Everything has its time and everything dies. Ultimately, this includes the universe itself. Not by some apocalyptic disaster or Omnicidal Maniac, but simply by the increase of entropy leading to heat death. Or expansion eventually reversing and causing a reality-collapsing Big Crunch. Maybe a Big Rip tearing everything apart, but the results are the same—the universe dies, or at least becomes uninhabitable for life. This is the Natural End of Time.
Plots involving the natural end of the universe will either deal with the time traveler being unfortunate enough to end up there, or deal with what's left of life struggling to survive and/or face the end of all things. Perhaps the end of the universe isn't shown, but a major plot is a character or group trying to stop the natural end of the universe. It may not be all doom and gloom though, since Eternal Recurrence may be in effect to create a new universe (or re-start the same).
Sub-Trope of The End of the World as We Know It, though on a larger and more natural/inevitable scale. Naturally a Class-X4 apocalypse at the least. Contrast Caused the Big Bang, which deals with the initial creation of the universe and what caused it, and Place Before Time, which is set before even that. This might be what can finally finish off someone with Complete Immortality. If The Grim Reaper can't be killed, he may finally die at this point due to death having no purpose or meaning at this point.
- JoJo's Bizarre Adventure: Stone Ocean: The plot of the Big Bad Enrico Pucci revolves around the fact the universe will eventually end, leading it to be reborn the same way. He plans to speed up time so souls can't die before the universe's end, allowing them to be reincarnated with knowledge of the future and through that peace of mind.
- Puella Magi Madoka Magica: Concern over the natural heat death of the universe is the reason why the Incubators set up the cycle of magical girls and witches—they wanted to avert entropy and prevent the universe naturally ending.
- Tengen Toppa Gurren Lagann: The Spiral Nemesis is a semi-natural Big Crunch scenario resulting from Spiral Beings (life that evolves and possesses willpower) producing enough Spiral Power that the sheer amount of energy-mass they create causes the universe to collapse. The Anti-Spirals repressed countless worlds of Spiral Beings and put a set limit on their population to make sure the Spiral Nemesis never happens.
- Alan Moore's "The Big Chill" deals with the heat death of the universe, where the last immortals gather to deal with the inevitable end of their existences.
- DC Comics: The End of Time is a recurring feature for time travelers in the DC Universe. Vanishing Point is established right before the end of time, serving as the base for time travelers. A null-bubble is used to freeze time so everything isn't destroyed around it.
- The Flash: When chased by the Black Flash, Death for speedsters, Wally West ends up running to the end of time to get away from him. As death has no meaning at the end of time, the Black Flash vanishes.
- Legion Of Superheroes: The Time Trapper rules the universe at its end, having existed from the 31st or previous centuries. While a Dimension Lord of this era, he has nothing to rule so seeks to rule the Legion's era.
- The Sandman: Death states she will be the only Endless to survive to the end of time, where she'll close the door on it. The Books of Magic shows this when Mister E goes to the end where Destiny is the last being to die, and he has to take The Slow Path backwards to return to the present.
- In order to get rid of Doomsday, Superman sends him to a point where he can't harm anything or possibly use his Adaptive Ability to save himself-the end of time. Brainiac later travels to this era to rescue Doomsday in hopes of implanting his mind into the creature.
- The Strange Visitor storyline features this in its final part, showing a Superman who has become the last hero and the last living thing in the entire universe. Notably, he's become so much more powerful that he's now the main thing keeping the universe together, only letting go when the last other being in existence, the Akkadian sky god Anu, passes away. As it turns out, him letting go also allows him to rescue a group of astronauts who have been caught in a Year Inside, Hour Outside higher-dimensional space, and shepherd them to the universe that will come next.
- Marvel Comics
- Fantastic Four: Galactus originates as Galan of Taa, a civilization that lived at the end of the previous universe. He was part of a crew trying to avoid the inevitable Big Crunch, only to fail. As the universe ended Galan was able to be reborn in the current universe as Galactus, some time after the Big Bang.
- Great Lakes Avengers: Craig "Mr Immortal" Hollis is stated to have such Complete Immortality that he will live to the end of the universe. It's implied that he'll be reborn into the next universe and function as the Galactus of that reality.
- Immortal Hulk: The end of Issue #24 is a Flash Forward to the end of the universe, where the sentience of the universe is waiting for Mr Immortal so it can merge with him as it did with Galan to bring forth Galactus. Unfortunately he and Franklin Richards have been killed the Immortal Hulk, at this point completely possessed by The One Below All, who eats the horrified sentience. Issue #25 sees the resulting Cosmic Entity tear through the new reality and eventually kill everything.
- Ultimate Marvel: The Ultimate Fantastic Four built a ship that can go to other universes, and explored the N-Zone, a universe that was about to die. They met Nihil (the Ultimate version of Annihilus), who tried to kill them to steal their ship and escape to their younger universe.
