This is the way the universe ends: not with a whimper, but a bang | Science

An artist’s impression of a black dwarf, a cooled-down stellar remnant that could form in trillions of years

MARK GARLICK/Science Source

In the unimaginably far future, cold stellar remnants known as black dwarfs will begin to explode in a spectacular series of supernovae, providing the final fireworks of all time. That’s the conclusion of a new study, which posits that the universe will experience one last hurrah before everything goes dark forever.

Astronomers have long contemplated the ultimate end of the cosmos. The known laws of physics suggest that by about 10100 (the No. 1 followed by 100 zeros) years from now, star birth will cease, galaxies will go dark, and even black holes will evaporate through a process known as Hawking radiation, leaving little more than simple subatomic particles and energy. The expansion of space will cool that energy nearly to 0 kelvin, or absolute zero, signaling the heat death of the universe and total entropy.

But while teaching an astrophysics class this spring, theoretical physicist Matt Caplan of Illinois State University realized the fate of one last group of entities had never been accounted for. After exhausting their thermonuclear fuel, low mass stars like the Sun don’t pop off in dramatic supernovae; rather, they slowly shed their outer layers and leave behind a scorching Earth-size core known as a white dwarf.

“They are essentially pans that have been taken off the stove,” Caplan says. “They’re going to cool and cool and cool, basically forever.”

White dwarfs’ crushing gravitational weight is counterbalanced by a force called electron degeneracy pressure. Squeeze electrons together, and the laws of quantum mechanics prevent them from occupying the same state, allowing them to push back and hold up the remnant’s mass.

The particles in a white dwarf stay locked in a crystalline lattice that radiates heat for trillions of years, far longer than the current age of the universe. But eventually, these relics cool off and become a black dwarf.

Because black dwarfs lack energy to drive nuclear reactions, little happens inside them. Fusion requires charged atomic nuclei to overcome a powerful electrostatic repulsion and merge. Yet over long time periods, quantum mechanics allows particles to tunnel through energetic barriers, meaning fusion can still occur, albeit at extremely low rates.

When atoms such as silicon and nickel fuse toward iron, they produce positrons, the antiparticle of an electron. These positrons would ever-so-slowly destroy some of the electrons in a black dwarf’s center and weaken its degeneracy pressure. For stars between roughly 1.2 and 1.4 times the Sun’s mass—about 1% of all stars in the universe today—this weakening would eventually result in a catastrophic gravitational collapse that drives a colossal explosion similar to the supernovae of higher mass stars, Caplan reports this month in the Monthly Notices of the Royal Astronomical Society

Caplan says the dramatic detonations will begin to occur about 101100 years from now, a number the human brain can scarcely comprehend. The already unfathomable number 10100 is known as a googol, so 101100 would be a googol googol googol googol googol googol googol googol googol googol googol years. The explosions would continue until 1032000 years from now, which would require most of a magazine page to represent in a similar fashion.

A time traveler hoping to witness this last cosmic display would be disappointed. By the start of this era, the mysterious substance acting in opposition to gravity called dark energy will have driven everything in the universe apart so much that each individual black dwarf would be surrounded by vast darkness: The supernovae would even be unobservable to each another.

In fact, Caplan showed that the radius of the observable universe will have by then grown by about e10^1100 (where “e” is approximately 2.72), a figure immensely larger than either of those given above. “This is the biggest number I’m ever going to have to seriously work with in my career,” he says.

Gregory Laughlin, an astrophysicist at Yale University, praises the research as a fun thought experiment. The value of contemplating these mind-boggling timescales is that they allow scientists to consider physical processes that haven’t had enough time to unfold in the current era, he says.

Still, “I think it’s important to stress that any investigations of the far future are necessarily tongue in cheek,” Laughlin says. “Our view of the extremely distant future is a reflection of our current understanding, and that view will change from one year to the next.”

For example, some of the grand unified theories of physics suggest the proton eventually will decay. This would dissolve Caplan’s black dwarfs long before they would explode. And some cosmological models have hypothesized that the universe could collapse back in on itself in a big crunch, precluding the final light show.

Caplan himself enjoys peering into the distant future. “I think our awareness of our own mortality definitely motivates some fascination with the end of the universe,” he says. “You can always reassure yourself, when things go wrong, that it won’t matter once entropy is maximized.”

Kent

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