Black dwarf supernovae might be the last event in the universe

The universe may have started with a Big Bang, but it will most likely end in an utterly anticlimactic way, slowly fading to black over trillions and trillions of years. Now, a theoretical physicist at Illinois State University has calculated what might just be the last interesting event that will ever happen – the explosions of stars called black dwarfs, which don’t even exist yet.

The ultimate fate of the universe is still up for debate, but one of the leading hypotheses is that it will undergo a “heat death.” Basically, all the stars will cool down and fizzle out, black holes will evaporate, and the never-ending expansion of the universe will stretch the fabric of reality so far that the remaining subatomic particles will rarely have the chance to whiz within a parsec of each other.

And now, thanks to theoretical physicist Matt Caplan, we have an idea of what might be one of the last things that will ever happen – black dwarf supernovae.

Currently, supernovae are explosive finales that are reserved for massive stars. When these huge thermonuclear reactors run out of fuel, the core will collapse and trigger a supernova, leaving behind a black hole or neutron star.

Smaller stars, such as our Sun, will instead expand into red giants then eventually shrink back down into white dwarfs. Since these white dwarfs don’t (normally) have the mass to go supernova themselves, they instead slowly cool down to the background temperature of space. When that happens, they’ll fade out and “freeze solid”, becoming cold, dark black dwarf stars.

It’s been calculated that this process would take trillions of years, and since the universe itself is “only” 13.4 billion years old, scientists don’t expect any black dwarfs to exist yet. The oldest known white dwarfs are still shining brightly.

A black dwarf was basically thought to be the end of the story, but according to Caplan, there’s still some life to be found in these objects. Fusion can still occur at very cold temperatures – it just takes an incredibly long time and requires some help from quantum mechanics.

The phenomenon of quantum tunneling means that occasionally, a particle will be able to “tunnel” through a barrier that normally it wouldn’t have enough energy to overcome. In this case, nuclei inside a black dwarf could spontaneously fuse together, even though they “shouldn’t” have enough energy to do so.

Eventually, those fusion products should build up enough to choke the black dwarf into a supernova, in a similar way to more massive stars. This explosive fate awaits as many as one percent of all stars shining today, Caplan estimates, while the vast majority will plod along silently as black dwarfs forever.

“Only the most massive black dwarfs, about 1.2 to 1.4 times the mass of the Sun, will blow,” says Caplan. “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.”

The physicist says that the most massive black dwarfs will be the first to go, followed by less and less massive ones within that range. But maybe hold off on whipping out your telescope to try to see one – you’ll be waiting an incomprehensibly long time. Caplan calculates that the first black dwarf supernova won’t happen for about 101,100 years. That’s a one followed by 1,100 zeroes, a number so big we don’t have a word for it.

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

And even if you could stick around to witness these events from the safety of a time machine, chances are slim that you would even be able to find them in the impossibly inky blackness of the universe’s Dark Era.

“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,” says Caplan. “It won’t even be physically possible for light to travel that far.”

But these black dwarf supernovae will still fire away, like trees falling in the woods with no one around, for an amount of time that’s even harder to fathom. Caplan says that the final black dwarf to go supernova will do so around 1032,000 years in the future.

“It’s hard to imagine anything coming after that,” says Caplan. “Black dwarf supernova might be the last interesting thing to happen in the universe. They may be the last supernova ever.”

The research was published in the journal Monthly Notices of the Royal Astronomical Society.

Source: Illinois State University

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