Artistic conception of a dark brown dwarf, which could resemble hypothetical black dwarfs, which are predicted to exist in the far future. Image: NASA / JPL-Caltech
“This is the way the world ends,” said T. S. Elliot in his famous poem, “Not with a bang but a whimper.” These days, scientists consider the heat-death of the universe to be the whimper, but a new theoretical analysis predicts that the cosmos will breathe its final gasp in the form of exploding black dwarfs.
Trillions upon trillions of years from now, long after the last stars have fizzled out, the heaviest black dwarfs will start to go supernova, according to new research published in Monthly Notices of the Royal Astronomical Society. Black dwarfs are the frozen remnants of white dwarfs, which themselves are the remnants of low-mass stars. The sole author of the study, astrophysicist Matt Caplan from Illinois State University, says these explosions will be “the last interesting thing to happen in the universe,” as he explained in an ISU press release.
The universe could end in any number of ways, but the current best guess is that it’ll continue to expand long after everything inside it has been torn to shreds, including galaxies, solar systems, stars, and even atoms. By the time black dwarfs are set to pop, the universe will be “cold and lifeless,” Caplan wrote in an email to me.
“The expansion of the universe will have long since separated all remaining objects by distances so enormous that no light will ever be able to reach from one to another,” he said. “Every object will find itself in a universe completely devoid of anything else in every direction. It will be cold and near absolute zero.”
When extant stars go supernova, it’s on account of excess iron in their cores—the result of internal nuclear reactions. The same cannot be said for smaller stars, which eventually burn out and shrink into white dwarfs. According to theory, white dwarfs will eventually lose their luster and freeze in the far future, transitioning into black dwarfs.
“Without a heat source, they simply cool off for all eternity, until they turn ‘black’ and no longer shine,” said Caplan. “It’s a bit like taking a hot skillet off the oven—all it can do is cool.”
These hypothetical objects would be roughly the size of Earth but with masses approaching that of our Sun. Importantly, nuclear reactions will still occur inside these dense, frozen worlds, but at appreciably slower rates than normal. And as the new study predicts, these reactions will result in a steady buildup of iron, though at cosmologically vast timescales. With this in mind, Caplan crunched the numbers to estimate how long it will take for these black dwarfs to produce enough iron to trigger a supernova explosion.
The answer, at 101,100 years, is “hilariously long,” said Caplan. The age of the universe itself is closer to 1010 years, so if you were to try to write out 101,100 it would have 1,100 zeros and “take up most of a paragraph,” he explained. Or as Caplan put it in the ISU release, “it’s like saying the word ‘trillion’ almost a hundred times.”
Importantly, these explosions will only happen among the largest of the black dwarfs, namely those around 1.2 to 1.4 times the mass of the Sun. These supernovae—the last to ever happen in the universe—will eventually stop around 1032,000 years from now, after which time the cosmos will truly be a quiet and uneventful place.
Caplan said his analysis took the effects of an expanding universe into account. However, “if dark energy is different than we currently suspect, then the expansion of the universe could destroy the black dwarfs long before they have a chance to explode,” he said. What’s more, Caplan’s calculations were based on our current understanding of nuclear physics, astrophysics, and cosmology, but to be fair, scientists can’t be certain if the laws of physics and the universal constants will remain the same in the far future. It’s possible, for example, that the universe won’t even exist at this future juncture.
“Some theories of particle physics predict that the proton is fundamentally unstable and will decay away, though this has yet to be observed or confirmed. If that’s the case, then all matter will sort of ‘evaporate’ long before any black dwarfs explode,” said Caplan. “That’s just one example. In a sense, our understanding of the far future is entirely dependent on our understanding of the laws of physics today, and small changes in physics as we know it can have enormous consequences for the final fate of the universe and its contents.”
Though Caplan said these black dwarf supernovae will be the “last interesting thing to happen in the universe,” we asked him if something of consequence or interest might happen after this phase.
“Depends on your definition of interesting,” he said. “If a cold iron ball floating in a universe where it is completely causally separated from all other objects is ‘interesting,’ then I suppose you could find something of interest.”
Okay, fair point. But if there’s any consolation in all of this, it’s that the universe will continue to expand forever, at least according to some theories. It’ll be dead, cold, and lifeless, but at least it’ll still be around.