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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 vacua 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."
But, Lehners continues, "the current leading measure proposals—namely, the global light-cone cutoff and its local counterpart, the causal diamond measure—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."
In narrative, pocket universes are fictional naturally or artificially-created universes that exist within the bounds of another universe. The term pocket universe is derived from its illustration in the actual universe where it is usually portrayed as an orb which would fit in a pocket.
It might 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...
The term should perhaps, therefore, be confined to two usages, 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 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 gamelike contrivances 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" (1955) or Philip K. Dick's Time Out of Joint (1958), whose protagonists are victims of artificial worlds shaped to delude and manipulate them; or (again trivially) like any fantasy game which involves role-playing 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 restraining artifice.
That is Clute's "broad" definition. In a narrower sense, Clute goes on to say, "the world initially perceived seems to be the entire world... 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..."
This sort of story, Clute argues, embodies "the purest form of the concept of the pocket universe." He cites such examples as Robert A. Heinlein's "Universe" (1941), Brian Aldiss's novel Non-Stop (1958), and Harry Harrison's Captive Universe (1969). Another example would be For the World Is Hollow and I Have Touched the Sky (November 8, 1968), a third-season episode of Star Trek.
Clute adds, "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."
Notable pocket universes include:
- Pocket realities are usually presented in the Marvel Universe as either comparatively small reachable but separate "dimensions", such as realms of magic with recurrently greatly diverging laws of physics, or as contained within/linked to outwardly small objects with tesseract qualities, that make them far larger on the inside than the outside, such as the (from inside apparently full-sized) universe within the Soul Gem or the Heroes Reborn sphere. There are also so-called "microverses" reached through microscopic nexes.
- The Pocket Universes in Walter Jon Williams' science fiction novel, Implied Spaces (2008).
- Hammerspace, a humorous concept used by fans to explain where cartoon characters are able to pull items from "out of nowhere"
- The place that the extra mass comes from/goes that Transformers gain/lose when they transform.
- The Pocket Universe in post-Crisis Superman comics, designed by the Time Trapper as the place from which the pre-Crisis Superboy originated.
- Dead Souls (1842) by Nikolai Gogol. The container in which Chichikov stores his deeds and other items.
- Fantastic Four (1961– ) – The Tesseracted Volume of Dr. Reed Richards.
- In Doctor Who (1963– ), the interior of a TARDIS.
- World of Tiers (1965–1993), a series of six books by Philip José Farmer in which humanoids have created a series of small pocket universes which are linked by gates, and populated by persons kidnapped in the far past from Earth.
- Dungeons & Dragons (1974– ) – The Bags of holding and portable holes are literally a "pocket" universe in some cases; also, the demiplanes in the D&D cosmology.
- The Land of the Lost (1974).
- Grandfather's pocket universe in the role-playing game Traveller (1977– ).
- Parasite universes attached to the Discworld universe (1983– ).
- The Pocket Universe created by Odin to simulate the Ragnarök in one arc of Neil Gaiman's Sandman (1989–1996).
- The Black Lodge in Twin Peaks (1990–1991).
- The Dunwich Room in the movie Cast a Deadly Spell (1991).
- In the Master of Orion series (1993– ), part of the story involves the Orion race imprisoning their enemies, the Antaran race, along with their entire home star system inside what is stated to be a "pocket dimension".
- Linking books and bubbles from the Myst franchise (1993– ). The D'ni used books to for many things: agriculture, refuge, prison, industry, etc.
- The MUD Alter Aeon (1995– ) has a weightless item container named "pocket dimension".
- The computer game Chronomaster (1995– ) revolves around adventures in 7 pocket universes.
- In the videogame Chrono Trigger (1995– ), the enemy creature Lavos has its own pocket universe.
- Terranigma (1995) features a small box which Ark carries with him, that he can climb into to find various rooms including an armory, inventory screen and study.
- The Ohtori Academy in the anime Revolutionary Girl Utena (1996– ) can be described as a form of pocket universe. Also, the dueling arena's metaphysical structure can be called a pocket universe within one. The construct of the academy in both the series, manga, video-game, movie and the manga based on the movie is shown to be a form of "another" world where extraordinary things happen without any 'logical' reason – also the whole storyline faces a theme to break free from the academy and go to the Outside World.
- Big Boo's Haunt (the haunted house) in Super Mario 64 (1996– ) and subsequent remakes.
- Buffy the Vampire Slayer (1997–2003) – It is never clarified whether the much-discussed "Hell Dimensions" and "Heaven" are pocket universes; the audience knows only that major characters must employ dark magic spells to enter into one of these dimensions or to be brought out of it.
- Men in Black (1997) – The Milky Way is revealed to exist "inside a marble being tossed around on a grand cosmic schoolyard."
- The Other Zone in Lexx (1997–2002), an "unstable partial universe" that consists of a shifting metaphorical landscape.
- In the 853rd century DC Universe, as seen in DC One Million (1998), every city in the Solar System is built into its own pocket universe, leaving the planetary surfaces almost untouched.
- In the TV series Andromeda (2000–2005), during the end of the second and beginning of the third season, and in episode "Delenda Est" of the third season, Trance Gemini states that the aliens attacking the Andromeda are from a pocket universe. In the final season, Dylan Hunt and the rest of the crew of the Andromeda discover that the Seefera System is an artificial Solar System created by the Vedrans and placed inside a pocket universe in order to hide Tarn Vedra and to create a braking system for Trance's sun when it returns to the system.
- In the Kirby: Right Back at Ya! anime series (2001–2005), rather than into any particular organ, what Kirby consumes seems to enter a pocket universe of sorts, depicted with a backdrop of outer space.
