Universe Today
(This is Part 4 of a series on primordial black holes. Check out Part 1, Part 2, and Part 3!)
The challenge is that nothing in this universe is simple. And if there’s one thing you take away from today’s episode, then let it be that. Don’t ever let yourself fall into the trap of simple answers for difficult questions. We’re cosmologists, we study the universe as it is, not as we wish it would be.
To test if primordial black holes might be a viable dark matter candidate, first you have to make them. Which means you need physics happening in the early universe; physics that you understand well enough to somewhat confidently say that the cosmos spontaneously creates black holes. Well, how many? And how big are they? And are they all the same size, or is there a spread? And if there is a spread, how big is that spread? Are we talking anywhere from microscopic black holes to ones rivalling the big black holes of the modern era? To advance a theory you have to provide a plausible formation mechanism and says something a little more concrete than “oh yeah there are a bunch of black holes.”
Next you have to model how these black holes interact with the early universe. We know they are capable of a few things: they accrete matter, they emit Hawking radiation, and they can merge together. And how you start them out affects all of these. Maybe your model produces lots and lots of really tiny ones, which evaporate before the present day. But in the early universe that evaporation would have provided a source of heat, which might throw off nucleosynthesis or the properties of the cosmic microwave background.
The same goes for merging. Depending on how you seed these primordial little beasts, they might merge frequently, or not at all. And this merger rate has downstream effects for their survivability and detection (and ability to serve as the dark matter). Too much merging and you essentially wipe out their numbers. Not enough and they might evaporate too soon.
Then once you move past the early universe these black holes, if they’re doing to be the dark matter, affect the rest of the cosmos through their gravity. Maybe they tend to sink to the centers of galaxies, which would throw off measurements of rotation speed. Maybe they really like to merge, which would mean they emit tons of gravitational waves when they do. Maybe only a certain subset of them survives as time goes on, which means their appearance as the dark matter might change.
It's all wonderfully messy which is why some people have devoted their entire careers to studying them.
But we are able to say some things with confidence. If the black holes are too small, less than large-asteroid-range, then they would have evaporated by now, which is no bueno if you need them to stick around and be invisible sources of gravity.
And if they have a certain combination of mass and abundance – say, a millionth the mass of the Sun to a couple times bigger than it, and abundant enough to explain the dark matter – then we should have picked up the twinkle of starlight from these big black holes just wandering around interstellar space. When they pass in front of a star, the bending of light causes a brief magnification. It’s called microlensing and we’ve used this to detect rogue exoplanets, but there have been no signs of abundant black holes.
And if they’re too big, like thousands or tens of thousands of solar masses, then in the early, hot, dense universe, then matter would have accreted onto them in a big way. This accreting matter would heat up and release X-rays, which would mess up the cosmic microwave background.
And lastly, if there are too many big black holes floating around in star clusters and dwarf galaxies, then their gravitational interactions mix up all the stars too much, making the dwarfs puffier than they ought to be.
All of this paints a rather grim picture on the prospects of primordial black holes. Observations across the board seem to disfavor them, at least as a significant fraction of the dark matter (and if your game is to explain the dark matter, then this is no fun at all).
But there are some intriguing observations that point in the opposite direction. The biggest of which is our first ever gravitational wave detection, made in September of 2015, involved two black holes with masses 36 and 29 solar masses. These are curious numbers, as the raw fact that we were able to make this detection suggests that these kinds of mergers are on the more common side of things, but it’s hard to get black holes formed from stars to merge often enough to get these kind of masses. Potential solution: maybe these are primordial black holes that we had the pleasure of witnessing merging.
Speaking of merging, there’s also the side problem of super-gigantic black holes appearing very early in cosmic history. These black holes are so big that, once again, it’s tough to form them so quickly through just the usual star-formation-and-eventual-merger process in enough time. Potential solution: maybe these are primordial black holes that already started out pretty beefy and bulked up from there.
And if that weren’t weird enough, there’s this hypothetical effect called memory burden. This would allow black holes to stop shrinking past a certain limit, which means even really small ones might still survive to the present day. It deserves its own episode so go ahead and ask.
Okay, so you’re telling me there’s a chance.
Yes, there’s a slim chance that primordial black holes can explain most, if not all, of the dark matter. If all the primordial black holes were made with the same mass, then there’s a narrow window, between 10^17 and 10^23 grams (so asteroid-to-moon-to-small-planet) range that is currently allowed by all observations. But these small black holes have trouble explaining the appearance of supermassive black holes in the early universe and the gravitational wave results. But that’s not a deal-breaker.
If primordial black holes can come in a variety of sizes, then a lot of options open up. It’s possible to tune their formation so they have the exact right distribution of sizes to escape all existing constraints while still explaining the dark matter. This is admittedly a little contrived and more complicated than most people would like, but like I said the universe is under no obligation to be simple.
So that’s where we are. Most cosmologists don’t consider primordial black holes to be a viable dark matter candidate, because of all the observations stacked against them. But they’re not entirely ruled out. And, come on, it’s really fun to think of exotic physics in the early big bang creating black holes out of thin air that then go on to dominate every galaxy.
So for that reason and that reason alone, they’re staying on my “maybe” list.
