In 2016, the LIGO (Laser Interferometer Gravitational-Wave Observatory) team surprised and delighted the world when they announced the first-ever discovery of gravitational waves, emanating from a collision of two black holes billions of years ago.
And along with the flurry of excitement (and a few Nobel prizes for good measure) came a strange little surprise. The black holes had very peculiar masses, peculiar enough that it opened up a fascinating possibility: The black holes that LIGO heard collide may have been forged when the universe was less than a second old.
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A guide to making black holes
We know how black holes are made in the modern-day universe. You start with a star, the bigger the better. At least eight times the mass of the sun should do the trick. Then you wait as the star burns through all its available supply of hydrogen. This should only take a few tens of millions of years. No big deal.
Then at the end of its life, the star will destroy itself in a cataclysm of energy, a supernova explosion. In the fires of that explosion, the densities in the core can reach an intense enough state that nothing — nothing at all — can resist the inward pull of gravity. So at the same time that most of the star is exploding outward, a fraction of it collapses inward on itself, folding end over end toward oblivion: a black hole.
The bigger the star, the bigger the black hole, which is what makes the LIGO results so interesting. Those colliding black holes had masses of 30 and 35 times the mass of the sun, respectively. To make a black hole that big, you either need to start out with a truly hideous monster of a star — somewhere north of 100 times the mass of the sun — or you need to build them up from mergers of lots of smaller black holes.
At the time, both scenarios seemed unlikely. Stars that big simply don't exist in the universe (at least, nowadays), and mergers aren't common enough to build up to that.
Hence: maybe these black holes had a different origin.
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The Big Bang black hole machine
The early universe was, to say the least, a crazy place. Temperatures and pressures unheard of in the eons since. Phase changes that rocked the entire cosmos. Transformations that rewrote the very laws of nature.
Back then, if conditions were right, any old patch of gas may have spontaneously shrunk itself to form a black hole of any size: from something weighing just a few kilograms to thousands of times the mass of the sun, and anything in between.
For every theoretical physicist working on the problem of these so-called primordial black holes, there is at least one hypothetical mechanism for generating them, involving everything from inflation theory to colliding universes.
So in one sense, it's easy for primordial black holes to explain the early LIGO results: you just find a theory that makes black holes in the right size range and abundance, wait a few billion years, and you're bound to get a merger event.
But if you want to populate the universe with black holes from the Big Bang, they're going to do more than make LIGO sing.
Hunting in the dark
What would a universe flooded with primordial black holes look like? That's the million-dollar question, which we need to answer if we want to test this hypothesis.
For one thing, the black holes may randomly crash into other things, gravitationally attract other things, and just generally cause mayhem. Kilogram-mass black holes hitting the Earth could trigger earthquakes. A silent black hole may pull apart binary pairs of stars or disrupt entire dwarf galaxies. A black hole ramming into a neutron star could ignite a terrible explosion. Even the hypothetical Planet Nine could be a black hole no bigger than a tennis ball.
And as a bonus when it comes to potential detectability, black holes aren't entirely 100% black: they might glow, ever so faintly, through the quantum mechanical process called Hawking Radiation. Big black holes hardly glow at all: one the mass of our sun radiates around one single photon every year, taking 10^60 years to lose all its mass. But smaller black holes can go off in much less time, releasing a burst of energy in the process.
Exploding black holes may have disrupted the early universe, changing the abundance of elements or the appearance of the cosmic microwave background. Or they may be responsible for some of the gamma ray bursts that we see in our skies.
Alas, despite all our attempts, we cannot reconcile the existence of primordial black holes with the universe that we see. For every possible observational avenue, the primordial black holes cause so much mayhem that it would be noticeable to us.
In other words, as difficult as it is to explain the masses of the merging black holes that LIGO witnessed, if you want a universe with those black holes to be primordial, it would be detectable in other ways.
Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of "How to Die in Space." He contributed this article to Space.com's Expert Voices: Op-Ed & Insights.
Learn more by listening to the episode "Did the Big Bang make black holes?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Robert K., Peter N., and Raul P. for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.