The end of the universe

Will the cosmos expand indefinitely, or will its expansion stop one day? What could stop it? It is clear only gravity could do it. The expansion of the universe, which makes galaxies separate, goes against the gravitational attraction, which tries to hold all bodies together.

If we look at Einstein's cosmic equation of general relativity, the question of whether gravity will succeed in stopping the expansion of the universe depends on the relative values and signs of the three terms in the equation. Depending on them, three things can happen:

1.      If the negative terms dominate, (if the curvature of the universe were positive, or if the cosmological constant Λ were negative) the attraction would first slow the expansion and then reverse it. Then the universe would enter into gravitational collapse and space would be compressed until it becomes as small as it was at its origin (Big Crunch). The universe would then be closed.

2.      If the curvature were negative or zero (k≤0) and there is no cosmological constant (Λ=0), gravity would not be able to stop the expansion and the cosmos would grow larger, ever more slowly. We would have in such a case an open universe.

3.      Finally, if the cosmological constant is positive (Λ>0), since a certain point in time the universe would expand at an accelerated rate. If there is no curvature (k=0), we would have a flat universe.

During the 1980s some cosmologists preferred the universe to be closed (see this post), but all attempts to prove it failed. In 1998, evidence was found that the expansion rate of the universe has been accelerating for around a billion years. This discovery, together with the detailed analysis of the cosmic background radiation by the COBE, WMAP and Planck satellites, has led to the current dominance of the standard cosmological model, in which the universe is flat and accelerating (case 3 above).

Suppose that the universe is indeed flat. If it were open, the result would be approximately the same. In these cases, in a hundred trillion years (10¹⁴), all matter capable of building stars will be exhausted. All stars will have become black dwarfs, neutron stars or black holes, and no more stars will appear. In a quintillion (10¹⁸) years, collisions between stars and galaxies will have scattered their components throughout space or integrated them into massive black holes.

If protons were to disintegrate after 10³⁴ years, as some think they will, baryonic matter would be unstable and disappear. After a very long time, matter would decay into an aggregate of neutrinos, photons, electrons and positrons. The last two particles, if they collided, would annihilate.

On the other hand, black holes are not stable. As Stephen Hawking pointed out in 1975, quantum mechanics predicts that their mass is gradually converted into energy, part of which escapes the gravitational pull in the form of radiation. After 10¹⁰⁰ years, black holes will also have evaporated. When the electrons and positrons destroy each other completely (perhaps in 10¹¹⁶ years), the cosmos will be occupied only by photons and neutrinos, apart from the mysterious dark matter, about which we know nothing. The temperature will be practically equal to absolute zero. Everything will be empty, there will be no normal matter. This is the thermal death of the universe.

If the cosmos were closed, the story would be different. When the collapse would begin, there would be many black holes concentrating most matter. As the Big Crunch approached, the universe would become so small that black holes would collide and merge. Three minutes before the end, the entire cosmos would become a single black hole. As we don't know exactly what happens inside a black hole, we can't predict what would happen next. Some believe that at that point the universe would bounce back and expand again. We would then have a cyclic cosmos, with a new expansion after a contraction, then another contraction, and so on. However, there are theoretical reasons to think that successive cycles would be damped, so that, after a certain number of cycles, the universe would become open or flat, and we would be back in the previous case. If this happens, the universe cannot have lasted an infinite time, and the question of the beginning would arise again.

Peter Higgs

There is another possibility: when the Higgs boson was discovered in 2012, it was pointed out that its mass and the mass of the top quark combine in such a way that the universe might not be stable, so that at any moment a change of state could take place, making our existence impossible. If this is true, the equilibrium of the universe could break down at any moment, passing to a different state, probably incompatible with life. But before panicking, let us remember that the Higgs field theory is a tentative theory, which has not been confirmed. Moreover, the masses of these two particles are not known with certainty, so the universe might be stable after all. And if the stability of the universe were as critical as it seems at first glance, we would have here a new case of fine-tuning.

The same post in Spanish

Thematic thread about Standard Cosmology: Previous Next

Manuel Alfonseca