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Humans colonized all the stars in the Milky Way in the first million years. In the next 200 million years they conquered all the planets in the Local Group. In the next few billion they conquered all the Virgo Supercluster.

At this point the accelerating expansion of the universe pushes further galaxies out of reach. This prevents humanity expanding further.

After billions of years, evolution has produced as many human species as there are planets conquered, some of them completely unrecognizable from the humans of today, while others had retained more or less the same appearance, like living fossils.

However, everything great will come to an end. In this fictional universe, roughly 91.8 billion years into the future, the increasing expansion of space will cause individual galaxies to be ripped apart.

175 million years before the end, ordinary galaxies are starting to have stars fly off into the void. The universe is expanding so quickly that it becomes impossible to ever get more than 200 million light-years from your starting point, without exceeding light speed.

Over the next few million years, astronomers everywhere will observe stars at the fringes of their own galaxy receding away at over a hundred km/s in every direction, and they'll also see most other galaxies starting to lose shape and get torn apart.

175 million years is a long time, and most future humans living in this time will probably go on their daily lives as the doomsday won't happen until they were long gone and their culture forgotten or evolved into something else. From their home planets, an average naked eye observer wouldn't notice anything wrong with the night sky other than their own galaxy completely disintegrating over the next 100 million years, but that's several orders of magnitude longer than an average human lifetime and therefore too slow to be noticeable. The first sign something is horribly wrong would be visible to the naked eye maybe a million years before the end: there are way fewer stars in the night sky than before, and many of them look way redder than expected. This is due to redshift, and this effect will be amplified the faster the object is receding away. 20,000 years before doomsday, most stars would've been redshifted out of the visible spectrum, leaving only a handful of neighboring stars twinkling creepy blood-red in the night sky, receding away faster than ever before.

Star after star in the night sky will redshift out of view and soon after get gobbled up by the Cosmic Event Horizon inching closer and closer. Less than a thousand years before the universe rips itself apart, the next closest star meets the same fate, leaving the night sky on any planet pitch black, except for other planets in the same solar system.

Even this won't last long. Starting a few decades before doomsday, planets will begin spiraling outwards from their respective orbits. At first, it starts slowly, barely noticeable, but as the Hubble parameter speeds up even more, the planets start gaining noticeable distance from their stars, and the climate on those planets starts cooling down, before reaching escape velocity around 9 months before the Rip, completely detaching itself from its star forever. Over the next month, what was once the host star will get dimmer and redder until it too fades from view. For the next 7 - 8 months, there will be nothing but complete darkness, except for maybe artificial lights on inhabited planets. Everything freezes over and gets snowed down, no one goes outside anymore as it's constant darkness and temperatures rapidly plunging down to below -100 degrees globally.

We've finally arrived on the day it happens. It's just like any other day for the past 8 months, except, if there are space stations in low orbit, they will start spiraling outwards today, and there's nothing one can do about it. A few hours later, they would've been yeeted into the void never to be seen again. Soon afterwards, the planet's atmosphere will start thinning, faster and faster. The planet might begin heating up slowly as the rapidly accelerating expansion are exerting forces on the planet's structure. People will also slowly start feeling lighter, then rapidly lighter, then eventually, about 30 minutes before the Big Rip, everything not directly attached to the planet will be lifted up from the ground and gradually start accelerating upwards like some dramatic Wingardium Leviosa spell straight out of a Harry Potter movie. This will be where human history ends as no one can possibly survive being yeeted into outer space in this manner. Over the next 10 minutes, the planet will start heating up to thousands of degrees and molten rock will be dripping off away from it, gradually tearing the planet apart, but as the Hubble parameter accelerates faster and faster, it'll start tearing larger and larger chunks from the planet before it eventually explodes into billions of tiny fragments.

Existence is futile at this point. Everyday objects held together by electromagnetic forces will get torn apart a few seconds before the universe ends, even subatomic particles will get torn apart too, only mere fractions of a second before the fabric of spacetime pops.

The big question is, are there any way for humanity to escape or hide from this event? Humanity has become extremely advanced and has had over 91 billion years to figure out a solution to survive this moment of doom. What could that solution be? Hiding inside black holes? Will black holes get torn apart too? What could they do afterwards if they somehow finds a solution to survive? If black holes are one of the solutions, then I would imagine an entire galaxy's population mass migrate to the central supermassive black hole of their galaxy a few million years before doomsday so they could ensure they won't get causally disconnected from it when they need it. I would imagine that for the last million years, a whole galaxy's worth of people will be concentrated within a light-year from the black hole, spawning racial conflicts between different human species of absolutely epic proportions.

On the contrary, what if there's no solution? I would imagine they need to shut everything down a generation before doomsday. Maybe they would enact policies that prohibits all people from reproducing so the last person dies a few years before the event happens.

