Primordial black holes may flood the universe. Could one hit Earth?

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." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.

Black holes sound pretty scary — dark, powerful, foreboding. And now, astrophysicists have cooked up something else: primordial black holes, forged in the earliest moments of the universe, that flood the present-day cosmos.

So, what are the chances that one of these ancient monsters will come wandering toward Earth? One astrophysicist has run the numbers. 

Related: Swarms of 'primordial' black holes might fill our universe

Birthed in the Big Bang

The early universe was a wild, complex time. Far different from the mild-mannered cosmos we inhabit today, the earliest moments of the Big Bang were marked by radical phase transitions, the splitting of fundamental elements and other wild events. While scientists understand the physics of the first few minutes, what happened before that is shrouded in mystery (and a bunch of complicated math).

You need some pretty extreme conditions to form black holes — say, a star collapsing in on itself during the final, catastrophic moments of its life. Stars weren't around in the first few seconds of the universe's existence, but there may have been just the right conditions to forge black holes; all you need is a lot of matter or energy crammed into a tiny enough volume.

In the unknown and uncharted reaches of the universe's distant past, the conditions could have been just right to flood the universe with primordial black holes, which could have any mass, depending on the conditions under which they were made. But interest in primordial black holes waned over the decades as searches for them turned up empty — that is, until we had the Laser Interferometer Gravitational-Wave Observatory (LIGO).

When LIGO detected its first black hole collision, the black holes had rather peculiar masses; each was a few dozen solar masses. That mass range is difficult to achieve with mergers of standard star-based black holes, because mergers would have to be a little too frequent to be plausible). And so, before you knew it, primordial black holes were back in the spotlight.

A dark encounter

The thing about processes in the early universe is that if there is some sort of exotic mechanism that can generate black holes, it's not going to make a few of them — it's going to flood the universe with them. In fact, there might be enough primordial black holes roaming the universe to explain at least a portion of dark matter, the mysterious substance that accounts for over 80% of all the matter in the cosmos.

Let's say a whole bunch of small black holes are swarming the cosmos, as presented in a paper published recently to the preprint database arXiv. What would happen to them?

Thankfully, black holes aren't 100% black, and they lose mass through Hawking radiation, the complex quantum mechanical process at the black hole event horizon that allows some particles and radiation to escape. The smaller they are, the faster they lose mass. Black holes less than roughly 100 million tons — slightly lighter than a typical asteroid — will lose about half their mass within the current age of the universe. Because of the way Hawking radiation works, black holes that are bigger than that will only lose a small fraction of their mass.

The total number of small black holes in each galaxy depends on how much of the dark matter you want to explain with them, and how big each one is. No matter how you slice it, though, there are a lot.

And each one is fast. Based on computer simulations and observations of galaxy dynamics, dark matter has a velocity of over a hundred miles per second. At that speed, an asteroid-mass black hole could cover the distance between Jupiter and Earth in just a couple weeks. So should we be scared?

Running the numbers

What would happen if an asteroid-mass black hole were to hit Earth? In short, catastrophe. The black hole would puncture our planet's surface like a hot knife through butter, but it would immediately begin to slow down because of its gravitational interaction with Earth. Any atom or molecule (or person) intersecting the event horizon — the boundary of the black hole beyond which nothing, not even light, can escape — would simply slip away from the known universe, never to be seen again.

In the best-case scenario, the black hole would exit through the other side of our planet, leaving the survivors to clean up the mess. In the worst-case scenario, the black hole would settle into the core of our planet, where its gravity would be enough to allow the black hole to begin feeding. Eventually, it would devour our entire planet.

Thankfully, according to the calculations in the paper, the chances of a black hole settling in Earth's core are rather minimal. Black holes are just too fast. 

On the other hand, the intersection of our planet with a black hole would lead to another unpleasant reality: heating. During its passage through Earth, the black hole would accrete matter, and that accretion would generate heat (the same heat that powers active galactic nuclei). The impact of an asteroid-mass black hole would end up releasing about the same amount of energy as the impact of a kilometer-wide asteroid.

You know, a dinosaur killer.

Thankfully, black hole collisions are likely rare. In the most "optimistic" scenario — optimistic by the scientists' standards, that is, so populating the galaxy with the maximum number of black holes — there might be one collision or so every billion years, according to the paper's calculations.

So, when it comes to black hole collisions, don't get too scared.

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