Wednesday, April 17

The future of the universe: what science tells us about the (far) end the cosmos is heading towards

“The cosmos is all that is, all that was, and all that will be. Our slightest contemplations of the cosmos make us shudder. We feel a slight tingling that fills us with nerves; a silent voice, a slight sensation of a distant memory, or even as if we were falling from a great height. We know we’re getting close to the greatest of mysteries».

At the beginning of last year I resorted to this reflection by Carl Sagan to start an article in which I proposed you to investigate the end of two of the most exciting objects that we can come across if we look beyond the protective environment that our planet offers us: stars and black holes.

The absence of certainties is the engine of our insatiable curiosity

Astrophysicists have managed to describe with amazing precision what happens to both objects at the moment when the laws of physics trigger their inevitable decline. The life of a star is conditioned by its initial composition, and, above all, for its mass. Because of the amount of matter that manages to accumulate through gravitational contraction.

The most massive consume their fuel very quickly. And when it runs out hydrostatic balance that until then kept at bay both the radiation and gas pressure, which pulls the star outward trying to expand it, and the gravitational contraction, which pulls the star inward trying to compress it, is lost.

The life of a star is conditioned by its initial composition, and, above all, by its mass

When this moment arrives, the star pours into the environment that surrounds it a good part of the matter that constitutes it. Some, the most massive ones, face this moment by triggering a supernova, while others, the ‘lighter’ ones, like our Sun, expel their outer layers to the stellar medium to give rise to the formation of a gas cloud known as planetary nebula in whose center will reside what remains of the star: a white dwarf.

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If the star was initially able to condense a large amount of matter and the white dwarf into which it degenerates when it runs out of fuel has a mass greater than 1.44 solar masses (this value is known as Chandrasekhar limit) will transform into a neutron star. And if the latter has a mass greater than 2.17 solar masses (this value is known as the Tolman-Oppenheimer-Volkoff limit) it will collapse to give rise to a quark star or a black hole.

Black holes emit particles of very low energy and a huge wavelength

In any case, astrophysicists suspect that nothing in the universe is immutable. Not even black holes. These exciting cosmic objects emit particles of very low energy and an enormous wavelength (in fact, it is similar to the size of the black hole itself). The only particles that manage to escape their gravitational confinement are very low energy photons.

Curiously, its emission is very slow, and this causes the loss of mass and rotational energy of the black hole to be slow as well. This form of radiation is known as hawking radiation because the physicist who described this mechanism was precisely the late Stephen Hawking.

The most massive black holes last longer, but the end of all of them is inevitably the same: they gradually return to the stellar medium the matter they have engulfed by emitting very low energy photons until completely evaporated. It is surprising that cosmologists have managed to describe the life cycle of stars and black holes so precisely, but we can still look a little further.

And it is that astrophysicists have spent many decades wondering about what the final fate of the universe will be. In this ambit there are no certainties. And, of course, no infallible answers either. What we do have are several reasonably strong hypotheses, although not all of them enjoy the same support from the scientific community.

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The universe moves slowly and inexorably towards its end

The hypothesis supported by most cosmologists predicts that dark energy, which is postulated to be responsible for the accelerated expansion of the universe, will continue to exert its effect as stars fade, the production of new stars declines, and, as a consequence of these two mechanisms, galaxies cool. Meanwhile, the supermassive black holes lodged in the galactic centers will continue to engulf the matter that surrounds them.

The most widely supported theory by scientists, known as the Big Rip, proposes that the expansion of the universe will continue forever fueled by dark energy.

But, as we have seen, even these objects will not last forever. Little by little they will return to the environment the matter they have accumulated through the emission of very low energy photons. The most widely supported theory by scientists, known as the Big Rip, proposes that the expansion of the universe will continue forever fueled by dark energy, leading to an increasingly cold and degraded cosmos in which each galaxy will move further and further away, being isolated from all the others.

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However, the other two hypotheses that I suggest we briefly delve into predict a very different fate. Some cosmologists have raised the possibility that the expansion of the universe is decelerating. This does not mean that it is not expanding, but simply that it is doing so more and more slowly. If so, we would be faced with two possible options.

The first of these is described by a hypothesis known as the Big Freeze, and holds that the expansion of the cosmos will slow down so slowly that the universe will continue to expand, but not in an accelerated way, for an inconceivably long period of time. In some way, this theory proposes that when dark energy stops promoting accelerated expansion, the universe will continue to expand, but it will do so under its own inertia.

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NASA black hole

All black holes will eventually evaporate due to the action of Hawking radiation. Astrophysicists are convinced that not even supermassive black holes at the center of galaxies will escape this inevitable fate.

The other hypothesis is known as the Big Crunch, and it argues that when dark energy stops promoting the accelerated expansion of the universe, there will come a time when it stops expanding and stops completely. At that moment, gravity will come into action and will gradually cause the matter to begin to agglutinate until it is concentrated in a point with such a high density that it is unimaginable. And, perhaps, this singularity trigger a new big bang capable of giving rise to a new universe.

These three hypotheses are inextricably linked to the nature and properties of dark energy, and our ignorance about this form of energy is total, so that currently no serious scientist dare to venture with guarantees which of these theories reliably reflects the final destiny of the universe. We may never meet him. Or perhaps scientific advances do allow us to find out. Whatever happens, all the effort we make along the way will have been worth it.

Images: POT

Bibliography: ‘The evolution of the universe’, by David Galadí-Enríquez | ‘Until the end of time’, by Brian Greene | ‘The evolution of our universe’, by Malcolm S. Longair

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