We are like butterflies who flutter for a day and think it is forever.
—CARL SAGAN, COSMOS
There will come, in the deepest of deep futures, a final, almost defiant transient event. Nobody will witness it. Or it might not happen at all if the universe manages to tear itself apart before then or if it turns out that protons have the ability to decay. But it’s interesting to speculate what that last event might be.
Here’s an incomplete list of what this event most assuredly will not be. It will not be the explosive death of a massive star. Those types of stars will all be long dead in an unfathomably ancient, rapidly expanding universe that can barely get two atoms to meet, much less enough atoms to build a massive star.
It will not be any kind of merging event either. All binary systems will have long since coalesced into a single object or been disrupted, the two dance partners gravitationally wrenched apart by a passing third object and cast adrift in a frozen, empty universe.
Supermassive black holes will have swallowed everything they could and collided as often as they could with each other, but there won’t be any of those left either.
Instead, what will most likely get the last laugh are the lonely remnants of mediocre stars: white dwarfs. Except by then, they won’t even be white. They will have spent the life of the universe cooling and cooling until their temperatures are nothing like the million or so degrees Celsius that they started with. It was this scorching temperature that gave them their intense white glow, a color that persisted even as they cooled to 100,000 degrees Celsius and grew a crystalline interior structure. But as the gigayears passed, they became yellow, then red, and then faded from sight altogether. At the time of their hypothetical swan song, they will be black, no longer radiating at all as they and the universe have reached the coldest possible equilibrium.
This admittedly will all take place a long time from now.
This scenario is so far into the future that each of these objects—however many there are—will occupy in a very real sense its own island universe. These will be nothing like the island universes debated by Curtis and Shapley in the 1920s, constrained by the limited imagination of humans. Instead, these frozen white dwarfs will each enjoy a personal space immeasurably larger than the volume of the currently observable universe. Moreover, each introvert’s bubble will be growing exponentially larger, making it literally impossible for any of these objects to have any impact on anything else.
Ever.
And yet, despite the fact that such things have exactly zero chance of ever being observationally verified, this has not stopped people like Matt Caplan of Illinois State University from thinking about them.
“It’s just so dang cool!” he exclaimed without a hint of irony that the objects he’s hypothesizing are literally at absolute zero. Dang cool, indeed. Sporting dark-rimmed glasses and bushy chaotic hair on which his sunglasses were perched, Caplan had that kid-in-a-candy-shop enthusiasm characteristic of every researcher I’ve encountered.
He was presenting a lecture remotely from Illinois through a Zoom meeting run by California State University, Long Beach, a colloquium I was viewing while preparing for a hurricane to hit Houston. Such is post-pandemic science communication.
In fact, such is post-pandemic science. A student in an online class at the beginning of the COVID pandemic had noted that the textbook equation for a white dwarf’s limiting mass depended on the proportion of electrons. “But what if it’s made of something different?”
“It’s the sort of thing I’d never even bothered to think about on my own,” Caplan admitted.
And so Caplan wondered, what if?
In the current universe, the hypothetical is pointless. White dwarfs aren’t made of anything different. They’re almost completely carbon and oxygen, or in rare instances oxygen, magnesium, and neon. Their behavior is fairly well understood, and their limiting mass is the same one calculated by Chandrasekhar in 1930: 1.44 times the mass of the Sun.
In the far future, though, dead stellar cores will slowly turn to iron, a transition that changes everything.
“I had the stroke-of-genius movie moment where I ran out of the classroom to write it all down,” Caplan said.
“Wasn’t this a Zoom class?” I asked.
He conceded, “Okay, that’s not what happened, but it can be fun to pretend it was.”
So while we’re having fun pretending . . .
Imagine the Sun. Now halfway through its hydrogen-fusing life, in about 5 billion years, it will swell to encompass Earth and everything that you ever cared about here. The atoms from your favorite car or cat or garden gnome will still exist, however, and maybe—while we’re pretending—those don’t all get swept out into space along with the Sun’s future planetary nebula. Maybe they settle onto the surface of the dense remnant.
And maybe, because the Sun is in the outskirts of the Milky Way Galaxy, the inevitable collision between our Galaxy and the Andromeda Galaxy will not simply catapult us into their merging supermassive black holes. Maybe in just a few billion years, the Sun’s white dwarf, along with the atoms of your favorite car or cat or garden gnome, will be nudged out to a safer distance instead.
And maybe, during all the gravitational jostling of the billions of stars in these merging galaxies, the Sun will win the gravitational lottery. Perhaps through exceedingly improbable, but not impossible channels, it might become paired up with a similar white dwarf. They might gradually spiral together—it’s not as though they don’t have the time, after all—and combine to form a white dwarf with about 1.16 times the mass of the current Sun.
Over the next many billions of years, the new and improved white dwarf will cool to a frozen black dwarf while the rest of the universe ties up the loose ends. In 100 trillion years, everything that can be processed in the universe will have been processed. Hydrogen fuel will have long been exhausted, and most of the matter in the universe will now be found only in the densest of remnants: black holes, neutron stars, and black dwarfs. There will be no shining stars, no constellations, no glowing gases decorating the night sky.