- Captain Future: Planets in Peril has the Captain help a universe on the brink of death, the last of the stars there barely shining. The remnants of the people are at war with an Always Chaotic Evil race created to survive in a starless universe, and he needs to help them last until a new Big Bang revitalizes the stars.
- Cthulhu Mythos: As reality is Azathoth's dream, it is destined to stop existing when it wakes up. Which could be at any time.
- James Blish has his series Cities in Flight end with the death of the Universe and the characters creating separate new ones.
- Michael Moorcock's "Dancers at the End of Time" series is about a group of superpowered decadent hedonists living in a Free-Love Future Post-Scarcity Economy in the final years before the heat death of the universe.
- In the Discworld novel Eric, one of the characters travels to the end of time, where Death is just about to metaphorically put all the chairs up and turn off the lights.
- In Poul Anderson's Tau Zero, a ship moving at near-light speed experiences an accident in transit and becomes incapable of deceleration. Due to Time Dilation the trip only takes a few years for them, even though outside the universe has ended and a new one begun by the time they get the ship repaired.
- Doctor Who Expanded Universe:
- In the Past Doctor Adventures, the end of time is described as "Event Two" (the Big Bang is "Event Zero"), taking place much farther in time from the examples in the series with the last proton decaying, and the universe collapses into a single point.
- In the Past Doctor Adventures novel The Infinity Doctors, the Doctor travels to a point near the end of time, and finds a small group of people still somehow clinging on to existence.
- The Hitchhiker's Guide to the Galaxy: The end of the universe is a popular destination that you can visit as many times as you want via Milliways, the Restaurant at the End of the Universe. The place is built on an asteroid and equipped with a shield to protect it from the end of the universe as well as engines that can move it back in time to relive the end of the universe over and over again. Mention is made of "the photon storms gather[ing] in swirling crowds around us, preparing to tear apart the last of the red hot suns", so presumably a combination of the Big Freeze and the Big Rip is intended here.
- Isaac Asimov's "The Last Question": Multivac is often asked if entropy can be reversed, forestalling the end of the universe. Despite turning into a Galactic AC, existing entirely in hyperspace, it fails to prevent the universe from dying. But it does figure out a solution, and is able to create a new one to replace it.
- Ben Bova's "Stars, Won't You Hide Me?" has a single human survivor of a light-speed intergalactic war fleeing in his ship from the pursuing enemies for an objective billions of years, living through time dilation right up until the Big Crunch, which he joins triumphantly, proof that humanity had survived until the end.
- Harlan Ellison's "The Wine Has Been Left Open Too Long and the Memory Has Gone Flat" is a surreal story set billions of years in the future, where reality is tired and worn and sapients are gathered for a symphony of sounds of things that shouldn't necessarily have sounds to stave off ennui. The main character plays the sound of the end of the universe, and, in response to the assertion that the universe will oscillate back to the beginning and begin again, another character plays the sound of the end of everything, which seemingly heralds the actual end of existence.
- Both H.G. Wells' The Time Machine and William Hope Hodgson's The Night Land have the world (and implied) the Universe ending with the Sun burning out a few million years in the future, before nuclear fusion was known to power it, in a case of Science Marches On.
- The World at the End of Time: Wan-To comes from a point so far into the future that most protons have decayed through heat death, and the protagonist reaches it through extreme Time Dilation.
- Doctor Who:
- "Logopolis" reveals that the universe should've ended due to entropy long ago, but the titular place has been keeping it in play. The Master ends up messing things up and puts the universe in danger. The Doctor sacrifices his fourth life to save it.
- "Utopia": In order to escape Captain Jack holding onto it, the TARDIS travels all the way to the year 100 trillion. They land on Malcassairo, at a point where heat death has led to all the stars' deaths, with the last of humanity trying to get off-world so they can go to "Utopia", believing it's their salvation. "Last of the Time Lords" reveals it was All for Nothing and humanity cannibalized itself into murderous cyborgs called the Toclafane to survive, eventually joining the Master in a Grandfather Paradox to take over the 21st century.
- "Listen": 22nd century time traveller Orson Pink accidentally ended up at the end of the universe, with the planet he was on the only place not to fall to the ravages of time.
- "Hell Bent": Ashildr, already The Ageless, has managed to outlive the rest of the universe. She dwells on the ruins of future Gallifrey, waiting for the inevitable.
- Supernatural: Similar to The Sandman, Death states that he will be the only being left at the end of time and will be there to reap God. He dies before being able to prove it.
- Orbital's song "There Will Come a Time" is a monologue set to music, encouraging you the listener to make the most of the time you have because eventually you will die, the Earth will be engulfed by the Sun's expansion, and the entire universe will tear apart into nothingness.
"But ultimately, nothing will survive. It will all be gone. In the far future, there will come a time when time has no meaning, as the universe expands and fades. Our descendants, isolated and adrift in an ocean of darkness, will watch as the galaxy evaporates away."
- Islam: According to some readings of the Muslim End of Days, after the Day of Judgement the universe will eventually collapse.
