This artist’s concept shows many small primordial black holes. Credit: NASA Goddard Space Flight Center
While black holes of any size are perplexing, some of the most mysterious are hypothetical primordial black holes, those that appeared in the first second after the Big Bang and some of which are featherweights on the black hole scale.
Because black holes collapse matter to nothing, a black hole with the mass of Earth would have an event horizon no larger than a hundredth.
That’s what gave two scientists, De-Chang Dai of National Dong Hwa University in Taiwan and Dejan Stojkovic of the University at Buffalo in New York, the idea for their recent paper, published September 20 in Physics of the Dark Universe. In it, they discuss how quickly moving primordial black holes might leave tiny holes in regular matter as they pass through, consuming all the material that falls into their tiny event horizons.
Specifically, they state that a black hole with a mass of 2.2 x 1019 pounds (1022 grams) would leave a 0.1 micron hole, large enough to be seen with a normal light microscope. The chances of this happening in any given space are astronomically small, but on cosmic timescales they are not impossible. In fact, the authors suggest in their article to look for “very old rocks, or even glass or other solid structures in very old buildings.”
The odds, they point out, are not good, perhaps incredibly small. But if people around the world could search for black holes with little or no specialized equipment: what a wonderful opportunity to spy on something incredible.
From the beginning
Primordial black holes are still just a theory. The reasoning goes like this: when the universe was still small, in the first second after the Big Bang, all the matter that ever existed was crammed into a tiny space. Since black holes form from an excessive density of matter, this would reasonably give rise to many black holes of all different sizes, both incredibly small and supermassive.
The problem is that we have never observed a black hole that cannot be explained at least mostly by other reasons. One wonders whether the supermassive black holes that reside at the centers of galaxies might have started out as primordial black holes (they seem to need some sort of push to get that big). But astronomers are sure that at least many black holes form when a massive star ends its life in a catastrophic explosion, and they are also sure that they can get bigger. However, there is no known way to form a black hole smaller than a star in our current universe. Such an observation – or even indirect evidence of one – would represent a major boost for the existence of primordial black holes.
“We need to think outside the box,” Stojkovic said in a statement, “because what has been done previously to find primordial black holes hasn’t worked.”
The research is important not only for understanding black holes, but also for the Big Bang. Primordial black holes are also a possible explanation for dark matter, the mysterious substance that makes up about 85% of the matter in the universe. Cosmologists can say that the universe is heavier than the stars, dust, and gas can explain, but we can’t see it. And so far he has evaded every attempt to make himself understood. Black holes, which are massive but invisible, are one possible answer, especially if they could be small and scattered across observable space – and even closer to home.
From the inside out
In addition to moving the black holes, which would form a tunnel through whatever material they encountered, the authors also consider what would happen if a primordial black hole encountered a celestial object such as a star, planet or asteroid. Such an encounter could occur during its lifetime, or the black hole could accumulate as part of the overall star, planet, or asteroid ensemble.
We tend to think that black holes are voracious and consume everything we see. But their range is limited by gravity and grows with the mass of the black hole. A black hole as massive as the Sun would have the gravitational reach of the Sun – and keep in mind that the Earth, although subject to the Sun’s gravity, still does not fall into our central star. It turns out that gravity has a shorter range than we tend to think.
So it is quite possible that a small primordial black hole resides, for example, inside a planet. Planets have denser cores than surfaces, so a black hole could quickly feed on that denser material, hollowing out a planet. Large planets would collapse with a hollow core, but asteroids or smaller planets made of tough materials like granite or iron could survive, as long as the primordial black holes were no more than ⅒ the radius of the Earth.
Again, while such a scenario is unlikely, it’s not impossible considering the age of the universe, and astronomers can often infer an object’s mass from how it orbits, making this an easy type of data to collect.
Closer to home
But there are many dense objects around us. If primordial black holes were common but invisible, we might not even need to look out into space.
Small black holes may have created or are creating even smaller regular holes in the substances around us. The authors recommend older objects, as they have accumulated greater chances of encountering a black hole. How much bigger? Not so much. It is estimated that the probability of a boulder even a billion years old encountering a black hole is only 0.000001. On the other hand, you might find it by simply walking to the nearest cliff and taking a look.
“You have to consider the cost versus the benefit. Does it cost a lot to do this? No, it’s not like that,” says Stojkovic.
And for anyone worrying about whether a black hole could tunnel through them right now, the authors assure readers that humans are much less fragile than a piece of rock, and the black hole would travel so fast you wouldn’t feel it. neither. “If a projectile moves through a medium faster than the speed of sound, the molecular structure of the medium doesn’t have time to respond,” says Stojkovic. “Throw a rock through a window; it will probably fall apart. Shoot a window with a gun; chances are it will simply leave a hole.
So, a small black hole won’t kill you. But it may have left a hole in your granite countertops. The only way to find out is to go and see. You could make history.
