with Adrian Liu
Recently, LIGO's discovery of gravitational waves has captured the imaginations of science enthusiasts of all ages and backgrounds. And it's extremely well-deserved attention! LIGO is a very, very hard experiment to do properly and detecting gravitational waves has been a physics Holy Grail for decades. One particularly surprising thing about LIGO's discovery was that the gravitational waves were so strong that they had to have come from two black holes -- each of them 30 times heavier than the sun -- merging and producing more energy in gravitational waves than is emitted by all the stars in the universe. Think about that for a moment: each star is basically a massive nuclear fusion reactor, and the LIGO event was more energetic than all the stars in the universe combined. That is pretty wild, and hardly anybody in the field was expecting this. Most models predicted that the kinds of events LIGO would see would be from much less massive objects merging.
The excitement that LIGO's discovery generated has led some folks in my field to wonder whether they could hitch a dark matter interpretation onto this surprising discovery. As you might know, dark matter has been a similarly mysterious thing that people have been trying to observe for decades. What if you could kill two birds with one stone?
It might sound crazy, but when you think about it, black holes have a lot of the observed properties of dark matter. They are dark (hence the name "black" hole) and heavy. The black holes would have to be made earlier than 1 second after the Big Bang in order to be consistent with nucleosynthesis. Hence, if dark matter is made of black holes, they have to be primordial black holes. Primordial black holes are actually predicted to form in some models of inflation, which is the "bang" part of the Big Bang. So primordial black holes are theoretically cool because they can give us information about dark matter *and* the beginning of the universe.
Being that these primordial black holes are so interesting, there has been a lot of interest to constrain them in the past, and lots of folks have come up with clever ways to do this. Recently, it has been argued that some of the existing constraints could be weaker than previously thought due to errors in the complicated astrophysical modeling that's required in order to make these constraints. So if you assume the constraints are weaker, it's possible that the LIGO gravitational wave signal came from two merging primordial black holes.
In this project, we showed that there is another observational handle on this question involving my favorite astrophysical objects. You guessed it: pulsars! I've previously worked on projects related to pulsars and written about them here. Basically, they are magically stable clocks in outer space, which we can use to detect any weird things going on due to general relativity. As general relativity taught us (or was it Dr. Who?), massive objects like black holes can make things all wibbly-wobbly timey-wimey. So if you have a really accurate clock, you can look for primordial black holes as they move around our galaxy!
In order to check whether pulsars could constrain primordial black hole dark matter, we ran a bunch of simulations. Basically, we simulated a bunch of Milky Way galaxies with primordial black holes scattered about, mimicking the dark matter distribution, and we checked what kind of effect these black holes would have on pulsar timing. And it turned out that pulsar timing does give a competitive constraint for masses relevant to the LIGO discovery! In this plot, we show the constraint you could have on the fraction of dark matter made of black holes (the y-axis) as a function of the primordial black hole mass in units of solar masses (the x-axis). For one simulation we used known pulsars and for another we used pulsars that could be discovered using the proposed Square Kilometre Array (SKA) and it turns out that you can get down to the level where black holes can't be more than 1-10% of the dark matter! Also shown on this plot are other methods of constraint, which give complementary results. We didn't use actual data, just a simulation, so stay tuned! Maybe we can use pulsars to learn more about what the universe is made of and we can make progress on one of the biggest longstanding mysteries in physics.