8 The future of the observable universe
It was established earlier this week that the observable universe – which is finite – is defined by the region of space in which light can travel to us since the Big Bang. What will that region look like in the future? This is quite tricky to think about. Several effects must be taken into account.
The hot plasma that is now visible to us as the background radiation – the effective edge of the observable universe, as discussed in Section 5 – will gradually cool, condense and give rise to stars and galaxies that will become visible to us. This will happen over the course of a few hundred million years.
At the same time, the ‘edge’ is moving outwards at the speed of light. Every year, the light from the background radiation is coming from further away and more galaxies are coming into view in front of it. The observable universe is getting bigger!
That would be true even if the universe were static. The next consideration is the expansion of the universe. As the galaxies continue to move apart (which you can think of as the expansion of space itself), the distances increase, and the observable universe becomes larger and emptier. At the same time, the light from these distant galaxies becomes weakened, moved towards the red end of the spectrum in a phenomenon known as the cosmological ‘redshift’. As time goes by, galaxies will become fainter and further apart – even as more of them come into view.
On top of all that, the expansion is accelerating. That changes things in quite a dramatic way. In the distant future, due to the rapidity of the expansion, the galaxies will be receding so quickly, and the redshift will be so significant, that they will effectively fade from our view. They will still be within the bounds of our ‘observable’ universe, but we won’t be able to see them!
Box 2 How to think of the cosmic expansion
Some of this week’s concepts may be making your head hurt! This is one part of a relatively short, wide-ranging course, so it’s not possible to explain every idea in depth. But here are a few thoughts that may ease your pain:
- The proper description of the expanding universe is a mathematical one derived from the theory of general relativity. The kind of maths required here is very difficult. But if you like to think visually, try this. Don’t imagine the galaxies flying outwards through space – think instead of space expanding uniformly, carrying the galaxies with it. This picture helps to answer a couple of puzzling questions.
- Where did the Big Bang take place? You might have a picture of a gigantic explosion with material flying off in all directions into empty space. But this is misleading. If space is expanding, then the answer to ‘where’ the Big Bang took place is: everywhere. There is no centre to the universe, and no edge. Wherever you are, you will see the galaxies expanding away from you.
- What is the universe expanding into? All of space came into being with the Big Bang, so there is no ‘outside’ space for the universe to expand into – and indeed, no need for it. Space is part of the universe and takes part in the expansion.