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    $\begingroup$ This question is long-winded. I thinned out the first few paragraphs. $\endgroup$
    – Daron
    Mar 27 at 0:29
  • $\begingroup$ It was long, but gripping. It asks for speculative science well beyond our current understanding - are you asking for us to fill out the details, because if so, there's a good chance that the answers will all just turn into technobabble which would be off topic. What can we help you with? $\endgroup$ Mar 27 at 0:38
  • $\begingroup$ Asking how a society would respond to an event is too broad and story based a question to be a good fit for this site. $\endgroup$
    – sphennings
    Mar 27 at 0:52
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    $\begingroup$ @sphennings "society" ? isn't this widening up a close rule ? "Asking how a -society- would respond to an event is too broad and story based" is there a meta topic supporting that interpretation ? As far as I understand sofar this rule is about characters in a story, not "society". When setting e.g. rules for a world, you can't ignore the collective, society is an element of creating appropriate rules imho. $\endgroup$
    – Goodies
    Mar 27 at 9:28
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    $\begingroup$ After 91 billion years, what makes you think that there will still be things that can reasonably be called "humans"? $\endgroup$ Mar 27 at 9:53

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They could live at the bottom level of a Birch planet within the event horizon for the final years of the universe before even the black holes are gone.

Another causal structure of space within their universe to hide within could be a cosmic string loop but this like black holes may not survive the Big Rip.

The only other option is to have a multiverse or cyclic universe. For a Multiverse simply finding a way to escape to another universe would be the solution via a wormhole or another FTL/ space-time manipulation method.

For a Cyclic universe, for any of the universe ending scenarios (Big Rip, Big Crunch or Big Freeze) the civilization will not be able to survive without extreme space-time manipulation techniques but they can leave a message in space-time for a Civilization in the next universe to decipher and learn how to revive their species.

This idea is part of Sir Roger Penrose CCC theory, the old universe will need to collapse down to a non singularity small scale for the birth of the new universe and its inflation. Messages will be imprinted in space-time by merging supermassive blackholes so that the gravitational wave background contains a complicated message which survives into the new universe.

Its not really a solution for that will please individuals that wish to live, as escaping the universe will be far more appealing but it is a way for the preservation of the species and their knowledge into the new universes although it does depend on a cyclic universe system and no singularities.

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Black holes, as currently understood, won't help much, simply because your maximum survival time after crossing the event horizon is only a few hours per billion solar masses. Also, it's unclear why the quintessence wouldn't be in there too.

Seven maids with seven mops

While we haven't detected quintessence except by its gravitational effects, it could certainly have other observable properties. It's more likely that it does if it evolves over time, which it must do to cause a big rip. (In contrast, non-big-rip quintessence could just be a cosmological constant term in the GR field equation, with no other properties.)

So maybe there's some way to control it. The main problem with this is that the stuff is everywhere; even if you found a way to do something about it locally, would it scale? I suppose it could be unstable in some way and you could find a way to trigger a transition to a less dangerous state that would expand outward on its own (and without destroying everything in its path in the process).

The nuclear option

The ultimate escape from the laws of physics is vacuum decay. There's a novel by Greg Egan named Schild's Ladder in which far-future (30,000 years) descendants of humans trigger vacuum decay and eventually manage to design bodies that can survive in the interior of the new-vacuum bubble (which follows completely different physical laws). It's implied that some of them end up living in the interior permanently. They triggered the decay accidentally, but perhaps it could be done deliberately.

The bubble in that story expands at $c/2$, giving them plenty of time to study it. Realistically it would expand at $c$, but then there would be no story, so that seems like an acceptable break from reality. Also, in that story, they've long since computerized their brains; there would be no way to transfer their minds to the interior otherwise. Of course it follows that they're effectively immortal. Your human descendants apparently never developed that technology, judging by the last sentence of the question.

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Quantum hair.

This is a Tough One. It hinges on a hypothetical Big Rip, based on the action of a "dark energy" nobody understands. But against this immovable object you have the irresistible force of 91 billion years of research. Stand in an open area free of obstructions, because we're going to be waving hands a lot.

First, we have to ask what happens if an object falls into a black hole. According to the Schwarzschild t coordinate, it falls forever - however, it falls so rapidly that light from another few microseconds later cannot ever catch up to it. The object's is frozen in time in those coordinates, experiencing nothing. In its "proper time" - if it does experience time - it simply falls into a singularity in a few microseconds.

So if the Big Rip affects objects according to the t coordinate, nothing falling into a black hole can escape it. If it doesn't, the object falls in and is torn apart. You might try to argue for a middle ground - the current Hubble constant falls in at light speed with the object, so that space continually expands in front of it and it never reaches an event horizon and comes out of the hole unscathed someday. This is 90% wishful thinking and 10% handwaving, with a dash of crank speculation on my part, but it would give you someplace to "hole" up for a trillion years while waiting for the Big Rip regime to turn into something else. That something else is likely to be much colder and much slower than anything we know, though, and you still have to figure out some way to mind upload into the slow "chemistry" of neutrino nuggets or something more tenuous.

The other way to go - with comparable wishful thinking and handwaving - is to say you have a special black hole that goes somewhere else. All the usual options for wormholes and Alcubierre drives apply. Can you move on to a better universe, or FTL to some part of space so far over our present cosmic horizon that the physical constants are different?

Oh, wups, I forgot the title. The most wishful thinking uses recent news that black holes might shed information from ingested objects as variations in gravitational field (the paper specifically addresses it in terms of gravitons). Now, slight variations in gravitational field are a rather tenuous way to try to continue your existence, but who knows? Are there any sort of interesting interactions in these gravitational radiation quanta? Is this a gravity radio for communicating with aliens? Can you use them to build a "propellantless" space drive that is more efficient than a photon drive? (I doubt it, but so long as I haven't made sense of the paper and don't understand the physics, I can be optimistic!)

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