“The cosmos is all that is, or ever was, or ever will be. Our contemplations of the Cosmos stir us–there is a tingling in the spine, a catch in the voice, a faint sensation, as if a distant memory, of falling from a great height. We know we are approaching the greatest of mysteries.”
This reflection by Carl Sagan captures all the awe, excitement, and curiosity many of us feel when pondering the universe and its nature, its birth, and its end. The observation of our world leads us to accept that everything around us has a beginning and an end; thus, it seems reasonable to assume that the universe we belong to follows this life cycle, too.
Science supports this view of a perishable cosmos. A universe with an expiration date. Observations by cosmologists and the physical models they use suggest that the universe won’t be around forever. However, we still have a long way to go, and many questions remain unanswered.
What science tells us today isn't set in stone, but astrophysicists believe it's reliable enough to reasonably predict the final fate of the universe. Albeit unsettling, this knowledge also encourages us to appreciate the beauty of our fragile cosmos, which lies precisely in its paradoxical ephemerality.
Slowly, Inexorably, the Stars Will Fade
The chemical elements that we (as well as all the matter around us) are made of are synthesized in stars through nuclear fusion processes. The hydrostatic equilibrium that keeps stars stable is possible because the radiation pressure and gas pressure, which try to expand the star, are counteracted by its gravity, which tries to compress it. This delicate balance keeps stars stable, as long as they have fuel to burn during the nuclear fusion processes.
However, not all stars are the same. If we study any two randomly selected stars, we will see that their compositions are not identical. Approximately 70% of their mass is hydrogen, between 24% and 26% is helium, and the remaining 4% to 6% is a combination of elements heavier than helium. These small variations in composition can significantly impact the life cycle of stars, although their life is primarily conditioned by their mass.
Less massive stars will gradually consume their fuel, moving continuously across the luminosity and temperature diagram. At the same time, their radius will increase and they will readjust and expand, acquiring a reddish color as a result of the cooling of their surface and becoming what we know as red giants. This is the fate that our Sun will face in some 5 billion years.
But things don’t end there. After the fuel of these relatively low-mass stars has been completely exhausted, they expel their outer layers, creating a gas cloud known as planetary nebula, with a degenerate star (also known as white dwarf) remaining at the center. Since their fuel is exhausted, energy production in their interior ceases, and this stellar object gradually cools down until it stops emitting any detectable radiation.
Less massive stars, like our Sun, are destined to become red giants. In the final stages of their life cycle, they generate nebulae with a stellar remnant known as a white dwarf at the center.
The life cycle of more massive stars follows a different path. The speed at which they consume their fuel is higher than that of the less massive stars, so those that turn into a white dwarf with a mass exceeding the Chandrasekhar limit (equivalent to 1.44 solar masses) become neutron stars. But there is yet another mass limit to exceed: the Tolman-Oppenheimer-Volkoff limit.
If the resulting neutron star has a mass greater than 2.17 solar masses, it collapses into a quark star or a black hole. As we've seen, the life cycle of a stellar object is conditioned by its initial composition and, above all, its mass, but its fate is always the same: It stops producing energy and emitting radiation. Even black holes gradually lose mass and end up evaporating.
But all is not lost. Some stars have the ability to reproduce: those with an initial mass greater than eight solar masses. These massive stars consume their fuel faster than the less massive stars, and when it's completely exhausted and energy production through nuclear fusion has stopped, hydrostatic equilibrium is broken.
At that moment, the outer layers of the star suddenly fall onto its iron core–from which no more energy can be extracted through nuclear fusion–and bounce back, ejected into the interstellar medium. This is a supernova.
For a moment, these violent explosions can emit more light than the entire galaxy that contains them. This gives us an idea of the enormous amount of energy released by this process. But the most interesting thing is that supernovas are responsible for synthesizing elements heavier than iron and for spreading the elements into the stellar medium that will later give rise to the clouds of dust and gas from which new stars and planets can form through gravitational contraction.
