Freeze, crunch, bounce or rip: how will the universe end?

Around 13.8 billion years ago, all the energy in the Universe today was held in one single spot, which erupted into existence in an event called the Big Bang.

Freeze, crunch, bounce or rip: how will the universe end?

Our understanding of the start of the universe is strong. Looking outwards, we can tell our galaxy is moving away from everything around us, in an expanding universe; peering back in time tells us more about the structure of the early universe, and how we got here.

Yet, while we have a widely-agreed theory of how the universe evolved, from a fraction of a second after the Big Bang to the present day, how the universe is going to end is an entirely different question.

big_bang_timeline

There are four main theories as to how the universe could meet its end, and each depends on something called the universe’s critical density.

The critical density is related to the average density of matter and it determines whether the universe can be described as ‘flat’, ‘open’ or ‘closed’. Put simply, if there is enough matter in the universe, it will cause it to eventually collapse back in on itself.

The Big Crunch                       

If the universe is ‘closed’, it means there is enough matter that, through gravity, will cause everything to move together. Gravity will become the most important force in the universe. It will eventually collapse in on itself, compressing back into a singularity like the Big Bang.

The Big Bounce

In another theory, similar to the Big Crunch, the universe collapses in on itself. But after forming a singularity, it triggers another Big Bang. This theory predicts our own Big Bang was not the beginning, but one in a series of crunch and bang cycles that will go on infinitely.

That is the question of a closed universe. But what if the density is not this high?

The Big Freeze

 If the universe is open, with a low critical density, it will keep expanding forever. Eventually, everything in it will reach a temperature of absolute zero, and this scenario is known as the ‘Big Freeze’. All of the stars and galaxies would run out of fuel and die and move further apart from each other infinitely.

If the critical density is not too low but not too high, the universe would keep expanding, but its rate of expansion would slow down eventually reaching a stop. This would take an infinite amount of time – and this scenario is a ‘flat’ universe. A flat universe would also lead to the Big Freeze.

Astronomers measured the critical density of the universe using NASA’s WMAP spacecraft and discovered that the actual density predicts a flat universe. You may think this solves the question of which scenario the future holds, The Big Freeze, but it is not that simple.

However, follow-up experiments showed that the rate of expansion of the universe, measured by something known as the Hubble constant, is not slowing down, as you might expect in a flat universe. In fact, it is speeding up. The mysterious driver behind this acceleration is still not understood and is given the generic name dark energy.

“We really don’t know whether the expansion is going to continue since we don’t understand why it’s accelerating,” physicist Freeman Dyson told the BBC

The Big Rip

By adding dark energy to the mix, the potential fate of the universe changes. In some theories, the force of dark energy increases in the future, causing the rate of expansion to continue to increase until it reaches the speed of light. This ends with all objects, even as vast as stars and galaxies, being ripped apart into their basic, elementary particles.

The end of the universe is not something you should worry about; it won’t happen for billions, or trillions, of years. But, if you were to put your money on any of these four possibilities, we’d recommend the Big Freeze. Most signs point to the Big Freeze as the most likely conclusion, yet of course, you wouldn’t be able to claim your bet even if you were correct. 

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