Black
holes are already mind-bending enough.
Matter and energy compressed into a region so dense that the escape velocity exceeds
the speed of light.
Places where space and time are tangled up so much that even if you could go faster than
light speed, you’d still end up at the singularity.
Astronomers know of two kinds of black holes in the Universe: stellar mass and supermassive,
and how we get them have been worked out pretty well.
But there’s a third class of black hole, one which has never been observed nor detected
indirectly, and yet it could explain some of the mysteries of the Universe, including
the nature of dark matter.
Primordial black holes.
To get a stellar-mass black hole, all you have to do is let a star with many times the
mass of the Sun run out of fuel in its core.
The star works its way up through the table of elements, fusing atoms with more and more
protons, desperately producing the energy to hold back the immense force of gravity.
When their process reaches iron, no energy can be extracted from the fusion process.
Fusion in the core shuts off, and the star implodes on itself, with its outer layers
falling inward at relativistic speeds.
In mere moments, all of this mass is compressed into a region dense enough that nothing, not
even the energy of the implosion, can escape.
A black hole is born.
Exactly how supermassive black holes are formed is still a bit of a mystery.
These can have millions or even billions of times the mass of the Sun, and form the hearts
of many galaxies across the Universe.
Maybe they’re the end product of merging stellar-mass black holes, which feed and merge,
feed, and merge their way up to billions of solar masses.
Or maybe supermassive black holes can form all in one cataclysmic event, as a gas cloud
directly collapses into a single monster black hole, without having to go through the whole
star-forming/star-destroying process first.
However it works, astronomers see both stellar and supermassive black holes across the Universe,
and they’ve worked out models supported by evidence to explain how they form.
But mathematically speaking, a black hole can theoretically be of any mass as long as
it’s compressed within a certain size.
A black hole with 10 times the mass of the Sun would have an event horizon about 59 kilometers
across.
But if you turned the Sun into a black hole - which can’t happen naturally - it would
only measure 5.8 km across.
If you turned the Earth into a black hole, it would measure only 18 mm across.
And Mount Everest turned into a black hole would be about a nanometer across.
Even if the math checks out, is there any theoretical way that smaller black holes are
actually possible?
Actually maybe.
And the idea for this goes back to 1974 with physicist Stephen Hawking.
Hawking proposed that in the earliest moments of the Universe, everything was kind of like
a black hole, with matter and energy compressed into an incomprehensibly dense region.
Unlike the Universe today, there weren’t vast differences in density where matter could
pull together into a single black hole.
But by looking at the temperature differences in the Cosmic Microwave Background Radiation,
we know there were regions of higher and lower density in the earliest Universe.
And maybe these over and under densities provided a spot where smaller, less massive black holes
could form.
Primordial black holes of all sizes, from little more than a few atoms squeezed together
to mighty supermassive black holes that could form the seeds for the even more massive black
holes that we see today.
And so far, these black holes have evaded our detection, because they’re just floating
free across the Universe.
Drifting through space, disrupting star systems and galaxies with their gravity, and creating
an enormous amount of mass that doesn’t interact with regular matter except through
its gravity.
Does that sound familiar?
Filling the Universe with tiny blobs of mass, which almost never interacts with regular
matter sounds a lot like the mystery of dark matter.
While regular matter accounts for 5% of the Universe, another 27% seems to be dark matter,
which dominates the Universe gravitationally.
Astronomers still aren’t sure what dark matter is, but they’ve mostly ruled out
that it’s actually gravity acting strangely at big distances, and are leaning towards
some kind of massive, non-interacting particle.
But one idea that just won’t go away is that dark matter is actually explained by
primordial black holes.
How could astronomers confirm or rule out that primordial black holes actually exist?
I’ll get to that in a second, but first I’d like to thank:
Chris Wheelwright Zoltan Somogyi
Brian Melcher Jason Sinn Photography
Don Griffiths Joseph Lee
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If primordial black holes really are scattered across the Universe, making up some or maybe
even all of the missing matter, how can astronomers find them?
In order to fully account for dark matter, black holes would need to be either asteroid
mass, a little less mass than the Moon, or intermediate-mass up to 1,000 times the mass
of the Sun.