In 10 million trillion years, galaxies themselves will dissipate. The higher-mass objects will bully the lower-mass objects to leave the galaxies altogether. The black dwarf containing the Sun’s remnant, along with the atoms of your favorite cat or car or garden gnome, might be evicted to safety. Over the next million quadrillion years, all remaining galactic orbits will slowly lose energy as they radiate gravitational waves and spiral inexorably toward the super-supermassive black hole at the core.
Now, fast-forward to 1050 years in the future, a number so great that we don’t have handy words for it. It’s 100,000 trillion quadrillion quintillion. The matter in every atom that currently makes up everything you now experience, including this book in your hands and the faintest star in the night sky, is now either part of those scattered supermassive black holes or tied up in the low-mass objects that were kicked out of their dying galaxies.
It’s very cold and very dark.
But the universe is still young.
After about 10100 years—a googol years—even supermassive black holes with masses of 100 trillion times that of the Sun will have slowly eroded away thanks to an incessant and infinitesimally small trickle of radiation, their lives just a heartbeat in the grand cosmic scheme of eternity. Meanwhile, those previously evicted low-mass objects endure.
Like their former selves, the frozen black dwarfs still have interiors abuzz with electrons that are incessantly doing battle with gravity. The previously agitated carbon and oxygen nuclei have now settled into a crystalline structure, lying dormant in preparation for the last big show.
Now, we wait some more.
At the inconceivably distant future 101100 years—that is a 1 followed by 1,100 zeroes, a number so large that it would take an entire page just to write out the zeroes—from now, things will finally begin to fall into place. The seemingly rigid crystal lattice in the black dwarf will turn out to be not quite so rigid after all. On the scales of the very tiny, particles frequently find themselves teleported to a different place, one that seems at first glance to be out of bounds. Protons do this all the time inside the Sun you see today, showing up beyond the repulsive wall of fellow protons and allowing for the production of sunshine. In the extremely distant future, entire nuclei will ultimately and inevitably do the same inside frozen black dwarfs. Two carbon or oxygen nuclei occupying nearby spots on the crystal latticework will become fused, not because the temperatures are so high, but because the universe simply finds a way if given enough time.
It has at long last been given enough time.
The slow, cold process known as “pycnonuclear fusion” will start where the pressure is the greatest, which is to say in the center of the black dwarf. From there, it will creep outward at a pace that makes glaciers appear impatient. Then, a new pycnonuclear fusion process will create even heavier nuclei, like nickel, each containing 28 protons and 28 neutrons. But this configuration is just a bit off, energetically speaking, so a couple of protons will adjust themselves to make neutrons. To make this happen, the two protons will shed their positive charges as positrons. Now we have iron, which has 26 protons and 30 neutrons. The antimatter counterpart to electrons, the positrons will race out into the buzzing sea of electrons and pick them off, one by one.
The situation seems wholly unfair. The electrons have been dutifully fighting for the past 101100 years to keep gravity from winning. Individually, an electron can’t hope to prop up a black dwarf against the crush of gravity, but collectively the trillions upon quadrillions upon quintillions of them have done the job faithfully for more years than can be counted.
Now, though, Caplan said, “it’s like knocking out the support beams in the basement of the star.” Once securely under the limiting mass of 1.44 solar masses, a black dwarf with a mere 1.35 solar masses, after 101100 long and thankless years, will be unable to prevent collapse. The least massive ones, containing just 16% more mass than the Sun, will manage a good run of about 1032000 years before this happens.
It makes no substantive difference if one uses the unit “microseconds” rather than “years.” They’re both pointlessly short units of time in the grand scheme of things. Even using the entire lifetime of the Sun—10 billion years—as the timekeeping unit merely changes the exponent to 31,990, which is really just 32,000. This is how ridiculously enormous these numbers are, and this is why people have asked Caplan, “Doesn’t it make you sad to have an idea about something that you will never, ever, ever observe?”
The universe is filled with such things. The joy, according to Caplan, is in imagining what is possible and seeing where the weird ideas take you.
So maybe, just maybe, in 1032000 years or microseconds or solar lifetimes, the remnant of the Sun, having long since merged with another similarly massive stellar core, will be part of a 1.16-solar-mass black dwarf, along with some of the atoms that made up your favorite car or cat or garden gnome.
The electron supports will be eliminated, and gravity will begin to win for the last time. Once the collapse begins, the elderly black dwarf is done for. It’s the story of a massive star core-collapse supernova all over again, but this time it will happen in the empty stellar zombie realm of the deepest, darkest future. The remaining electrons, succumbing to the new intense pressure, will combine with protons and spit out the universe’s last flood of neutrinos, and the floor will drop out from underneath the rest of the star. A full foe of energy will blast the rest of the star into space in the universe’s last transient event ever.
No observatory will ever detect light or neutrinos, cosmic rays or gravitational waves from this event.
Nothing new will ever form from its remnants.
But maybe, just maybe, at least part of our mediocre star will have the last word.