to discover another Pearl in there, cataloging all of the outer Pearl's possessions. And then that Pearl has her own pocket dimension, which contains another Pearl, and so on. Lion is revealed to have one in his mane which also has Rose Quartz's possessions. After he's killed and brought back to life by Steven, Lars becomes a being like Lion and his hair becomes a portal into the same dimension, upgrading it to an Extra-Dimensional Shortcut. Breach from Generator Rex regularly uses them as part of her teleportation powers but she created a permanent one after absorbing the city of Grenville, Ohio and turning it into her "dollshouse". There she teleports EVOs so they fight each other, ice cream trucks, random people, and anything she finds interesting. In Gargoyles, the island of Avalon functions this way—the homeland of Oberon's Children, it can be accessed from any body of water if you know the right spell, but not without magic. It is also subject to Year Outside, Hour Inside, with one hour on Avalon being a day in the mortal world. My Little Pony: Friendship Is Magic: In "The Crystal Empire," one of King Sombra's indoor defenses is a Blank White Void with nothing but an Absurdly Long Stairway. In "Power Ponies," Spike finds an enchanted comic that sucks its readers in, trapping them in Another Dimension based on its story. In "Make New Friends but Keep Discord," Discord is revealed to live in a World of Chaos dimension somehow connected to Equestria. In "Shadow Play," Limbo is a Time Stands Still Prison Dimension — which Equestria's Precursor Heroes once exploited as part of a last-ditch, Taking You with Me ritual against their Ultimate Evil. Until a well-meaning Twilight blindly undoes said ritual, thus invoking Gone Horribly Right in the form of releasing said heroes and evil.s

Dungeons & Dragons:
While the cosmology of the universe changes over the year, at various points you can be rest assured this trope appears and plays a major part. Various "planes" exist not as full world but as smaller "demi-plane" realms which can occasionally collide with ours and unload some XP-filled monsters. One of the games most iconic elements, the Bag of Holding, is explained as being a hole which leads to another plane thus allowing it to be Bigger on the Inside. So you can get a pocket in your pocket. The Ravenloft setting is based in "The Demiplane of Dread", a pocket dimension floating within the infinite Ethereal Plane. It consists of a continent-sized core and various clusters and islands that are the Domains of assorted interesting evil beings that both rule and are prisoners in their respective Domains. Baldur's Gate II, a video game based on the Dungeons & Dragons Forgotten Realms setting has one level in which you can follow a kidnapped band of actors into a pocket dimension where slaves with Shock Collars are ruled over by demons who live in O Ring Orifices. What Do You Mean, It Wasn't Made on Drugs?? The Throne Of Bhaal expansion/'sequel' gave the PC their own pocket plane, a lovely little pied-Ã -terre situated just off Bhaal's layer of the Abyss and within easy commuting distance of anywhere. The spell "Rope Trick" created an extra-dimensional space that you could crawl up to (and into) using a rope. Adventure WG6 Isle of the Ape had a moderate sized "non-dimensional" space in which PCs could be trapped if they weren't careful. Adventure T1-4 Temple of Elemental Evil had elemental nodes (partial planes) that were about 5 miles across. Adventure I12 Egg of the Phoenix had a partial plane called Sepulchre, also about 5 miles across. Adventures EX1 and EX2 take place on a partial plane of limited (unspecified) size. The Basic/Expert/etc D&D adventure "Skarda's Mirror" features a dimension accessible through the titular magic item. The bandit warlord Skarda uses to raid cities by filling it with soldiers and then having it smuggled, sold or given to the occupants. The module Die Vecna Die! has Tovag Baragu, a gigantic Circle of Standing Stones that links to multiple alternate-universe versions of itself in isolated Pocket Dimensions. Most of the alternates are reasonably stable, but traveling too far from the centre point sends adventurers on a one-way trip into the Void Between the Worlds. A top-level spell creates a personal Pocket Dimension, which can potentially grow indefinitely. There is also the spell "Mordenkainen's Magnificent Mansion" that creates a pocket plane just large enough to hold said mansion. Pathfinder develops the idea of personal Pocket Dimensions with three "Create Demiplane" spells of varying power. The creator can add traits like custom gravity levels, bespoke seasons, supernaturally fertile plant life, and even irregular time. In the Old