More often than not, when we talk about the big events that have changed the universe we focus very much on the past. There’s the Big Bang, the emission of the Cosmic Microwave Background, and sometimes even the moment when dark energy began to dominate the cosmos. But there’s another direction we can look at: the future.

The future of the universe is an extremely interesting topic and just as hotly debated as its past. What will happen to the universe? And more importantly, how will it all end? Well, T.S. Elliot was probably right in suggesting that the way the world ends is with a whimper rather than a bang. The leading cosmological and physical theories all suggest that it will be a long drawn out death. Let’s look at a few scenarios.

Big Bang, Big Crunch, Big Bounce

The fate of the cosmos rests on one key value, its density. The idea is that if there’s more "stuff" around in the cosmos than a certain value, the universe will collapse in on itself. This is known as the Big Crunch, a very popular theory even if it does sound like a delicious and filling snack. This is also connected to the Big Bounce, the idea that the universe is a cycle of expansions and contractions with a series of Big Bangs followed by Big Crunches repeating.

And A Possible Big Rip

If instead, the density of the universe is that specific value or less, then the universe will expand forever. The value is calculated by measuring all the components of the universe that we know of, light and matter, and the ones we believe exist but are yet to prove, dark matter and dark energy. Dark energy especially is the big player in the future of the universe, being responsible for the accelerated expansion of the cosmos.

In some scenarios, this acceleration will eventually become so fast that it will break the fabric of space-time. This scenario is known as the Big Rip. 

OK, Probably None Of These

The general consensus is that these "Big" scenarios are likely. The value for the density of the universe is extremely close to the critical value, suggesting that the universe will indeed expand forever. This might not have the soundbite appeal of the Big Crunch or Big Rip, but it is just as unsettling. So let’s pretend we are some eternal observer sitting comfortably, watching the future unfolding.

The Expanding Universe

In this eternally expanding universe, we are lulled into a false sense of security at first. For the next 100 billion years, things proceed as normal. Stars form, galaxies merge (Andromeda and the Milky Way in about 4 to 8 billion years), and we move around the Local Group, the ensemble of galaxies in which we are gravitationally bound. Between 100 billion years and a few trillion, anything outside the Local Group will stop having any influence on us. The group will merge into a single massive galaxy and slowly but surely all the other galaxies will be so far from us that their light will never reach us.

The general gist of this epoch is correct as far as we know, but the details are vague. The most likely scenario is that stars will continue to form for billions of years but each generation will have more and more red dwarfs, which will burn steadily but dimly for up to 1 trillion years. Based on this, the last star could form even 100 trillion years from now. But instead of bright galaxies, we would be looking at a large reddish object, peppered with glowing embers slowly dying.

Between 1 million billion and 100 million billion years into the future, stars and planets will either escape galaxies or they will collide and then fall into black holes. Most objects will escape, but up to 10 percent will end up in the black holes.

(So far, if you think this has been easy, then good. Now is when things get complicated.)

Decay, Or Not Decay, That Is The Question

The future of the universe beyond 100 trillion years depends on quantum mechanics and, in particular, if protons decay or not. If protons decay, then all objects in the universe (bar black holes) will crumble. Planets will go first. Heavy elements will break down into lighter and lighter ones until they are hydrogen. The hydrogen will just turn into light and electrons. The same fate will happen to stars. Without proton decay, dark energy will be responsible for breaking down matter in about 100 thousand billion billion years.

Black Holes Rule, But Not Forever

After that, black holes will reign supreme, slowly emitting "black body" radiation, otherwise known as Hawking radiation, until they completely evaporate. At that point, the universe will be in its lowest energy state. The physics get even more nebulous then. Now it just comes down to temperature and entropy. With nothing in it, the universe has reached absolute zero and with no energy available it has reached maximum entropy.

The End.

Or, at least the end as far as we can tell. We as humans, and especially physicists, don’t do well with endings. We have invested a lot in understanding the universe, it can’t all be for nothing. And perhaps it isn’t. A few different theories suggest the “dead universe” might generate a new Big Bang. Unfortunately, we wouldn't be there to witness it.