An Astrophysicist Calculated the Last Big Explosions Before the Universe Dies

The last supernovas before the universe dies in a heat death will be twice-dead ancient relics on a mind-boggling time scale, according to astrophysicist Matt Caplan.
August 14, 2020, 1:00pm
​Artist concept of a supernova. Image: ESA/Hubble
Artist concept of a supernova. Image: ESA/Hubble

In case you hadn’t heard, the universe is likely to end in a heat death, meaning that it will keep expanding over googols (10 with 100 zeros) of years until the galaxies break apart, the stars burn out, black holes evaporate, and nothing is left but a gloomy and lonely abyss.

Now, for the silver lining: Before the universe plunges completely into shadow, the aged corpses of long-dead stars, known as “iron black dwarfs,” may blow up in one last supernova light show, according to a forthcoming study in the Monthly Notices of the Royal Astronomical Society.

“It’s not impossible that other explosions could happen,” said Matt Caplan, an astrophysicist at Illinois State University who authored the study, in a call. “But I think I can confidently say these would certainly be the last supernova-like explosions.”

This bizarre class of supernova does not yet exist because our universe is only 13.8 billion years old—far too young to have produced such ancient relics.

A star like the Sun would have to endure two deaths just to begin an afterlife as a black dwarf. First, it has to collapse into white dwarf, a type of super-compact dead star. The Sun is expected to make this transition in about six billion years. After that, so the theory goes, a white dwarf will cool and fade over an unimaginably long period of time until it becomes a frozen black dwarf.

These double-dead stars of the far future have been speculated about before. The renowned theoretical physicist Freeman Dyson touched on the topic in a 1979 paper, for instance. However, Caplan’s forthcoming study is the first to tackle the questions of if, and when, such objects would blow up.

“That’s the new thing,” he said. “I rigorously calculated out how long you have to wait and the exact conditions to achieve collapse, and what these sorts of supernovae might be like compared to things we see today.”

Black dwarfs would be shells of their former radiant selves, yet they could still eke out a tiny bit of nuclear fusion in their cores at an extraordinarily slow rate. This process would cause iron to gradually accumulate inside the objects until they evolve into cold lumps of metal. For stars the size of the Sun, that’s the end of the story: an iron chunk forever.

However, stars a few times more massive than the Sun could transition from white dwarfs, to black dwarfs, to black dwarf supernovae. Once a black dwarf deteriorates to a mass of about 1.2 to 1.4 times that of the Sun, the repulsive pressure generated by its electrons would buckle under the force of its gravity causing the entire object to collapse, crash into the core, and ignite into the last transient light-burst in the universe.

This vision of the future has some caveats. For one, we don’t know for sure that the universe will die in a heat death. Though it is the favored scenario, many other possible cosmic Doomsday models have been proposed.

Scientists are also not sure if protons, which are subatomic particles, will eventually decay into smaller parts over time. If they do, black dwarfs will evaporate long before they have the chance to explode.

“Experiments that have gone looking for proton decay have yet to see evidence for it,” Caplan said. “That doesn’t mean that proton decay doesn’t exist.”

Assuming that protons remain intact and the heat death rolls along as planned (more or less), about one percent of stars may eventually die as black dwarf supernovae, though it would take a truly inconceivable amount of time for them to blow up.

Caplan suggests imagining an hourglass filled with all the protons in the observable universe, which is estimated to be a number like 10 with 80 zeros behind it. One of the protons drops to the bottom of the hourglass every 10 billion years. When all of the protons have dropped, you remove one, flip the hourglass, and start the process over again.

“If you do this until you are out of protons, you will have waited about as long as you need to to see a black dwarf supernova,” Caplan said. “It’s a big number. It’s hard to get your head around.”

The number, which is about 10 with 1,100 zeros behind it, is the amount of years it would take for the first black dwarf supernova. It would then take googols upon googols more years for all of them to burst. By that point, the dimensions of the universe would be so large that Caplan called its predicted size “the biggest number I’m ever going to see in my career in a serious calculation.”

In other words, you’d have to be a very long-lived and adaptable lifeform to survive to the age of black dwarf supernovae. Even then, you probably couldn’t capture a glimpse of any explosions. At this extreme far point in the future, the vast expansion of space will have isolated everything in the universe behind a “cosmic event horizon,” beyond which no interactions or observations are possible.

“The cosmic event horizon is similar to the event horizon of a black hole,” Caplan said. “Once you’re past that point, the expansion of space is creating more space or distance between [objects] every second than light can travel.”

“That happens relatively soon,” he noted. “Soon” here means somewhere between 10 to the power of 20 up to a googol years, which is still billions of times longer than the age of the universe. When compared with the brain-numbing timescales of black dwarf supernovae, however, our slide into solitary cosmic bubbles is near-term.

Given how long it will take black dwarfs to evolve, there’s no chance of spotting one in our era of the universe. That said, Caplan hopes to keep exploring the future of iron black dwarfs in models and theories.

“It would be really cool to try and run some of these [black dwarf supernovae] in simulations just to see what kind of weird differences you might expect from supernovae today,” he said.

“This is the reason that people become theoretical physicists,” he concluded. “It’s because of the big questions: the Big Bang, black holes, and the end of time.”