It may be that for every star in the universe there are billions of microscopic black holes
streaming through the solar system, the planet, even our bodies every second.
Sounds horrible - but hey, at least we’d have explained dark matter.
80% of the mass of our universe is completely invisible to us - its existence only revealed
through its immense gravitational influence.
This is dark matter, and it is one of the universe’s most perplexing mysteries.
These days most dark matter hunters are trying to hypothesize or detect exotic new particles
to explain the stuff - and we recently discussed some of the possibilities for brand new particle
physics that might explain dark matter.
Others are trying to find holes in Einstein’s theory of gravity - the general theory of
relativity - that might explain dark matter’s influence without the need for actual matter.
But there is one explanation that may require no new physics whatsoever - an explanation
that is so bafflingly simple it almost feels overlooked.
What if dark matter is just black holes?
These hyper-dense holes in the fabric of spacetime seem to be great dark matter candidates - being
so black and holey and all.
As we’ve discussed many times before, black holes are regions of gravitational field so
intense that not even light can escape.
Evidence for the reality of black holes is now pretty convincing - and we’ve talked
about this evidence before.
The fact that we know black holes are actually real seems like a significant point in their
favor as an explanation for dark matter - it’s more than we can say for any of the other
dark matter particle candidates.
So what would it take for this to be true?
Well, dark matter makes up roughly 80% of the mass of the universe,
but it’s much more spread out than regular matter - for example, in our galaxy it forms a vast
halo around twice the diameter of the Milky Way’s spiral disk, where most of the stars
are found.
So for black holes to be dark matter they’d need to be abundant enough to make up all
of this mass, and they’d need to be spread out in the same way that dark matter is.
In other words, most of the physical universe needs to be vast swarms of black holes that
outweigh all the atoms in the universe by a factor of four.
These are the non-negotiables for black-holes-as-dark-matter.
The main remaining variable is the mass of the individual black holes.
We could get to the required dark matter mass with lots of massive black holes, or ludicrously
many smaller black holes.
We have, of course, been trying to find evidence for black hole dark matter for some time.
Any given study is sensitive to a particular range of black hole masses.
If a study doesn’t find enough of black holes in that range, then that mass range is ruled out as a main
contributor to dark matter.
Scientific method 101 - form a hypothesis, then try to prove it wrong.
Our hypothesis is that dark matter is made of black holes.
Today we’re going to go through the mass spectrum of black holes, and close one window
after another - we’ll see at the end whether there are any gaps left - whether it’s still
possible for black holes to explain dark matter.
But before we start eliminating specific black holes masses, let’s rule out an entire class
of black holes. In face we're going to rule out black holes from the only reliable astrophysical source - dead stars.
We know black holes form from the remaining cores of the most massive stars, after they
explode as supernovae.
We can make a pretty good estimate of the maximum possible number of these black holes
by estimating the number of stars that formed and died through cosmic history.
When we look into the distance we’re actually looking back in time, so we can literally
see star formation happening in the early universe.
We can also see the products of the supernova explosions- not so much the black holes produced in
those explosions, but the heavy elements forged in the
cores of these stars during their death-throes.
That star formation history and heavy element abundance tells us there haven’t been anywhere
near enough supernovae to give us enough black holes to make up all of dark matter.
Also, if dark matter is produced as stars die, you’d expect its influence to increase
over time.
But we know that dark matter has been with us from the very beginning.
The density fluctuations in the cosmic microwave background tell us that the gravity of dark
matter was pulling matter together long before the first stars ever formed.
So if dark matter is made of black holes then those black holes must have been with us from
the beginning.
Fortunately for our hypothesis, there is a reason to think that colossal numbers of black holes
may have formed in the very early universe.
We call these primordial black holes.
Now, we’ve talked about them before, but let’s dig much deeper into the question of whether
primordial black holes could explain dark matter.
There are a few ways primordial black holes could form.
But the most mainstream mechanism is that they collapsed from density fluctuations.
We know there were regions of the early universe that had a bit more matter than other
regions - we see that in the cosmic microwave background from around 300,000 years after
the big bang.
Those fluctuations would have been much stronger at earlier times and perhaps strong enough that
the most massive of them would have collapsed into black holes.
Now those black holes should have all formed at around the same mass - but that mass depends
on the details of the state of the early universe, and could be anything from a grain of salt
to tens of thousands of suns.
Because of this, to really falsify the primordial black hole as dark matter hypothesis, we need
to rule out this entire mass range.
OK, let’s get started.
We’ll begin with the extreme ends of the mass spectrum - those are easy.
The most massive black holes in the universe weigh in at millions to billions of times
the mass of our Sun.
We see these “supermassive black holes” in the centers of essentially all galaxies.
Dark matter can’t be made of these or anything close to because they tend to fall to the
centers of their galaxies pretty quickly.