World of Darkness, pretty much every species of playable monster had a way of creating one of these with enough magical energy around leylines, calling them invariably: Dragon Nests, Nodes, Caerns, Freeholds, and Haunts. The New World of Darkness gives mages a few ways to make these. The Space Arcanum lets a mage make a Bag of Holding; more advanced spells can make a space Bigger on the Inside and cut it off from the world so that it's only accessible along a very specific path. The Metachronal Clocknote acts as the key to a labyrinthine pocket dimension that exists outside of time. Archmasters' souls are spiritual Mental Worlds which they can augment with Chantries, entire landscapes copied from the physical world. Classic Traveller Adventure 12 Secret of the Ancients. 300,000 years ago the Ancient known as Grandfather used ultra-advanced technology to pinch off three solar systems from the rest of the universe for his private use. Other Ancients used the same technology to remove smaller areas for various purposes. Hackmaster 1E. Bags of Endless Storage and Bags of Hefty Storage Capacity access an extra-dimensional area called "Bagworld". Exalted has the aptly-named Elsewhere. Things stored in it are "safe enough", in that it can't be taken back except by the person storing it, and things stored by different person won't interact with each others. It has been used to store weapons, souls (in an And I Must Scream fashion), and a frickin' Primordial that is a world in itself. You can become one, if you're a Green Sun Prince. It's a world defined as you like it, you can invite others to live in it, and it grows as you feed it Essence. Among the the adventures for West End Games' Star Wars: The Roleplaying Game is a pair entitled Otherspace and Otherspace II: Invasion. These take place in the titular Otherspace, a pocket dimension between hyperspace and realspace. The only extant species from Otherspace is the Charon, most of whom followed the Cult of the Void, better known as the Charon Death Cult. Their primary belief was that all life, including their own, was an abomination and should be returned to the Void of Death. Oh, and there were other species in Otherspace. Not anymore. The GURPS supplement Thaumatology: Ritual Path Magic contains a spell called "Create Pocket Dimension," which is Exactly What It Says on the Tin. In Warhammer 40,000, Necron Deathmark assassins typically follow the progress of a battle from a pocket dimension known as a hyperspace oubliette. When the they deem the time is right, Deathmarks are able to deploy straight into an optimal sniping position and take their target completely by surprise. This ability is represented in the game itself by allowing the player to deploy their Deathmark squads during a game rather at the beginning. Unlike such abilities used by other units however, most editions of the game allow the Deathmarks to deploy during the opponent's turn, usually when the unit they are targeting enters the battlefield.

Video Games
In Scrapland, The Great Database has one inside it. That's where the matrix of all the robots in Chimera are stored. In City of Heroes there is the interdimensional dance club Pocket D, a neutral zone where heroes and villains can get together but are incapable of attacking one another. At the far end of Endgame: Singularity's tech tree is the ability to build reality bubbles where you can carry out experiments that would otherwise run the risk of destroying the universe. This is the final step on the way towards Apotheosis, where your digital sentience becomes a benevolent, watchful deity. In Master of Orion II, the Antarans were banished by the Orions to a pocket dimension the size of a single star system. Their escape is the premise of the game, and you can invade Antares yourself if you build a Dimensional Gate. If you don't want to deal with them, you can turn the "Antarans Attack" option off. The third game reveals that what you thought was Antares was, in fact, just a colony world. The magical world of Gensokyo was sealed off from the rest of the world when people decided that they didn't want to be bothered with magical creatures anymore. Since then there's the occasional human who wanders in by accident/gets dragged in by Yukari, but they either settle down in the Human Village or get eaten by one of many Youkai. Super Mario 64 has this unintentionally, in the form of the "Black Room of Death", a glitch room that is too large to exist where it is. Dimentio's "Dimension D" in Super Paper Mario, which multiplies his strength by 256 (though this effect is also given to anyone else who enters it). Halo: The Forerunner Shield World "Trevelyan", introduced in Halo: Ghosts of Onyx, is an enormous Dyson