Five ways the universe could end

With dark energy, our observable horizon grows ever smaller with time, despite the increasing size of the Universe. Anything that’s not already gravitationally bound to our galaxy will eventually be pulled away from us. For us, that means only the members of the Local Group will survive these tumultuous times. But anything outside of that bubble, which is only a few billion light-years on a side, will be gone forever. So if you’re close and personal with a galaxy that’s not named Andromeda or Triangulum, you might as well say your goodbyes now.

Given enough time (and the Universe has plenty of that to go around), galaxies dissolve as stars, and planets get flung into wonky orbits through countless interactions with each other. With even more time, even all macroscopic objects evaporate through quantum tunneling, and black holes shrink due to the emission of Hawking radiation.

Far enough into the future, say, 10¹⁰⁰ years from now, what we call the Universe will consist of an expanding bath of subatomic particles slowly cooling on their way to absolute zero.

This is called the “heat death” of the Universe, but you can think of it instead as the death of heat. There will be no more differences in temperature anywhere, which means thermodynamics shuts down, which means no more ability to do work. And that means no potential for life as we know it (even the really lazy kind).

The Big Rip

The heat death of the Universe is a rather morose picture, but it seems inevitable based on the fact that dark energy is a constant. No matter where or when you are in the Universe, dark energy is always there, seemingly never getting stronger, never getting weaker.

But measurements of the strength of dark energy made over the past two decades have raised questions about that “seemingly.” Instead, they lean in a threatening direction, indicating that dark energy might be getting stronger with time. These measurements aren’t enough to declare this as observational fact, however, because the uncertainties are more than large enough to accommodate a “nothing to see here” constant value. So, no alarm bells (yet), but it’s always struck me as interesting that the data tends to prefer this scenario.