A few days ago, the person working the register at Trader Joe’s asked me how the Universe will end. Sort of an odd question to ask a stranger but I suppose if you ask enough people, eventually one will be a cosmologist… (or perhaps my NASA t-shirt and general “physicist-y” look labeled me as a person who might have an answer)
On the surface, this question seems completely speculative and of the “tree falling in the woods” variety, designed to be mental Drano — but we have theories about this rooted in honest-to-goodness science!
We live in a Universe which experienced a Big Bang. Essentially, this means there was some kick (Inflation) which started an expansion of the Universe. There is also matter in our Universe, both “normal” (baryonic) and “dark,” which interacts via gravity; creating clumps of matter, then larger clumps of matter, then stars, galaxies, etc. These two effects oppose each other, the expansion pushing everything apart and gravity trying to pull everything back together. We can then imagine three scenarios: gravity wins, the expansion wins, or they both shake hands and call it a draw.
Gravity 1, expansion 0
If the force of gravity exhibited by the matter in our Universe is more powerful than the expansion, then this will counteract the expansion, slowing it down. Eventually, the expansion will stop and then reverse!
Imagine throwing a bucket full of marbles into the air. [Note: do not attempt, or at least wear a helmet!] At first, the marbles will fly upwards with giddy abandon, laughing in the face of Earth’s gravity. These marbles will reach a maximum height, stop, and then fall back to Earth, defeated.
As all the matter in the Universe is attracted via gravity, we would eventually collapse into one giant cosmic black hole. A Big Crunch. The physics in this cosmic singularity break down, but it is theorized that we could end up with another Big Bang and thus a cyclic, bouncing Universe.
Now if the expansion is more powerful than the gravitational interaction of all the matter in the Universe, we end up with a very different (though no less terrifying) cosmic death.
Here, the Universe expands and expands. As it expands it cools. Photons traveling while the Universe expands around it become redshifted until they are practically imperceptible. The space in between galaxies will eventually expand so fast that the all the galaxies will disappear from each other’s skies. Far in the future, all the Universe’s stars will fizzle out. Eventually, the Universe will equilibrate to one extremely low temperature, ceasing to do anything particularly interesting and being completely inhospitable to life. This is known as the Big Freeze.
Ok, we’ll call it a draw
If the gravitational attraction and expansion balance each other out, we would still have the same end as above. As before, all the stars will burn out eventually. The Universe will equilibrate at a very cold temperature and no life will be able to exist.
In my simplified model of the Universe that I’ve used so far, I’ve ignored a major component (though it has been implied): Dark Energy. Dark Energy is a mysterious “anti-gravity” which is driving the accelerated expansion of our Universe. If the expansion of space-time accelerates exponentially, eventually all the galaxies will be alone. Then solar systems. Then planets. Eventually, atoms themselves will be ripped apart due to the rapid expanding of space-time in their subatomic structure. Everything as we know it will be destroyed by this accelerating expansion. This is aptly named the Big Rip.
e) none of the above
Ok, so there are more theories as to what could potentially happen, some extremely strange. Perhaps the most entertaining is that the vacuum of space is actually a “local minimum” in energy — that is, our Universe sits on a plateau and we could roll down the hill at any moment, and into oblivion. Although I’m sure the Mayans knew this all along, we primitive space-men will have to use our particle accelerators to probe for evidence that we live in a stable Universe. Spoiler alert: we probably do.
So what’s our prognosis?
The good news is (aside from the “false vacuum” death), all these unceremonious ends are tens of billions of years away at the earliest.
From cosmological measurements of the Universe, we can see that the expansion is winning, and is even accelerating. So it seems like it’s a Big Freeze or Big Rip for us. But considering all the catastrophic cosmic catastrophes that are likely to happen to our tiny little blue dot in the next few billion years, I wouldn’t lose sleep over it.