These may be the last explosions before the universe becomes dark

The last chapter in the history of the universe is expected to be rather pale. Physicists believe that countless billions of years from now, after all the stars have been burned out, the universe will be a cold, dark space where nothing of importance happens, or even can happen. As space itself expands, and matter becomes thin straw, less and less energy is obtained. Over the ions, the universe simply descends into a scenario known as heat death.

But before the lights go out for good, there could be one last display of fireworks. Astronomers believe that compact stars, known as white dwarfs, will be among the last remaining objects to survive in an aging universe. Now, accept a paper for publication in the Monthly announcements from the Royal Astronomical Society finds that these stars can continue nuclear fusion at a mind-bogglingly slow rate, eventually leading to supernova-like explosions.

The idea of ​​exploding white dwarfs comes as a bit of a surprise, as scientists generally think of these burnt stars “as if they were cooling off forever”, says Abigail Polin, an astrophysicist at the California Institute of Technology and Carnegie Observatories not involved in the study.

Based on the new model, the first of these explosions of white dwarfs is not least to be thanked¹¹⁰⁰ years. That’s a 1 followed by 1,100 zeros – a number so large we have no name for it. “As you write it, it’s just a whole page with zeros,” says study author Matt Caplan, an astrophysicist at Illinois State University. (The current age of the universe is a whopping 13.7 billion years.)

“It’s out of reach of any time scale we normally think about,” Polin agrees. But if Caplan is right, these bursts would be the last great astrophysical event before the definitive slips into darkness.

Stars burn by fusing hydrogen into helium in their nuclei. As an average star, about the size of our Sun or slightly heavier, has used all of its hydrogen, there is not enough energy to counteract the star’s own gravity, and begins to shrink its core, while the outer layers expand drastically. As the core shrinks, pressure and temperature increase, allowing heavier elements to fuse together. The star eventually throws up its outer layers, and what remains is an ultra-dense object just a few thousand kilometers away – a white dwarf.

Over a period of trillions to hundreds of trillions of years, white dwarfs radiate all the remaining heat, and the frozen remains are sometimes called black dwarfs. But although black dwarfs are cold and small, allowing them to remain stable for enormous periods, Caplan’s calculations show that nuclear fusion can still occur through a phenomenon known as quantum tunneling.

Within the nuclei of black dwarfs, the nuclei of individual atoms each have a positive charge, so that they repel each other like the pulse of a magnet. But according to quantum theory, each nucleus acts as a wave as a particle. Thanks to this wave-like property, a nucleus will sometimes “tunnel” through the repulsion barrier that separates it from its equally charged neighbor.

“We think of white dwarfs as these completely inert objects,” says Marten van Kerkwijk, an astrophysicist at the University of Toronto who was not involved in the study. “It’s really fun to think that these silent, dead stars can continue to fuse.”

Over many trillions of years, these super slow fusion reactions will produce the heavy element iron, according to Caplan. The process will also release positrons, which are similar to electrons but have a positive charge. When these positrons encounter electrons in the core of the star, they will destroy each other. Without those electrons and the pressure they exert, the white dwarf itself will no longer be able to overcome the tug of gravity. It will continue to shrink until it “bounces” outward in an explosion, similar to a traditional supernova.

Caplan notes that only the heaviest white dwarf stars – those with a mass more than about 1.2 times that of the Sun – can undergo such an explosion. Even so, a white dwarf explosion would be the fate of about one percent of roughly 10²³ stars that exist today, he says.

Before the explosions, the quietly fusing black dwarfs would not release any visible light. “You wouldn’t even see it in front of you until it exploded,” Caplan says.

However, if the matter itself is unstable, then stellar remnants such as white dwarfs may not sit long enough to carry out this slow fusion process. Physicists have speculated that subatomic building blocks of matter called protons can decay over enormously long periods of time – from 10³¹ to 10³⁶ years. If they do, white dwarfs can evaporate before they have a chance to explode.

But as long as protons hang around, ”the physics of [Caplan’s] paper, and its results seem legitimate, ”says Fred Adams, an astrophysicist at the University of Michigan and co-author of the 1999 book The five centuries of the universe: Within the physics of eternity, which explores the longer future of the universe.

Although heat death is currently the most widely accepted theory for how the universe will end, astrophysicists continue to think about a number of alternatives. The universe could return on its own, with all matter compressed to a single point, which could then be followed by another big bang. Or perhaps the accelerating expansion of the universe will continue in such a way that it destroys space itself, in which case individual atoms will eventually fall apart.

By the time white dwarfs start popping up, the universe will be unrecognizable. Galaxies will have lost their structure, with the remains of individual stars floating freely through space. Even the largest known black holes are probably evaporated by 10¹⁰⁰ years from now, because of a process known as Hawking radiation. Although this is a very long span, the peanuts are comparable to the time scale of explosions of white dwarfs.

Dark energy – the mysterious force that serves gravity and sends everything away from everything else – will have separated all remaining objects, including white dwarf stars, to the extent that no object will be in the sight of another.

With no stars burning to produce heat, it is strangely unlikely that anything would survive at this point – but if there was such a creature, it could only see one white dwarf explosion, because everyone else is outside its “cosmological horizon” would occur. maximum distance at which information of any kind, including light, can be retrieved.

Although a span of 10¹¹⁰⁰ years the imagination disappears, this only marks the beginning of the end, when the heaviest white dwarfs inflate. The lighter ones will last longer – up to about 10^32,000 year, according to Caplan’s calculations. And despite these bangs, the hot death of the universe cannot stop. Exploding white dwarf stars may well be the last hurray of the cosmos.

“After this, the universe will be cold and dark and sad forever,” Caplan says. “Unless there are new physics we have not discovered.”