- The Big Finish Doctor Who audio drama Singularity features this as the origin of the mysterious Somnus Foundation, a point in the far-distant future when all other forms of life are extinct and all but a handful of stars are dead - as Turlough discovers when the Foundation use his body to house one of their brethren in 21st-century Earth, leaving him trapped in the future, in one of the barely-preserved Somnus bodies, with "a ringside seat to the Heat Death of the Universe." As with the Toclafane in the later TV episode Last Of The Time Lords, the Somnus are actually the last remnants of the human race, and are trying to take over 21st-century Earth in order to change the past in their favor.
- Chrono Trigger has The End Of Time, which also functions as a Place Beyond Time. Gaspar, The Guru of Time was flung there by Lavos.
- In Outer Wilds, you're stuck in a "Groundhog Day" Loop, and one of your biggest goals is finding out why the Sun keeps exploding 22 minutes into each time loop, and how you can stop it. The ultimate answer is the Sun is going nova because it's reached the natural end of its life cycle. All the other stars are burning out as well, because you're living in the final moments before the heat death of the universe. There's nothing you can do to stop it. At most, you can complete the Benevolent Precursors's research to find the "Eye of the Universe" and leap into it, and from there you can play a role in the creation of a new universe.
- By the time of Dark Souls 3, the First Flame is finally about to go out and bring about the Age of Dark. It's also subtly implied that a sort of fantasy Big Crunch is occurring during the Unkindled's journey: the "convergence of the lands", where both locations from past games and new locations are being smashed together as the mountains at the edge of the world push in towards the Kiln of the First Flame at the centre of Lothric.
- A Beginner's Guide to the End of the Universe: The series takes place after the heat death of the universe, with only the pocket universe the Everyman lives in left.
- Homestuck: Lord English emerges at the end of a universe's natural lifespan, though as an immortal, time-travelling demon this doesn't inconvenience him. He then goes back in time in hopes of destroying everything far earlier and more unnaturally.
- It Hurts!!: On the orders of God, Pasqualo takes the long path to the very end of the universe, when every star in the universe finally runs out of fuel. His reward, straight from the avatars of Harmony and Chaos, is a seed that can create the next universe. He decides to make a universe where his existence is erased and retconned to become Aurora's, since she can't exist without some kind of tragedy.
- Cat Ghost: Played with. Elon discovers that her universe is ending and is powerless to do anything but wait with Naarah. The universe does indeed explode... and the characters get transported to a strange room where they do nothing but answer questions. When that falls apart, they go to a new universe entirely, even changing form.
Elon: The universe never truly ends. It just changes.
- "Time Keeps On Slippin" manages to make the natural end of the universe a more immediate doomsday scenario. Thanks to taking chronotons out of where they're supposed to be, the universe suffers from time skips where time passes but people don't remember what happened. Because of this the skips threaten to skip to the natural end of time, but without any memory of what happened, it simultaneously functions as an immediate threat.
- In "The Late Philip J. Fry", Fry, Bender and Professor Farnsworth end up going too far on a forwards-only time machine and arrive on a desolate Earth in the year 1 billion with no hope of finding a backwards time machine to return home. With this, they decide to go forward and witness the end of the universe. Once the last proton decays, to their shock a second Big Bang occurs and the universe starts over. When they miss getting back to their present time, they simply go forward past the end of the universe again.
- The fate of the universe is up for debate, but there are three popular scientific theories as to how it happens. it might A) collapse back in on itself, B) Peter out, or C) tear itself to shreds. To elaborate:
- The Big Crunch: The expansion of the universe gradually runs out of momentum, causing gravity to win over and reverse the expansion. Everything gets attracted to one another and the universe collapses back into a singularity in a reverse Big Bang. Some believe this would lead to a "Big Bounce" where a second Big Bang starts from this new singularity, and a similar scenario may explain the universe's origins and Big Bang in the first place. This theory is seen as less credible nowadays due to the fact the expansion of the universe seems to be accelerating, not decreasing.
- Heat Death or "The Big Freeze" (to greatly simplify things, the name "heat death" comes from the fact that heat itself will have died in this scenario): The universe continues to physically exist forever, but due to the Second Law of Thermodynamics all usable energy is reduced to waste energy. Star formation eventually stops, protons are thought to eventually decay and even black holes eventually decay away from Hawking Radiation. The upside is if the universe physically exists forever, by the laws of quantum fixes, particles will, through sheer random probability, eventually be organized again in a way that could start the universe over, though it would take approximately 10^10^10^56 years by most estimates
- The Big Rip: The opposite of the Big Crunch, where the expansion of the universe accelerates beyond what gravity or any fundamental force could counter. Galaxies, stars, planets and eventually even subatomic particles and even space itself are eventually torn part as everything expands away from everything else, to the point where all distances within the universe become infinite. Like the Big Crunch, this has since been deemed less probable due to the amount of dark matter being formed not being enough to cause problems with the universal expansion.