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.