At the opposite end of the mass spectrum we have the black holes under a billion tons
or around the mass of a small mountain.
We can rule these out because by now all of these would have evaporated as Hawking radiation.
Now, there’s a terrifying caveat here that I’ll save for the very end.
We can also rule out black holes a bit larger than this as dark matter.
For dark matter to be made of black holes with masses around that of a larger asteroid
or small moon, we’d need truly ridiculous numbers of them to get all the dark matter
we need.
I’m talking somewhere between a billion to a billion billion times more of them than
there are stars in the universes..
Even though these black holes would have microscopic event horizons, at those insane abundances,
they’d be wreaking absolute havoc in various astrophysical objects.
For example, they’d puncture white dwarfs and neutron stars on a regular basis.
A primordial micro-black hole would pass right through a white dwarf, but it would deposit
enough heat to allow the stars' ultra dense carbon and oxygen to undergo nuclear fusion.
Like striking a match, this would ignite a cataclysmic chain reaction, exploding the
star as a type 1a supernova.
Neutron stars are a bit different.
They're so dense, that they would stop a microscopic black hole in its tracks.
The black hole would then swallow the neutron star from the inside out.
But we see too few type 1a supernova and too many neutron stars for this to be a common
phenomenon.
Probably no more than a few percent of the dark matter mass can be from these micro black
holes.
Ok, let’s move up the black hole mass spectrum to masses around that of a planet.
At this point, gravitational lensing becomes the go-to method for dark matter hunters.
When a compact mass like a black hole passes in front of a distant light source, the warped
spacetime around the black hole acts like a lens, magnifying the source in an effect
called microlensing.
Microlensing doesn’t necessarily tell you whether the lensing object is a black hole
or some other dark compact mass like a brown dwarf star, neutron star, Dyson sphere or
a cluster of Reaper capital ships - as long as they’re massive, compact, and dwell in
vast numbers throughout the halo of the Milky Way, watching and waiting.
Broadly we call this breed of dark matter candidate “MACHOs” - massive, compact
halo objects.
The best way to search for MACHOs in our galaxy is to monitor the stars in the galactic bulge
or in our neighboring galaxies to see if they fluctuate in brightness due to microlensing.
A dedicated telescope on Mt Stromlo in Australia pioneered this work throughout the nineties.
The very first MACHO microlensing events were detected, but there weren’t nearly enough
to account for dark matter.
Since then, further surveys have really nailed down the constraints.
The Magellanic Clouds are especially good for this - the low level of microlensing of
stars in these mini-galaxies has allowed us to rule out MACHOs between roughly the moon’s
mass to 10 or so times the mass of the Sun as a main contributor to dark matter.
But even if MACHOs aren’t all of dark matter, studies of the Magellanic Clouds and Andromeda
have found enough microlensing events to suggest that 20% of the mass of the Milky Way halo
may be dark, compact objects with the masses of a small star.
And we can’t rule out these as primordial black holes, nor as Reapers - but there’s
no good evidence of either.
So far we’ve mostly ruled out black holes around the Sun’s mass or lower as an explanation
for dark matter.
Larger black holes are tricky, because you need fewer of them to make up the mass of dark
matter - which means they're less likely to spotted through microlensing studies.
As I mentioned, the most massive black holes trickle to the center of our galaxy, but there’s
an intermediate mass range from tens to thousands of solar masses that could still be abundant
throughout the Milky Way.
One way to test that is to look at dwarf galaxies.
These galaxies are so small and dense that even black holes with tens of solar masses
should have trickled to the center by now, and in the process flung less massive stars
into higher orbits.
Studies of the structures of dwarf galaxies tells us that no more than 4% of the dark
matter could be black holes of tens to thousands of solar masses.
The existence of loosely bound binary star systems throughout our galaxy gives us similar
constraints - if there were lots of black holes of several tens times the Sun’s mass
then these binaries would long ago have been torn apart by close encounters.
Ok, so there we have it.
We have some evidence ruling out most of the black hole mass spectrum as the main source
of dark matter.
There are a few narrow windows remaining, but these are just the windows where our studies
are less sensitive, and frankly it's unlikely that it just so happens that all dark matter is
packed into the spots we haven’t properly looked at yet.
So dark matter probably isn’t black holes - but don’t be sad - that means dark matter
is probably something that physics hasn’t explained yet, which means when we figure
it out we’ll have a window into NEW physics, and who knows what we’ll find there?
But speaking of new physics - one last option.
It’s speculated that an evaporated black hole might leave behind a tiny naked singularity
- a speck of infinite density weighing less than a grain of salt.
These so-called Planck relics are bad news as dark matter - because they’re essentially
undetectable.
And also because there would have to be a few in the room with your right now.
Whatever the case, dark matter is freaky stuff, fitting as the main material ingredient of
our generally freaky space time.