system
Given the following post, respond as if you're commenting on Reddit:
human
Which theory about the end of the universe is most likely to happen?

I remember when I was reading about the end of the universe, I saw that there are several theories about it. There's:

- The Big Crunch
- The Big Rip
- The Big Freeze/Heat Death

I don't know if I'm missing anything but I am just wondering, is there a theory which is most likely true or most widely accepted?
gpt
Based on the data we have today, there's no reason to believe there will be a Big Crunch or a Big Rip. This isn't to say these things won't happen, just that the simplest available theory which fits all the data suggests these won't happen, but rather that the Universe will keep expanding at an ever-faster rate ad infinitum, as everything inside it cools down and decays.

The key factor here is what's driving the present phase of cosmic acceleration, the nature of the so-called *dark energy*. Before we knew that the expansion of the Universe was accelerating, we'd thought that it was decelerating (this should make sense; gravity is usually attractive, so all these galaxies and dark matter particles pulling on each other should slow down the expansion). In this case, a Big Crunch was certainly possible. Imagine launching a rocket into the sky; if it's launched below the escape velocity, then it falls back down. The question of whether there would be a Big Crunch was essentially the question of whether our Universe was expanding "at escape velocity" - or, more accurately, whether or not its spatial geometry was closed like a sphere.

This all went out the window when we found that the expansion was actually speeding up. A Big Crunch now seems very unlikely, because you'd need dark energy - i.e., whatever is causing the acceleration - to turn off and then leave you with a closed universe. (This is tricky because even if we got rid of dark energy, we'd be left with a spatially *open* universe, which doesn't recollapse.)

What about a Big Rip? This is more plausible because it requires you to have dark energy, and we know we have some. For this to happen, the dark energy needs to actually get denser as the Universe expands. That would be *weird* and violate a lot of our understanding of physics, but it's not impossible, and people have built theories where this happens. (For comparison: normal matter of course dilutes as the Universe expands, while the simplest models of dark energy have a constant density. This might sound weird, but something like the energy of empty space - which doesn't care about the size of the Universe - would fit the bill.) The data we have right now aren't good enough to confirm this picture, but neither are they good enough to rule it out. Big Rip models are allowed, but not preferred, and because they're much more complicated than non-Big Rip models, Occam's Razor tells us to disfavor them.

In the simplest dark energy model, it's just a constant energy density throughout all of space and time, and so neither of those bombastic scenarios happen. The resultant future is more like a Big Freeze.