Institute of Astronomy

End of the Universe

Published on 07/11/2012 
Question: 

Many astronomers predict that the universe will continue expanding till eventually all life will end the stars will go out and the universe will be cold and lifeless.
If gravity is the strongest (as well as the weakest, we can all pick up a pen) force known, why does everything have to end.
The astronauts showed, with the particles in the bag, that even in a vacuum objects with mass attract, so with this in mind, why won't the universe eventually slow to a stop and then contract to its starting point and a new big bang happen.
Is this possibly what has been happening since before this time began?
Matter cannot be unmade apparantly, only changed into something else, so even with infinite distances even the minutest amount of gravity would attract a smaller object to a larger one even at a molecular level, surely.
I would be interested in your comments, although I am not an astronomer or otherwise involved in space science, I am interested to find out if this has been put forward by greater minds than mine and if so who, when and where can I read about it?

If the Universe only contained matter then the expansion of the Universe would indeed gradually slow down due to the gravitational attraction and eventually start to re-collapse down to a 'Big Crunch', the opposite of the Big Bang.

The fly in the ointment is what we currently call Dark Energy.  Astronomers and physicists are sometimes not the most imaginative of people, so rather like with Dark Matter we tend to just call something 'Dark' if we can't see it and don't know what it is.  Dark Matter is essentially just matter that we can't see, it still interacts through gravity in just the same way as normal matter.  Dark Energy on the other hand is different, rather than attract things together Dark Energy drives them apart and accelerates the expansion of the Universe.  There are an extremely large number of ideas about what Dark Energy could be but none of them are obviously any more likely than the others and indeed some physicists are unconvinced whether it really exists at all, though a lot more think it does than doesn't.

Anyway, the eventual fate of the Universe is down to the balance between Dark Energy and matter (including Dark Matter), and to what exactly the Dark Energy is, so there are various different scenarios:

  • One option is the Big Crunch I mentioned earlier, depending what the Dark Energy is and how it behaves, this could still happen, though it currently seems less likely.
  • The one that you were asking about is commonly referred to as 'Heat Death', which is essentially that if the Universe continues expanding forever then eventually all sources of stellar fuel will be exhausted, so all stars will eventually go out, and after long enough the Universe reaches a uniform temperature near absolute zero.  The subtly here that makes this a problem for life is that any form of life requires temperature gradients in the background, so if there are none that is a problem.
  • Another option is what is known as a Big Rip, which happens if the rate of expansion accelerates indefinitely.  In this scenario the Universe is eventually expanding so fast that it rips apart, first galaxies and stars, and then eventually molecules and atoms.

There are variations on these three broad scenarios as well depending on the flavour of Dark Energy used.

As there is currently no clear idea of Dark Energy might be there is also no clear idea of exactly what the eventual fate of the Universe will be, however a Big Crunch scenario does seem less likely.

Page last updated: 7 November 2012 at 15:19