Explosions of black dwarfs in supernovae may mark the end of the universe

Explosions of black dwarfs in supernovae may mark the end of the universe

The “ultimate destiny of the universe”, as the end of our cosmos is called, is a topic of physical cosmology, and there are many hypotheses about how and when it will happen. While it may take a few billion – or trillion – years, it is highly likely that the end will happen, and the answers will depend on the amount of energy in the universe. But it may all end slowly, according to theoretical physicist Matt Caplan.

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According to him, most of the stars will slowly disappear as their temperatures reach zero. "It will be a little sad, lonely and cold," said the physicist, adding that there will be no one to witness the end of the cosmos.

Most think that everything will be dark when the universe comes to an end, and that the universe will be full of black holes and stars reaching the end of their nuclear fusion cycles. However, Caplan imagined something different after calculating how some of these dead stars can change over the ages. His new work points out that many white dwarfs may still explode in supernovae in the distant future, long after everything else in the universe has died.

Depending on the size of the stars, their evolutionary journeys can be quite different. Stars much larger than the Sun, for example, end up exploding in supernovae after their nuclear materials fuse into iron – and iron cannot be fused inside the stars. When it is the only element to be burned, it ends up causing the star to collapse, creating a supernova.

A timeline according to the Big Rip hypothesis, which says that if the expansion of the universe reaches a speed above the critical level, it will cause the displacement of all types of matter, the galaxies would isolate, and the atoms themselves would disintegrate (Image: Jeremy Teaford / Vanderbilt)

Smaller stars have a different destiny. The Sun itself, for example, will become a red giant, when it will become very unstable and will rapidly lose all of its outer layers. These layers will expand through space and become a planetary nebula – by the way, one of the most beautiful objects in existence. In 100,000 years, the Sun will pass to a "white dwarf" and, when its main nuclear fuels – hydrogen and helium – are exhausted, it will no longer be able to produce energy, as it does not have enough density to produce other elements, such as iron.

At this point, astronomers say the star cools down to become a "black dwarf", a dark, invisible object. And it may be that at the end of the universe, only these dead objects remain. However, Caplan says that this is not the last breath of the smaller stars. Like today's white dwarfs, black dwarfs will basically be made of light elements like carbon and oxygen and will be the size of the Earth, but will still have almost the same mass as the Sun. In other words, they will be very dense. So something can still happen to them.

The stars shine because of thermonuclear fusion and, according to Caplan, fusion reactions can still happen even in white dwarfs, due to quantum tunneling. “The fusion happens, even at zero temperature, it will only take a long time”, concludes the physicist. He says this is the key to turning black dwarfs into iron and triggering a supernova.

According to the theorist's calculations, these nuclear reactions of black dwarfs will start to happen in a very, very long time – you would have to say the word "trillion" almost a hundred times. If you wrote this time estimate, the number would take up almost one page. "It is frighteningly distant in the future," he concluded.

The image of the planetary nebula Helix, in the Aquarius Constellation, suggests what the outer layers of the Sun might look like when they transform into an object like this (Image: NASA / NOAO / ESA / the Hubble Helix Nebula Team / M. Meixner / TA Rector )

Well, this will not happen with all black dwarfs, only the most massive ones, with about 1.2 to 1.4 times the mass of the Sun, which means that up to 1% of all the stars existing today will have this destiny. It seems little, but there are about a billion trillion stars. The rest will remain black dwarfs. The physicist's calculation also suggests that the Sun will not explode in a supernova, even after becoming a black dwarf.

This entire black dwarf explosion process will also take place between large intervals. The most massive ones will explode first, followed by the least massive ones, until there are no more stars to explode. The last black dwarf will explode a long time after the first – a long time, imagine the number 10 to 32 thousand, and transform that number into years.

Only then can we say that the universe will be really quiet, with no activity going on, with no energy remaining. "It's hard to imagine anything coming after that," says Caplan. "The galaxies will have dispersed, the black holes will have evaporated and the expansion of the universe will have pulled all the remaining objects so far that none will ever see any of the others explode," says the physicist, saying that it would not even be physically possible to light travel so far.

It is certain that a black dwarf supernova is something that will remain only in our imagination, even if the possibility that they will happen someday is proven. However, perhaps scientists can create a computer simulation, based on the physics we know today. "If we can't see them in the sky, at least we can see them on a computer," thought Caplan. His study was accepted for publication in the Monthly Notices of the Royal Astronomical Society.