Gradually, the most massive stars in a galaxy will disappear, which means that supernovas will cease and, with them, the production of elements heavier than iron. The most massive stars emit the most light, so their extinction will cause the combined light of all the stars in the galaxy to adopt a more yellowish hue.
In this stage of development, the galaxy’s matter is confined to low-mass stars, which, as we have seen, end their days as white dwarfs, as well as sparse clouds of dust and gas that don’t allow for the formation of new stellar objects.
Astronomers have seen giant elliptical galaxies, composed essentially of hydrogen and helium, where the production of new stars has stopped. However, as long as low-mass stars and stellar remnants remain active, the galaxy can keep emitting light for billions of years.
In the end, however, they will all die out, and the galaxy will be populated only by white dwarfs, neutron stars, brown dwarfs, black holes, and planets. All of them will gradually cool to near absolute zero. In a way, the galaxy will die.
Black Holes Are Inevitably Destined to Evaporate
At the center of each galaxy there’s a supermassive black hole. All the star systems that make up the galaxy orbit this black hole. The final fate of many of these objects (stellar remnants, planets, and smaller black holes) is to be swallowed by that supermassive black hole, due to the energy loss caused by gravitational radiation. This phenomenon causes the orbits of stellar systems to drift toward the central black hole, feeding it.
But not even the supermassive black holes at the center of the galaxies are eternal. They gradually lose mass due to the existence of virtual particles in the vacuum. It sounds complicated–and it is–but, even if we don’t need to delve deeply into this mechanism, it's still a good idea to learn a bit about it.
To achieve a precise understanding of how this phenomenon works, we would have to delve deep into the complexities of quantum mechanics, so for now, suffice it to say that black holes are not as black as they seem; they emit very low-energy particles with enormous wavelengths, similar to the size of the black hole itself.
The mechanism that explains the process by which black holes slowly lose mass and rotational energy is known as Hawking radiation, and it was originally proposed by Stephen Hawking in 1974.
Typically, the only particles that escape the gravitational confinement of the black hole as a result of the formation of pairs of virtual particles are very low-energy photons. Their emission is very slow, which causes the black hole’s mass and rotational energy loss to be slow too. This form of radiation is known as Hawking radiation, because the first scientist to propose its existence, in 1974, was the British astrophysicist Stephen Hawking.
What we have discovered so far suggests that as a black hole’s mass increases, it takes longer to evaporate due to Hawking radiation. Not even the voracious supermassive black holes at the centers of galaxies are immune to this effect. Gradually, many of the objects orbiting them will fall into them due to gravitational radiation, and the black hole will slowly return the matter it swallowed by emitting very low-energy photons until it finally evaporates.
A Universe Headed for Two Possible Ends
As we've seen throughout this article, the production of new stars will eventually stop and galaxies will gradually cool, losing their brightness as the remaining active stars exhaust their fuel.
Moreover, many cosmic objects will be gradually swallowed by supermassive black holes, which will very slowly return all the matter they had accumulated by emitting very low-energy photons, before evaporating completely.
These mechanisms tell us that in a very long time–much longer than the time that's already passed, which, according to astronomers, is about 13.8 billion years–all the matter in the universe will disappear.
The protons and neutrons of the objects that escape the black holes’ voracity will eventually disintegrate into lighter particles, so the universe, in its final stage, will consist only of whatever photons remain from the cosmic microwave background and those once emitted by stars, as well as any electrons that haven’t been annihilated by contact with the positrons that result from proton decay.
Once the universe reaches this level of evolution, scientists consider that two possibilities are likely. One suggests that gravity could stop its expansion and reconcentrate all the space and matter that it contains into a single point, possibly causing a new explosion that could give birth to a new universe.
The second theory, increasingly supported by scientists, proposes that the universe’s expansion will continue indefinitely under the influence of dark energy, leading to an increasingly cold, degraded universe. But that’s a whole other story.
Images | Philippe Donn | Alex Andrews
Related | Stephen Hawking Fell Short in His Prediction: The Universe Is Doomed To Evaporate
View 0 comments