Analysis / Natural End of Time

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. While this is usually considered an actual end of time, as space and time would cease to exist, 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, due to dark energy (barring the, as of now unlikely, possibility of dark energy becoming attractive instead of repulsive, ultimately causing the Big Crunch) and there seems not to be enough matter to counter such accelerating expansion with its gravity too.
• 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,^note Current studies suggests we're already on the tail-end of the star formation era, since more and more free hydrogen is being locked up inside low-mass red dwarf stars that live for trillions of years protons are thought to eventually decay and even black holes eventually decay away from Hawking Radiation leaving just photons, neutrinos, and some stray electrons and positrons behind (and dark matter, assuming it does not decay). The upside is if the universe physically exists forever, by the laws of quantum flux, particles will, through sheer random probability, eventually be organized again in a way that could lead to another Big Bang, giving birth to another universe, 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-time 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 energy being formed not being enough to cause such kind of problems with the universal expansion, and its most likely form to exist not being the one that would trigger this scenario.
• There may also be a fourth possibility. A vacuum metastability event, also known as The Big Slurp, would effectively destroy the current universe and replaced it with a new one with slightly different laws of physics. If space is not in its lowest possible energy state, then if that energy were be be ignited, reducing it to the true lowest possible energy state, it would would set off an unstoppable chain reaction that would expand through the universe at the speed of light. The lowered energy state would cause changes to the ways that matter behaves. Life might still be possible in the new universe but nothing from the previous one would survive because laws as basic as chemistry would not work they way we are familiar with anymore^note The original paper suggested such new Universe would be unstable and collapse into a singularity (ie, a Big Crunch) instead. Since then, other authors have proposed milder variants of this scenario in which even chemistry as we know it thus life could still be possible. It's even possible such event has already happened, just after the Big Bang corresponding to what triggered cosmic inflation, and we're living on the post-vacuum metastability event Universe. A vacuum metastability event may have already started somewhere else in the universe and could instantly kill us all when it reaches us at any moment, though this is extremely unlikely (basically if it was remotely likely to happen it probably would have already)^note Either that or it's so unlikely that it would not happen until a very distant future, long after even black holes had evaporated away, or while such quantum vacuum collapse could have already started it would have taken place in a corner so distant of the Universe that by the time it arrived here we'd have been long gone, assuming the accelerated expansion of the Universe did not mean "us" would outrun such bubble and we'd not have to worry about it until a very distant future in which the decay of the false vacuum took place in our corner of the Universe too.. If that sounds scary, don't panic. Because it travels at the speed of light we'd never know it's coming, and our deaths would be instantaneous, so just go about your day and don't worry.