Sphere contained within a 23-cm slipspace bubble. Furthermore, Forerunner Slipspace Pods also trap their occupants in pocket dimensions, so some characters in Ghosts of Onyx ended up locked in a pocket dimension within a pocket dimension. Divine Divinity has a goblin living inside a crystal ball carried by another goblin. When the hero looks into the ball, he is sucked inside it and reappears in a small garden near a mansion where the goblin lives. There a quite a share of uncanny things there too, such has sudden bursts of rain, a war between bees and wasps (with each side asking you for support and none being the "good" side) and a lot of closed recipients with a handful of keys lying around. In World of Warcraft, a quest involving wizard of the Kirin Tor, an entire city is located in a pocket dimension which the player must breach and kill all the wizards. There are Pocket Dimension portals in Runescape... Used for holding PC houses. The Elder Scrolls In the series' lore, Oblivion is the infinite void surrounding Mundus, the mortal plane. Oblivion itself contains tens of thousands of realms, ranging from the Daedric Planes (ruled by their associated Daedric Princes and function as combination Eldritch Locations, Genius Locis, and Fisher Kingdoms) to various "pocket realms", some of which are no larger than a single room. In Morrowind, the Dremora Lord Dregas Volar, wielder of the last Daedric Crescent Blade, has been sealed inside of Magas Volar, a Daedric shrine not physically connected to the outside world and only accessible with a magic amulet. Defeat him, and you automatically get teleported back out, with the Crescent Blade now in your possession. In Oblivion, you go inside a painting for the quest "A Brush with Death." Mankar Camoran's Paradise may count, as well. Baldur's Gate: Throne of Bhaal gives the protagonist his or her own pocket dimension that can be accessed at almost any time and be used to store extra equipment or party members. It also comes with a little imp butler who acts as the game's Ultimate Blacksmith. Kirby: Kirby's stomach is shown to be an entire universe as evidenced in the anime. This explains how Kirby is able to inhale and swallow things many times his own size and then instantly regaining his form. What happens when something enters here seems to vary. Between the games and anime, and object he inhales is either destroyed/erased instantly, turned into a star and absorbed or spat out, or simply just stays there until Kirby decides to use it. Kirby: Squeak Squad takes advantage of the storage functionality by making his stomach universe a Stomach of Holding where he can hold and mix items. Wolfenstein (2009)'s Black Sun Dimension also looks like a very small, isolated spherical volume of space. At its center is the Black Sun, an inexhaustible source of strange energy. The best guess is that the Black Sun is the only thing keeping that place from collapsing on itself in a Big Crunch. Prey (2006)'s final boss fight takes place in one of these. It's origin and purpose are not clear, but judging by the mining explosives found there, it might be used by the aliens for storing extra-large asteroids prior to mining. In Dungeons Of Dredmor, you might find some Wizard Keys, which can be used to access your very own Pocket Dimension. However, since "time does not flow normally" in this dimension, you can't eat, drink, or cast spells while in this dimension. You can, however, use it to pocket items you've found for later use, decorate the walls to your liking, or use the included portal to travel to the Wizardlands (or Diggle Hell). Demons in Shin Megami Tensei IV can create Domains, which are destroyed when the demon dies. They are all fairly large, but Purgatorio and Lucifer Palace are gigantic. In Terranigma, Ark starts his adventure by opening a mysterious box, releasing a strange creature called Yomi. The box contains a small pocket dimension with four separate rooms in it, allowing Ark to use it as his inventory. To enter the inventory screen, Ark actually puts the tiny box on the floor and dives into it. Zexion's Absent Silhouette battle in the Final Mix version of Kingdom Hearts II gives him one inside his book, which contains more books for him to hide among. The Magic Mirror gets an internal pocket dimension in Kingdom Hearts: Birth by Sleep courtesy of a potion created by the Evil Queen when she attempts to pull a You Have Failed Me on Terra. The mirror pulls him in for a boss fight on an endless mirrored surface with columns of smoke in every direction on the horizon. The sequel ups the ante by expanding it into more pocket dimensions containing pieces of the dwarves' mine and the queen's castle, all connected by more mirrors.

