These may be the last explosions before the universe goes dark

If new calculations about the remnants of sunlike stars are correct, everything will end with a series of bangs and then a whimper.

Tuesday, August 18, 2020,
By Dan Falk
Ancient white dwarf stars in the Milky Way, as seen by NASA's Hubble Space Telescope in ...

Ancient white dwarf stars in the Milky Way, as seen by NASA's Hubble Space Telescope in 2012. These stars are between 12 to 13 billion years old, making them among the oldest stars in the universe, and according to new research, white dwarfs will also be the last stars, lingering on until they finally explode in the unimaginably far future.

Photograph by Image by NASA and H. Richer (University of British Columbia)

The final chapter in the history of the universe is expected to be rather bleak. Physicists believe that countless billions of years from now, after all the stars have burned out, the universe will be a cold, dark expanse where nothing of interest happens, or even could happen. As space itself expands, and matter is stretched thin, less and less energy is available. Over the eons, the universe simply runs down in 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 persist in an ageing universe. Now, a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society finds that these stars can continue to undergo nuclear fusion at a mind-bogglingly slow rate, leading eventually to supernova-like blasts.

The idea of exploding white dwarfs comes as something of a surprise, as scientists generally think of these burned-out stars “as just cooling off forever,” says Abigail Polin, an astrophysicist at the California Institute of Technology and the Carnegie Observatories who was not involved in the study.

Based on the new model, the first of these white dwarf explosions isn’t due for at least 101100 years. That’s a 1 followed by 1,100 zeros—a number so big that we don’t have a name for it. “If you write it out, it’s just a whole page of zeros,” says study author Matt Caplan, an astrophysicist at Illinois State University. (The universe’s current age is a measly 13.7 billion years.)

“It’s beyond the scope of any time scale we usually think about,” Polin agrees. But if Caplan is right, these bursts would be the last major astrophysical events before the final slide into darkness.

Running on cosmic fumes

Stars burn by fusing hydrogen into helium in their cores. When an average star, about the size of our sun or a bit heavier, has used up all its hydrogen, there isn’t enough energy to counteract the star’s own gravity, and the core begins to contract while the outer layers expand drastically. As the core shrinks, pressures and temperatures increase, allowing heavier elements to fuse together. The star ultimately sheds its outer layers, and what’s left forms an ultra-dense object just a few thousand kilometers across—a white dwarf.

These white dwarf stars were imaged during an astronomical survey conducted by NASA's Hubble Space Telescope in 2006.

Photograph by Image by NASA, ESA, A. Calamida and K. Sahu (STScI), and the SWEEPS Science Team

Over a period of trillions to hundreds of trillions of years, white dwarfs radiate away any remaining heat, and the frozen remains are sometimes called black dwarfs. But even though black dwarfs are cold and small, allowing them to remain stable for immense periods of time, Caplan’s calculations show that nuclear fusion can still take place thanks to a phenomenon known as quantum tunnelling.

Inside the cores of black dwarfs, the nuclei of individual atoms each have a positive charge, so they repel each other like the poles of a magnet. But according to quantum theory, each nucleus acts like a wave as well as a particle. Thanks to this wave-like property, a nucleus will occasionally “tunnel” through the repulsion barrier that separates it from its similarly charged neighbour.

“We think of white dwarfs as these totally inert objects,” says Marten van Kerkwijk, an astrophysicist at the University of Toronto who was not involved in the study. “It’s really neat to think that these quiet, dead stars can keep fusing.”

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 star’s core, they will annihilate each other. Without those electrons and the pressure they exert, the white dwarf itself will no longer be able to overcome gravity’s tug. 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 will be the fate of about one percent of the roughly 1023 stars that exist today, he says.

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

If matter itself is unstable, however, then stellar remnants such as white dwarfs might not stick around long enough for this slow fusion process to take place. Physicists have speculated that subatomic building blocks of matter called protons might decay over enormously long periods of time—from 1031 to 1036 years. If they do, then white dwarfs could evaporate before they have a chance to explode.

But as long as protons stick around, “the physics of [Caplan’s] paper, and its results, seem to be legit,” says Fred Adams, an astrophysicist at the University of Michigan and co-author of the 1999 book The Five Ages of the Universe: Inside the Physics of Eternity, which explores the universe’s long-term future.

While heat death is currently the most widely accepted theory for how the universe will end, astrophysicists continue to debate a number of alternatives. The universe could collapse back in on itself, with all matter compressed to a single point, which might then be followed by another big bang. Or perhaps the accelerating expansion of the universe will proceed in such a way that it destroys space itself, in which case individual atoms will ultimately be torn apart.

The last lights amid endless dark

By the time white dwarfs begin to pop off, the universe will be unrecognisable. Galaxies will have lost their structure, with the remnants of individual stars whizzing freely through space. Even the largest known black holes are likely to have evaporated by 10100 years from now, due to a process known as Hawking radiation. While this is a staggeringly long span of time, it’s peanuts compared to the timescale of white dwarf explosions.

Dark energy—the mysterious force that counteracts gravity and pushes everything away from everything else—will have separated any remaining objects, including white dwarf stars, to the extent that no object will be within sight of any other.

With no stars burning to produce heat, it’s staggeringly unlikely that anything would remain alive at this point—but if there were such a creature, it could see only one white dwarf explosion, because all others would occur outside its “cosmological horizon,” the maximum distance over which information of any kind, including light, can be retrieved.

Though a span of 101100 years defies the imagination, this only marks the beginning of the end, when the heaviest white dwarfs would blow up. The lighter ones will take longer—up to about 1032,000 years, according to Caplan’s calculations. And in spite of these bangs, the heat death of the universe cannot be stopped. Exploding white dwarf stars may well be the last hurrah of the cosmos.

“After that, the universe will be cold and dark and sad forever,” Caplan says. “Unless there’s new physics that we haven’t discovered.”

Dan Falk (@danfalk) is a science journalist based in Toronto. His books include The Science of Shakespeare and In Search of Time.
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