If we are to talk about ends, why don’t we talk about the end? The ultimate end: the end of the universe itself. I admit this is a bit of speculation. Physicists are scientists, not fortune tellers…but what we can say is “based on the laws of physics that we know…how should the universe end?”
For a physicist, the universe “is over” when the things we can measure are no longer happening. When physics ends, because there are no physical phenomena. Or if you want to say it in simpler words, when nothing more happens.
The problem is that the words here mislead us a bit. We say “the universe is expanding”, but really we should say “space is expanding”. For us to understand this, I have to ask you for a little stretch of the imagination: I need you and the listeners to imagine the space itself.
Is easier than it looks. Space is a place where we are all placed, as if it were a shelf where we place books. We are each placed on a shelf in this kind of “three-dimensional shelf”. The only difference with a real library is that it cannot be seen or touched. So we have to imagine the universe as crossing many shelves, with legs supporting and connecting them, and the stars and planets placed there. If we have that picture, we’ve already done all the hard work.
The expansion does, neither more nor less, that this plateau grows. The shelves are getting longer. The legs of the bookcase are higher. Every inch of this “scaffolding” on which we have climbed grows. But we don’t see the library, we don’t see any of that. What do we see? May the things that are placed on the next shelf move away from us. As the invisible shelf grows, the objects on it move further apart. It is the expansion of the universe. What we actually see is galaxies moving away from us.
As they move away, the galaxies disconnect. Think that every inch of the universe is growing. Every inch separates things. If the galaxies move away, it means that there are more and more centimeters between them and us… and therefore, the speed at which they move away is greater. There will come a time when this galaxy will move away from us faster than light. And that moment is over. There is no longer any way to access this galaxy, or to know anything about it. We are disconnected: it is as if we live in different universes.
The universe is not over yet. But our galaxy will age: there will be less and less gas to form stars. Stars die and become black holes, or black dwarfs, which are cold balls of matter that can no longer ignite. The matter remains locked up in these “stellar corpses” and is no longer used to make new stars. After a very long time, the galaxy literally shuts down. There are only millions of dark balls left that revolve around the center. But we haven’t reached the end of the universe yet: there are still things going around, and so there is physics.
some of these balls are black holes, and they literally devour the rest. Sometimes they eat them, and other times they push them out of the galaxy and throw them into a totally empty universe where they won’t find anything. The other galaxies have been lost: there is nothing to be found in the universe. This process of “predation” of black holes continues for millions and millions of years, in total darkness. And it ends… when there is only one left. A single gigantic black hole instead of a galaxy. And this black hole can no longer do anything: it can no longer eat anyone and it is alone in the universe.
We have gone from a universe made up of millions of galaxies to millions of “bubble universes” disconnected from each other and made up of a single black hole which is a sterile object, nothing changes or happens to it. It’s the end of the universe. This is where physics is kaput.
Eye, it will not happen tomorrow. If the physics we know is correct, it’s 1030 years from now: one one followed by 30 zeros.