The Epoch of ‘Black Dwarfs’

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“The ‘big bounce’ –a cyclical expanding and contracting Universe–is more popular with general audiences than among working cosmologists,” theoretical physicist Matt Caplan told The Daily Galaxy. “The current best models,” he noted, “suggest the universe will expand forever and undergo a ‘heat death’. While people write papers about the possibility of a big bounce (and these get lots of press coverage) they should be regarded as speculation. Until we have a measurement that demonstrates unequivocally that the equation of state of dark energy is changing with time then a big bounce is no more likely than a big rip or big crunch (all of which are inconsistent with the standard model of cosmology).”

At its death, our universe will be unrecognizable and, unfortunately, no Homo sapiens will exist to witness it or capture the event in poetry or film: “Galaxies will have dispersed, black holes will have evaporated, and the expansion of the universe will have pulled all remaining objects so far apart that none will ever see any of the others explode. It won’t even be physically possible for light to travel that far,” says Caplan, about the far off heat death of the cosmos which will then be mostly black holes and burned out stars.

“It will be a bit of a sad, lonely, cold place, with most stars slowly fizzling as their temperatures fade to zero,” he says, punctuated the silent fireworks of black dwarf supernovas, explosions of the remnants of stars that were never supposed to explode.

Theoretical work by Caplan, an assistant professor of physics at Illinois State University, inspired by a 1979 paper by the recently deceased theoretical physicist Freeman Dyson then at the Institute for Advanced Study in Princeton, finds that many white dwarfs may explode in supernova in the distant far future, long after everything else in the universe has died and gone quiet.

As white dwarfs cool down over the next few trillion years, they’ll grow dimmer, eventually freeze solid, and become ‘black dwarf’ stars that no longer shine.”

Caplan uses large scale computer simulations to study astro-materials, the solids that form inside dead stars. “Stars freeze,” he writes. “At the end of their lives stars cool and contract forming white dwarfs and neutron stars. In these extremely dense environments nuclei can be packed so closely that they freeze solid, forming materials many trillions of times denser than anything on earth.”

“Stars less than about 10 times the mass of the sun do not have the gravity or density to produce iron in their cores the way massive stars do, so they can’t explode in a supernova right now,” said Caplan. “As white dwarfs cool down over the next few trillion years, they’ll grow dimmer, eventually freeze solid, and become ‘black dwarf’ stars that no longer shine.”

Like white dwarfs today, he says, they’ll be made mostly of light elements like carbon and oxygen and will be the size of the earth but contain about as much mass as the sun, their insides squeezed to densities millions of times greater than anything on earth.

“The Galaxy at the End of the Universe”

Time Beyond Googols

“Stars shine because of thermonuclear fusion—they’re hot enough to smash small nuclei together to make larger nuclei, which releases energy. White dwarfs are ash, they’re burnt out, but fusion reactions can still happen because of quantum tunneling, only much slower, Caplan said, noting the key for turning black dwarfs into iron and triggering a supernova: “Fusion happens, even at zero temperature, it just takes a really long time.”

Caplan’s new work, accepted for publication by Monthly Notices of the Royal Astronomical Society, calculates how long these nuclear reactions take to produce iron, and how much iron black dwarfs of different sizes need to explode, calling his theoretical explosions “black dwarf supernova”. He calculates that the first one will occur in about 10 to the 1100th years.

“In years, it’s like saying the word ‘trillion’ almost a hundred times. If you wrote it out, it would take up most of a page. It’s mindbogglingly far in the future,” he notes, a number far beyond googols.

A Dead Silent Universe

“Only the most massive black dwarfs, about 1.2 to 1.4 times the mass of the sun, will blow,” he observes, which that means as many as 1 percent of all stars that exist today, about a billion trillion stars, can expect to die this way. As for the rest, they’ll remain black dwarfs. “Even with very slow nuclear reactions, our sun still doesn’t have enough mass to ever explode in a supernova, even in the far far future. You could turn the whole sun to iron and it still wouldn’t pop.”

Caplan calculates that the most massive black dwarfs will explode first, followed by progressively less massive stars, until there are no more left to go off after about 10^32000 years. At that point, the universe may truly be dead and silent. “It’s hard to imagine anything coming after that, black dwarf supernova might be the last interesting thing to happen in the universe. They may be the last supernova ever.”

Matt Caplan and Illinois State University

Image credit: YouTube

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