How Is the Universe Going to End?SClick to view As far as cosmic questions go, it's as good a one as any: When will our great universe cease to be, and shuffle our great^10^75-grandchildren off the mortal coil? This concern goes beyond the death of the Sun in 5 billion years, or even the theorized crash of the Milky Way into the Andromeda galaxy in 7 billion. No, the death of the universe is truly giant on every scale - unavoidable, and fascinating.As Neil DeGrasse Tyson puts it in his book Death by Black Hole and Other Cosmic Quandaries:
With or without warp drives, the long-term fate of the cosmos cannot be postponed or avoided. No matter where you hide, you will be part of a universe that inexorably marches toward a particular oblivion.
Yikes. So what are the particular oblivions that we can expect? 1. Total heat death If you're like me, chances are your thermodynamics class stopped being pure fun right around the time you learned about the second law. The second law introduces the concept of entropy, often described as a measure of disorder in a system, but really a measure of the system's unavailability to do work — the evenness with which energy is distributed. Once an ice cube melts in a glass of water, for example, there's no way that ice cube will ever be able to go back and do any more work. It's finito. The same goes for most of the processes in the universe. The second law of thermodynamics tells us that any process, when it occurs, can either increase the entropy of the universe or leave it unchanged; it can't ever decrease the entropy. So when you think about it that way, it's only a matter of time until the entropy in the universe is at its maximum, and there's no room for any more processes to occur — including any kind of life. 2. The big freeze As the Big Bang was occurring, the temperature of all the matter in the universe was extreme, on the order of trillions of degrees. In the approximately 14-billion-year history that's followed, however, the universe has continued to gradually expand, and so its average temperature has decreased along with it. Today astrophysicists estimate it to be about 2.7 degrees Kelvin. Astrophysicists, however, will also tell you that the universe's expansion hasn't stopped. And physics mandates that with expansion comes cooling. Eventually, the average temperature of the universe will get all the way down to absolute zero; no matter how fantastic parka technology gets by that time, life will have to stop. In case 2.7 degrees sounds to you like it's pretty close to zero, here's a nice thing to remember: Experts in the field agree that the heat in the universe should last us at least 10^10^26 more years. 3. A big crunch / big bounce Our universe started with a big bang, didn't it? Well, perhaps that big bang was just the end of a previous universe's big crunch — it existed for some amount of time, happy and innocent, and then began to slowly shrink into itself until it collapsed. Collapsed, mind you, with a bang. If this theory is true, it could mean that we're living in an oscillatory universe; whenever one clump of galaxies has run its course, it contracts and explodes to form another. The major difficulty astrophysicists have had supporting this theory is that it doesn't explain how a recurring universe like this would avoid total heat death. There's also the increasing evidence that the universal expansion will continue indefinitely, but our universe has certainly surprised us before. 4. The big rip So the universe is continually expanding. Some postulate that it's expanding with increased speed toward a moment where everything in the universe will tear itself apart. First, gravity will become too weak compared to the overwhelming cosmological forces pulling everything out, and galaxies themselves would separate. Then, individual stars and planets would unbind; in the final end, every atom in the universe would lose that essential gravitational imperative holding their elements together, and all of creation would separate into total destruction. For this, my friends, we've got about 50 billion years to go. Honestly, though, despair is not the proper reaction to these theories. If you feel like studying quantum mechanics (and who doesn't?), there are plenty more exciting ways to look at the fate of our existence — you could be the one with the next great proposal for what will happen when we drop into a lower energy state, or what exactly we can expect from the supremely mysterious dark energy that occupies 73% of our universe. And if you've been reading io9, you know we've got your back: we're fans of the multiverse theory, and not just because it means that Doctor Who's Billie Piper is having her way with David Tennant in another plane of existence. All of this is a problem for your distant future relatives anyway. Turn your brain on and give it your all, but you know these cosmic solutions will truly find their answers when the kids are listening to the 10^10^75 offspring of the Jonas Brothers. Image from Wikipedia.