How will the universe end?
Yasmine Surendramohan from Lancashire (age 15-24)
There are 3 major theories concerning how our universe and everything in it will ultimately end up: the Big Crunch, the Big Chill, and the Big Rip.
Firstly, then, let us look at the Big Crunch. This is the theory that states that the Universe will eventually stop expanding and start to collapse in on itself – kind of like running the Big Bang in reverse. Now imagine, if you will, that we’re at the moment of the Big Bang. There’s an immense explosion, and matter, energy, and forces are created almost instantaneously – in the first few nanoseconds of one gigantic Universal cataclysm.
After the Bang, the Universe and all its matter continues to expand outwards for a few billion years or so. However, there is a problem. Between all the bits of matter in the Universe, there is Gravitational Attraction.
And the bigger and closer together two lumps of stuff are, the more attraction there is. So while the force of the Big Bang has thrown matter outwards and further and further apart, gravity is trying to bring it all back inwards and closer together. Now here is the important thing: if the average density of all the matter in the Universe is large enough – more than the so-called Critical Density – then gravity will overcome the force of the initial Bang, and the expansion of the Universe will slow down…then stop…then reverse…then begin to contract at an ever-increasing rate.
Imagine it – as the billions of suns and planets get closer together they heat up, until eventually they all implode into a white-hot singularity. This of course means the end of the Universe as we know it.
On the other hand, if the average density of matter is below the critical density, then gravity is too feeble to overcome the force of the bang, and the Universe just keeps expanding forever.
If this happens, then as the matter gets more and more spread out, it gets colder and colder, until the whole Universe chills to a temperature of absolute zero. So the Universe does not so much end; rather, its contents reach a uniform equilibrium which renders life impossible. This is theory number two, and we call this “The Big Chill”.
Now for most of the 20th century, astronomers knew that the universe was expanding – by measuring the rate that other galaxies seemed to be moving away from us. So this would be consistent with either the Big Crunch (if it was yet to slow down and reverse), or the Big Chill theories.
But it wasn’t until the beginning of the 21st century that they discovered that the Universe that the expansion of the universe was actually speeding up – apparently accelerated by some unknown force, which they christened Dark Energy.
Dark Energy directly opposes the force of gravitational attraction, pulling all the matter in the Universe apart in all directions. If this happens, then gravity will eventually become too weak to hold things together, and everything will be ripped apart at the seams.
First galaxies will be separated from each other, then – a few months before the end – solar systems would rip apart. In the last few minutes, suns and planets follow, and finally – a few nanoseconds before the end of time – even atoms would be destroyed. Ouch. Hence we call this the Big Rip.
The Punk Scientists at the Science Museum
Filed under: age 15-24, Answered Big Questions, Astronomy Big Questions, Bryson Gore's Big Answers, Cosmology Big Questions, Edd Edmondson's Big Answers, Physics Big Questions, The Punk Scientists' Big Answers