If a massive enough primordial black hole passed through the Solar System, we’d certainly
notice the gravitational effects as the planets were pulled from their orbits.
Let’s hope we don’t get to run that observation.
But astronomers have come up with clever ideas to search for them en masse across the Universe.
In early 2020, an international team of astronomers attempted to search for primordial black holes
acting as gravitational lenses.
When a black hole passes in between us and a more distant background object, its gravity
acts as a natural lens, distorting the light from the distant object.
The carefully studied the light coming from the Andromeda Galaxy, which is 2.5 million
light-years away.
They used the powerful Subaru Telescope in Hawaii to take 190 consecutive images of Andromeda.
In these images, there were 90 million individual stars, each of which could have a black hole
produce a lensing event from our perspective.
After two years of data analysis, filtering out all the noise and non-gravitational lens
events from the data, they were only able to identify a single star that brightened
and then dimmed.
Only one, possible primordial black hole.
And that’s not enough black holes to fully explain dark matter.
Another way to search for primordial black holes is to detect when they disappear.
In addition to proposing the idea of these tiny black holes in the first place, Hawking
calculated that black holes should slowly evaporate over vast periods of time.
The less massive the black hole, the more quickly it gives off radiation, and the faster
it loses mass.
In fact, at this point in the age of the Universe, any black hole less than 100 billion kilograms
will have already evaporated.
As a black hole’s mass decreases, its temperature increases, and in the end, it’ll disappear
in a sudden flash of gamma radiation and other particles.
One of these particles, for example, is the neutrino.
In 2019, researchers published a paper proposing that the IceCube neutrino detector could theoretically
detect the neutrino from an evaporating primordial black hole during the last 1,000 seconds of
its life.
You could also detect the flash of gamma radiation.
In fact, one theory is that a class of ultra-short gamma-ray bursts are actually explosions of
primordial black holes.
Space and ground-based observatories could detect these explosions.
Astronomers recently pored through 2,700 hours of observations from the High Energy Stereoscopic
System and didn’t find and events yet.
Another way to search for primordial black holes is through gravitational waves.
In fact, in September 2020, astronomers announced the detection of two black holes creating
a combined mass of 142 solar masses.
One of the black holes going into the collision had 85 times the mass of the Sun, which is
a very interesting number.
Exploding stars can create black holes up to 65 times the mass of the Sun.
And the most massive stars can generate black holes greater than 120 solar masses.
But there shouldn’t be a way to get an 85 solar mass black hole.
So where did this black hole come from?
Obviously, it could have been the result of a previous merger, but another intriguing
idea is that it’s a primordial black hole.
If more of these collisions are detected, it could help astronomers constrain the number
of intermediate-mass black holes in the Universe, and figure out if they’re a candidate for
dark matter.
Another possibility is that you could detect the gravitational waves of the primordial
black holes that evaporated at the beginning of the Universe before even hydrogen and helium
were formed.
Primordial gravitational waves caused by primordial black holes.
I’m sure you’ve heard the theory that the mysterious Planet 9 could actually be
a grapefruit-sized primordial black hole with 5-10 times the mass of the Earth.
That would nicely explain why a huge planet in the outer Solar System hasn’t been discovered
yet.
But Dr. Avi Loeb and his collaborators have figured out a way to detect its presence.
In theory, this tiny black hole will have a tiny accretion disk of gas around it from
the interstellar medium it’s feasting on.
This will heat up, melting nearby icy objects in the Kuiper Belt.
In theory, the upcoming Vera Rubin Observatory will be able to detect the flashes from icy
objects getting too close to the black hole.
At this point, primordial black holes are still entirely theoretical.
But search the arXiv server for papers and you get 1,525 focused on primordial black
holes.
I just mentioned a fraction of them in this video.
Many astronomers are seriously considering them as a candidate for dark matter.
Either they’ll get discovered or ruled out.
I’ll let you know which way it turns out.
Ask for Comments: What do you think?
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How do we even know regular old black holes are real?
Here’s a video where I talk to Dr. Paul Matt Sutter, and lay out all the different
lines of evidence to suggest that black holes are, indeed, a thing.