It’s sad to say, but if we choose star formation as the criterion, our universe is already on the decline. From this point of view, it reached its climax about 10 billion years ago, before its expansion (which is, let us remember, incessant), combined with the fact that the quantity of matter remains constant, means that the nascent stars are more and more dispersed – and it’s not going to get better, explains Popular Mechanics.
The formation of a star requires the compression of a small mass of matter, so the more the universe goes and expands, the less often this will happen. And one day, in about 1,000 billion years (1012 years for friends), the last star will see the light of day. And in 100,000 billion years (i.e. 1014 years), the last light will go out.
Dissolution
As the expansion of the universe is constantly accelerating, some galaxies that we can currently observe from Earth will eventually no longer be visible. We can currently observe the light they emitted when they were much closer to us. That which they produce at the time when these lines are written will never reach our solar system.
In fact, eventually, only the Local Group of galaxies (consisting of the Milky Way, Andromeda and about 60 others, all gravitationally connected to us) will end up being visually accessible to us. And our universe will come down to that, since we will be isolated from everything else. The sequel is even less pleasing: it will end up dissolving, or self-digesting, and it will soon be complete darkness (but count 1020 or 1030 years before it happened).
Before that, with the death of the last star, an era of degeneration will begin. This will last more or less 1 billion billion years, or 1018 years. The planets will survive it, but they will be deprived of any source of heat. Ditto for asteroids, comets and space debris.
The death of everything
Gradually, everything will crumble, atom by atom, so that after 1065 years (this number does not even have a name), there will no longer exist any macroscopic objects in the universe. Only black holes will survive, but under the effect of what is called Hawking radiation, they will also eventually evaporate gradually. After 10100 years (unofficial denomination: a googol of years), they too will cease to exist.
The particles will then dominate the universe. Protons will perhaps be the greatest objects of this era, but nothing is less certain, because their stability through the ages has not been proven. Anyway, after 10200 years, they will eventually disintegrate. It will then be the reign of dark energy, all in an ever-expanding universe, which will then experience thermal death: it will no longer have any thermodynamic energy, which will prevent it from ensuring any movement or any life.
What remains of it will then reach an absolute thermal equilibrium. The temperature will continue to fall, tending towards absolute zero – but without ever reaching it. This will coincide with the end of all life. Then we don’t really know. A new Big Bang? An unexpected phenomenon? We don’t know, but we can’t wait to see it.