The Universe Will End With a Black Dwarf Supernova

The last explosions to ever take place in the universe will be "black dwarf supernova," with these silent fireworks going off long after everything else has died, trillions upon trillions of years from now.

That is according to a study that is about to be published in the Monthly Notices of the Royal Astronomical Society. In it, Matt Caplan, assistant professor of physics at Illinois State University, has looked at a theoretical type of explosion that could take place when the universe ends.

It is based on the idea that the universe will eventually run out of energy in what is known as heat death—also known as the big chill or big freeze. Eventually, as the universe continues to expand, the supply of gas required to form new stars will run out, meaning no new ones form. After this, existing stars will start dying until the point where all that is left are remnants, including black holes, neutron stars and white dwarfs.

In his study, Caplan has looked at what will happen to white dwarfs long into the future. Unlike the most massive stars in the universe, white dwarfs do not explode as supernovas. Instead, they shrink away for millions of years, eventually becoming "black dwarf" stars that no longer emit light or heat.

These stars will have shrunk down to a size roughly the same as Earth, but with as much mass as the sun. At this point nuclear reactions will be taking place within black dwarfs, just at a far slower rate than the stars existing today, like our sun.

These reactions could eventually lead to explosions through quantum tunneling. "Nuclear reactions, via quantum tunneling, turn the star to iron over a very, very long time," Caplan told Newsweek in an email. "And then, once the star contains enough iron, it will explode very quickly like supernova today. 'Quantum tunneling' makes it possible for 'forbidden' reactions to happen if you wait long enough."

In stars like our sun, fusion reactions take place because nuclei are bouncing around and have a good chance of bashing into one another and fusing. In black dwarfs, there is not enough energy for these thermonuclear reactions to take place. "Still, nuclei have a small chance of spontaneously fusing and basically 'passing right through' the barrier from electric repulsion—that's quantum tunneling," he said.

It takes an extremely long time for these reactions to take place. Caplan estimates the first "black dwarf supernova" will take place 10^1100 years from now. This figure, he said, is like saying a trillion almost 100 times. The last ever black dwarf supernova will happen in around 10^32000 years. "If you wrote this number out, it won't fit in an article—it would fill a short book," he said. "It's the largest number I'll probably ever use in any serious calculation in my career, and I don't have any real way of picturing it."

Caplan said you could wait a million years before seeing a single fusion reaction in a black dwarf. Eventually, however, enough will take place for a supernova. "Without the internal pressure from the electrons to support the star and prop it up, gravity causes a runaway collapse," he said. "The star implodes and heats to incredibly high temperatures, and ejects much of the outer material."

Caplan's findings show the biggest black dwarfs will be first to explode, with smaller ones taking longer to reach this point. Once these last explosions take place, there will be nothing left and the universe will be "cold, dark and boring," he said. "So much time will have passed by the time the last black dwarf supernova occurs that I can't imagine anything interesting happening anywhere ever again. Ever."

He now plans to try to simulate black dwarf supernova. "No one will ever see one of these explosions in real life, but maybe we can on a computer," he said.