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 weâd
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
universeâs 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 Einsteinâs
theory of gravity - the general theory of relativity - that might explain dark matterâs
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 weâve 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 weâve 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 - itâs
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 itâs much more spread out than regular matter - for example, in our galaxy it forms a vast halo around twice the diameter of the Milky
Wayâs spiral disk, where most of the stars are found. So for black holes to be dark matter theyâd
need to be abundant enough to make up all of this mass, and theyâd 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 doesnât 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 weâre going to go through the mass
spectrum of black holes, and close one window after another - weâll see at the end whether
there are any gaps left - whether itâs still possible for black holes to explain dark matter. But before we start eliminating specific black holes masses, letâs 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 weâre 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 havenât 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, youâd 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, weâve talked about them before, but letâs 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, letâs get started. Weâll 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 âsupermassive black holesâ
in the centers of essentially all galaxies. Dark matter canât 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, thereâs a terrifying caveat here that
Iâll 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, weâd need truly ridiculous
numbers of them to get all the dark matter we need. Iâm 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, theyâd be wreaking absolute havoc in various astrophysical objects. For example, theyâd 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, letâs 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 doesnât 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 theyâre 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 âMACHOsâ - 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 werenât 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 moonâs mass to 10 or so times the mass of the Sun
as a main contributor to dark matter. But even if MACHOs arenât 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 canât rule out these as primordial black holes, nor as Reapers - but thereâs no good evidence of either. So far weâve mostly ruled out black holes around the Sunâs 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 thereâs 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 Sunâs 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 havenât properly looked at yet. So dark matter probably isnât black holes
- but donât be sad - that means dark matter is probably something that physics hasnât
explained yet, which means when we figure it out weâll have a window into NEW physics,
and who knows what weâll find there? But speaking of new physics - one last option. Itâs 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 theyâre 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.
Wow, such a good episode. And I actually understood most of what they talked about!
Also the last one was great, about the Magnetic Dipole Moment experiment! PBS Spacetime krushing it wutttttt đ
Some spiders live in holes, maybe it could be a black spider web hole?