Eli5 what will exist when everything in the universe ends/ dissipates?


True nothing? One final all-encompassing element or molecule? Same as before the big bang?

In: 7

We don’t know for sure if the universe even will “end” in the sense of heat death, but signs are pointing that way. The current speculation is that all particles will decay into energy and radiate away until the presence of anything in any region of space is negligible. But also, none of us can truly fathom how absurdly long it will take to even get close to such a state.


Depends on how you define the universe. For example, where does the earth end? Does it end at the ground below your feet? Or are you still “on earth” until you’ve left the upper atmosphere, meaning the gasses that make up the atmosphere are considered part of the planet earth too. If you then look at the universe in the same way, where does it end? Does it end where the vast expanse of space that all matter once contained in the big bang is expanding into potentially ends, or does the universe end at the furthest point matter from the big bang has reached, and the empty void beyond that is what would be considered “outside of the universe”.

The current best guess is that everything will get trivially close to absolute zero (but not actually reach it, because energy cannot be destroyed, so it will just be evenly distributed among everything). But maybe it will collapse, after which there may be a new Big Bang on a new universe. Or maybe time is cyclical, and the universe will replay itself infinitely. It’s not the kind of thing there can be a definitive answer to unless we somehow manage to observe it – perhaps at a cafe with cows who volunteer to be eaten.

energy and iron. heat death does not mean no matter or energy, it means that there is no usable energy (maximum entropy). the thing most people misunderstand about the concept of work, is that you have to have a differential in order to do work. if everything is at the same temperature, you can’t extract any work from it, without adding energy to the system, and at heat death there is no way to get that. also, iron, because all elements transmute to it over time, so ya, iron stars are still floating around.

If we imagine the universe continues to age ‘normally’ like it seems to be aging now, then the stars will slowly die out and many will collapse into black holes. Those black holes will travel the universe until they’ve consumed all matter. This happens because on the scales of cosmological deep time, sooner or later, *everything* will have a chance encounter with a black hole and be consumed.

So all that’s left is black holes. Stephen Hawking showed us that black holes ‘leak’ energy via quantum fluctuations. This means that occasionally, a photon of light can escape a black hole. Yes, the pop-sci phrase that ‘not even light can escape a black hole’ is actually wrong, or at least incomplete. Eventually, after indescribably long time scales (something like a quadrillion, quadrillion, quadrillion times the age of the universe, but even that’s nowhere close) black holes evaporate into light. So the final configuration of the universe could well be an endless sea of light, forever expanding apart. Nothing but photons, drifting in the dark. This is the heat-death of the universe. Cosmologist Roger Penrose suggseted that in this state, the universe becomes mathematically identical to the moment of creation, and that another big bang may then occur. This is called Conformal Cyclical Cosmology.

However, there are a few observations at the moment that suggest Dark Energy might be increasing in strength over time. This means the expansion of the universe may be accelerating at an ever faster rate. If that’s true, then the observable universe will get smaller and smaller continuously. It will shrink down to the size of the galaxy cluster, then a galaxy, then a solar system, and smaller and smaller, until expansion is happening so fast that individual atoms will be pulled apart, their parts moving away from each other at faster than the speed of light, and all interactions will end. This is the big rip scenario. If it’s correct, the universe will disintegrate in about 22 billion years.

Either way, there seems to be an interesting symmetry here between the beginning of the universe and the end.

Some scientists have posited that the energy in a gravitational field is *negative*, and that once all the energy in the universe is infinitely far away from itself, the negative gravity energy would be equal the energy of the big bang, giving the universe an overall zero-energy level, kinda like the end of the universe returning the energy back to its original state. And all existence would be just a quantum fluctuation, springing from nothing, and returning back.