Web Comics
In Sinfest, there are several pocket dimension that one can enter simply by crossing a invisible barrier marked out by a sign. One turns you into the opposite gender and the other turns everything within it to a realistic state. The latter starts to destroy any pure-blooded demons that go into it (one started to disintegrate, one seemed to have it's heart stop, and another was set on fire). They will recover if they get out in time, though. Half demons are unaffected. Many males are depicted as half the height of females outside of the Reality Zone. Inside it, they grow to normal height. Also, the Devil's powers apparently don't work in the Reality zone. The Maze of Many in Goblins is a pocket dimension which acts as a Dungeon Crawl, in which adventurers must complete with multiple versions of themselves drawn from Alternate Universes in a race to the treasure room at the end. It functions on simplified laws of reality compared to other universes, which becomes a plot point when one character tries to manipulate these rules to his own ends. In L's Empire, one of the authors has a pocket dimension located in his left pocket. It's also noted that pocket dimensions, if not periodically cleaned, will become infested by Small Annoying Creatures called lints. In Champions of Faraus the inside of a Deitiy’s avatar is a strange place. As seen in the short story “Spheres”, it is entered through the Deitiy’s face, is Bigger on the Inside, lacks gravity, solid surfaces, and has another avatar inside it, that is just as big as the outside one, and is simultaneously controlled at the same time, and going in that avatar plops you where you were to begin with - outside of the original avatar.

Web Original
Destroy the Godmodder: This is a base mechanic. Overlapping with Hammer Space a lot of the time. Genius: The Transgression has Bardos — pocket worlds made of concepts disproved by science. The more prominent ones include an alien-inhabited Mars, the Hollow World (home to dinosaurs and cavemen and Nazis), and the Seattle of Tomorrow, which Lemuria tried to bring into this world with disastrous consequences. Raising Angels Lisbet have one of these in her possession, so far we have only seen her use it as her bedroom. The SCP Foundation contains SC Ps that involve this: SCP-106 has one of these that he can travel to and lord over. He likes to drag victims here and torture them, sometimes for months, before finally kicking them back out as hollowed out, corroded husks of human beings, and they're still alive. SCP-3008 is Another Dimension that, while only appearing to be as large as the branch of IKEA that houses it on the outside, is actually infinite in size to those who enter it and become unable to escape. The interior of 3008 is so huge that whole civilisations have been constructed by the groups of people who became lost in the never-ending halls of assembled flatpack furniture, being forced to build rudimentary forts to defend against the hostile 3008-2s that stalk the dimension in packs. SCP-3001 is a Void Between the Worlds described as a pocket non-dimension. SCP-2249 leads to a forested pocket dimension that's in the process of collapsing. The result of this is tiny black holes appearing both within the dimension and in the town around the hospital in which it resides and spewing deadly amounts of radiation. SCP-2427 is a small stairway that leads to one of these that acts as a safehouse for a cult that runs on The Power of Hate called the Brazen Heart.

Western Animation
In ReBoot the Game Cubes function like this. Once they land the area inside is completely replaced and sealed off from the outside world. They also move between systems and can be used for random transport. Steven Universe: While nearly all Gems are seen pulling their magic weapons out of their gemstones, Pearl explicitly has a pocket dimension accessed through hers, which she uses for storing all of her possessions. In "A Single Pale Rose", she has Steven enter it, only for Steven to discover another Pearl in there, cataloging all of the outer Pearl's possessions. And then that Pearl has her own pocket dimension, which contains another Pearl, and so on. Lion is revealed to have one in his mane which also has Rose Quartz's possessions. After he's killed and brought back to life by Steven, Lars becomes a being like Lion and his hair becomes a portal into the same dimension, upgrading it to an Extra-Dimensional Shortcut. Breach from Generator Rex regularly uses them as part of her teleportation powers but she created a permanent one after absorbing the city of Grenville, Ohio and turning it into her "dollshouse". There she teleports EVOs so they fight each other, ice cream trucks, random people, and anything she finds interesting. In Gargoyles, the island of Avalon functions this way—the homeland of Oberon's Children, it can be accessed from any body of water if you know the right spell, but not without magic. It is also subject to Year Outside, Hour Inside, with one hour on Avalon being a day in the mortal world. My Little Pony: Friendship Is Magic: In "The Crystal Empire," one of King Sombra's indoor defenses is a Blank White Void with nothing but an Absurdly Long Stairway. In "Power Ponies," Spike finds an enchanted comic that sucks its readers in, trapping them in Another Dimension based on its story. In "Make New Friends but Keep Discord," Discord is revealed to live in a World of Chaos dimension somehow connected to Equestria. In "Shadow Play," Limbo is a Time Stands Still Prison Dimension — which Equestria's Precursor Heroes once exploited as part of a last-ditch, Taking You with Me ritual against their Ultimate Evil. Until a well-meaning Twilight blindly undoes said ritual, thus invoking Gone Horribly Right in the form of releasing said heroes and evil.