Fire or Ice? - How Will the Universe End?

There are countless galaxies in the universe. How the universe ends depends on if they continue to move apart or if they start falling in towards each other.

We live in a universe that is 13.8 billion years old. But how much longer does it have left? And how will the universe meet its ultimate end? We can’t yet know for sure how the universe will end, but we can use our current understanding of the cosmos to predict what might happen.

Cosmic Tug-of-War

The universe began from the Big Bang nearly 14 billion years ago. It was a period of extreme expansion in the universe where it rapidly grew from a single point outwards. The universe continues to expand to this day, as if it were coasting outwards from the Big Bang. To understand the ultimate fate of the universe, we first have to understand this expansion of the universe.

When Edwin Hubble first determined that the universe was expanding, the natural assumption was that the rate at which the universe expanded would be decreasing over time. If you think of the Big Bang as a traditional explosion, then as the various shock waves and debris from the explosion move outwards they’ll be slowed down over time by forces like friction until the outward expansion stops altogether. In the case of the universe, the slowing force would be gravity. The mutual gravitational attraction between galaxies would slow their movement apart from one another as time went on.

However, that’s not what turned out to be true. Further observations suggested that rather than slowing down, the expansion of the universe was speeding up. This meant that some unknown force had to be adding energy into the expansion of the universe to not only counteract the slowing effect of gravity, but also to speed it up further. This unknown source of energy has been labeled as dark energy. We don’t yet know what dark energy is, but we do know that it is the single most important factor in determining the ultimate fate of the universe and how it will end.

The fate of the universe is a tug-of-war between gravity and dark energy. The universe will end in a completely different way depending on which force wins out.

Ending in Ice: The Big Freeze

If dark energy wins out, that means that the universe will continue to expand outwards forevermore, which would eventually lead to its doom. There is only a finite amount of matter and energy in the universe. It may be a lot, but the more the universe expands, the more spread out the matter and energy becomes. Eventually, after enough expansion, the entire universe will reach a point where the available energy is spread so thin that the average temperature of the entire universe would approach absolute zero. At this point, it would be impossible for any kind of complex structures, such as life, to exist. This would be the heat death of the universe, which we call the Big Freeze.

The Big Freeze would begin in earnest some time from 1 to 100 trillion years in the future, at which time there won’t be enough fuel for new stars to form. Over the following tens of billions of years, the existing stars would die out one by one without being replaced. Eventually, long-lived black holes would dominate the universe, but even black holes have a finite lifespan and the largest of them would die out after a googol years (that’s a 1 followed by 100 zeros). At this point in time, there would be nothing of substance left in the universe. It will just be one massive frozen wasteland. The universe will have died in ice.

Ending in Fire: The Crunch

However, if it turns out that gravity can overpower dark energy in the end and the expansion of the universe begins to slow, then another option is possible. The mutual gravity of all the galaxies across the universe could begin to reverse the expansion of the universe and everything could fall back inwards. This would essentially be the Big Bang, but in reverse. We call this version of the end the Big Crunch. The universe would end by returning to the state in which it began. Within 100 billion years, all the matter and energy of the universe would be compacted back down into a singularity of infinite density and infinite temperature. Much the opposite of the Big Freeze, this would be the universe ending in fire.

While this may be the end of the universe as we know it, it may not be the ultimate end of everything. The Big Crunch would leave the universe in the exact state it was preceding the Big Bang, meaning that another Big Bang could occur, giving birth to a new universe. It’s possible that the very nature of the universe is cyclic and the Big Bang that gave rise to our universe was simply the most recent in an infinite cycle and that the Big Crunch of a previous universe set the stage for it.

Given our current understanding of the universe, a Big Freeze scenario seems more likely than a Big Crunch, but with so much that we don’t know about dark energy and its impact on the universe, it’s impossible to say for sure how the universe will end. But it’s oddly poetic that we seem to have the possibility of ending either in fire or ice.