system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What will happen next after the universe collapses in itself?

Just curious.
gpt
If the Universe collapses, then we could only guess as to what happens next. Presumably a new Big Bang will start the party right back up.

Unfortunately, it doesn't look like a Big Crunch is in our future. Currently the Universe is still expanding, and its rate of expansion is accelerating, due to largely unknown factors currently dubbed Dark Energy.

So, if the Universe continues to expand, one of two possibilities currently exist. If the rate of expansion of space continues to accelerate then eventually gravity will not be enough to hold matter together. First galaxies will be flung apart, then stars, planets, and other bodies will be ripped into an expanding cloud of atoms and compounds. All that remains to hold them together are the strong and weak nuclear forces. Soon even that will be overcome and atoms will be torn apart, followed by the remaining subatomic particles, until all that remains of the Universe is the most basic foundation of matter. This is known as the Big Rip.

The other possibility is Heat Death. If the rate of expansion slows but does not reverse, then gravity will still be strong enough to hold galaxies together. But in this case stars will cease to be born, existing stars will die, and planetary bodies will cool, all due to the different forms of energy given off by all matter. Eventually the Universe will dissolve down to an atomic soup and cool to just above absolute zero.

Not to worry, though! All this will happen long after any of us are dead. Likely the human race will be either extinct or have transcended to some higher plane of existence. Or who knows? Maybe we'll still be around and with sufficient technology to stop the degradation of the Universe.