- RuneScape (2001) – This MMORPG contains a number of pocket dimensions.
- The Matoran Universe inside the body of the Great Spirit Mata-nui from the Bionicle (2001–2010) timeline.
- The surfaces of each world in the popular game Kingdom Hearts (2002– ).
- Anime TV series Magical Shopping Arcade Abenobashi (2002– ) – Omnyou Mystics's main responsibility was to create an alternative universe for people when something tragic had happened to them. Though not directly explained as a pocket universe, it is presumed that the universe is limited to the imagination and knowledge of the Omnyou Mystic who created it.
- In the Futurama episode "The Why of Fry" (2003), Fry sent the Infosphere into a pocket dimension to keep it from destroying the universe.
- In the Futurama episode "The Farnsworth Parabox" (2003), Professor Farnsworth tries to dispose of one of his experiments which turns out to be a parallel universe in a box that is small enough to hold.
- Lost (2004–2010) – The Island is contained in a pocket dimension.
- The night club Pocket D, found in City of Heroes (2004– ) and City of Villains.
- Dark Aether in Metroid Prime 2: Echoes (2004– ) is a dark version of the planet Aether, located in an alternate dimension.
- In the television series Ben 10 (2005–2007) there is a pocket dimension called the Null Void, in which criminals are kept. It is later explained to have been used originally as a new home for people who have ruined their lives.
- The Pocket Universes created by the different Proxys in the anime Ergo Proxy (2006– ).
- The Flat-Space dimensional compression technology in Kurt Wimmer's UltraViolet (2006).
- Photon Nanotrance technology found in Phantasy Star Universe (2006– ).
- The MUD Arctic MUD has a rare ring with a gem that is, itself, a pocket universe into which you can toss items for storage. Upon touching an item to the gem, it would vanish in a flash of light. To retrieve the item, the player would think of it and it would reappear.
- In Superman continuity the Phantom Zone is often depicted as a pocket dimension.
- The Peach's Castle ( Bowser's Galaxy Reactor stage in Super Mario Galaxy).
- The kakeras, or shards, in Umineko: When They Cry (2007– ) are the hosts to different versions of the same universe, resulting in billions of them, enough for the characters to refer to the collection of the shards as 'The Sea of Fragments.'
- A fifth-season episode of the television series Fringe (2008– ) involves Dr Walter Bishop creating a pocket universe located within the membrane between the show's prime and parallel universes.
- In Shin Megami Tensei: Strange Journey (2010), members of Jack's team create a pocket universe for the purpose of establishing an HQ inside a space abnormality known as the Schwarzwelt.
- In Doctor Who, the Doctor's TARDIS slips through a "Charged Vacuum Emboitment" (CVE) created by the Logopolitans and into a pocket universe termed "E-space," whose interstellar space has a green tinge and "negative co-ordinates." It possesses planets with human life. Between E-space and our normal "N-space" is a third "zero co-ordinate" realm termed "the Gateway" by its inhabitants. These events are played out from the episodes "Full Circle" through to "Warrior's Gate."
- In the Doctor Who episode "The Doctor's Wife" (2011) (written by Neil Gaiman), the Doctor and his companions travel to what Amy Pond calls "a tiny bubble universe sticking to the side of the bigger bubble universe?" to which the Doctor confirms "Yeah. ...No! But if it helps, yes."
- An indie supernatural horror video game SCP – Containment Breach (2012) features a pocket dimension which can only be accessed by SCP-106, along with any prey it takes. It is a bizarre, green-lit, labyrinthine place.
- In another Doctor Who episode, "Hide" (2013) (written by Neil Cross), the Doctor finds out that, what is mistaken for a ghost haunting the Caliburn mansion in 1974, is, in fact, the image of a human time-traveling pioneer from the far future, who was trapped in a pocket universe during an early time-travel experiment.
- The Lost Room exists in a pocket dimension in a motel.
- In the Doctor Who episode, "The Day of the Doctor" (2013) (written by Steven Moffat), Gallifrey is saved from the Last Great Time War by being placed in a pocket universe by all of the known incarnations of the Doctor.
- In the online multiplayer game Warframe, the 'Orokin Void' is a pocket dimension players can travel to which contains advanced, Corrupted versions of the game's primary three factions – the Grineer, the Corpus and the Infested.
- In the anime Naruto both of Obito Uchiha's eyes (including the one given to Kakashi Hatake) can access a pocket dimension using the jutsu called Kamui (meaning Authority of the Gods in Japanese), items and people can be transported in and out of this dimension including certain body parts, while in the dimension the body parts become intangible(unable to be touched) in the outside world. The interior of the pocket dimension is what appears to be many stone block platforms amidst total darkness.
- "A New Theory of Cosmic Origins" by I. M. Oderberg (review of "The Inflationary Universe" by Alan H. Guth)
- Alan Guth (1998). The Inflationary Universe. ISBN 978-0-201-32840-0.
- Lehners, Jean-Luc (15 August 2012). "Eternal Inflation with Noninflationary Pocket Universes". Physical Review D. Ridge, NY: American Physical Society. 86 (4). doi:10.1103/physrevd.86.043518.
- Clute, John (1995). "Pocket Universe". The Encyclopedia of Science Fiction (Updated ed.). New York: St Martin's Griffin. pp. 938–939. ISBN 0-312-09618-6.
- Semley, John (May 23, 2012). "Men in Black III". Slant Magazine. Retrieved January 22, 2013.
- Silman, Jeremy. "Lexx 4.18 The Game". Retrieved January 22, 2013.