with Tongyan Lin, Ben Safdi, and Chih-Liang Wu
We used Gaia’s first data release to look for a dark matter disk, with the basic idea being that if there is a dark matter disk then stars will be attracted to its gravitational tug. Thus a dark matter disk would leave a distinct imprint on the positions and velocities of the stars being monitored by Gaia. We did a super fancy analysis that you can read all about here if you’re a nerd, but I promise we were really really careful. 😉 In all seriousness though, we did try to be very careful and take all sorts of confounding factors into account. This was my first time really analyzing data in a “professional” context, and to do this we ended up using almost a *CPU century* of computing time to search over all the possibilities that could be affecting our analysis. And guess what we found? Zippo. At the levels currently allowed by our analysis, there cannot be a dark matter disk big enough to cause the extinction of the dinosaurs or really appreciably affect the baryons (this benchmark is shown as a star in the plot, and it sits comfortably in the "ruled out" area.) Is this kind of a bummer? Maybe. We failed to discover dark matter. But to quote Edison, "I have not failed. I have succeeded in finding ways that will not work. When I have eliminated the ways that will not work, I will find the way that will work." In this spirit, we have nudged a bit further in the quest to understand dark matter by constraining its properties.
Most of the stuff in the universe bears little resemblance to the stuff we are made out of. The stuff we are made of (baryons) is what makes up stars, planets, and ice cream. All the awesome pictures of space, all the achievements of mankind, all the pictures of cats on the internet: baryons, baryons, baryons. Clearly baryons do excellent and interesting things with themselves, ranging in complexity from an interstellar cloud of hydrogen gas to the collective works of James Joyce. But what I’ve been interested in recently is what’s going on with the dark matter, which is what makes up around 80% of the mass of the universe.
We’re pretty sure dark matter exists and that it’s not just that we don’t understand how gravity works. We see independent evidence for dark matter in a range of environments, from relatively tiny dwarf galaxies to the largest cosmological scales, and saying “gravity doesn’t work” just does not explain the range of observations. Based on how the dark matter is distributed (which we can only determine through its gravitational interactions, since we cannot see the dark matter because it is dark), we can infer something about its properties. Occam’s razor, which says that the simplest explanation for something is the most likely one, would tell you that the dark matter doesn’t have any complicated interactions— in this case, the dark matter would be chilling out in space in clouds feeling only its own gravity. However, we know that the world doesn’t always obey Occam’s razor. For instance, ice cream isn’t simple but rather is made of a complicated mix of molecules which are fundamentally made of different combinations of electrons, quarks, and gluons. In principle, the dark matter could also be complicated in which case it could clump in various ways out in space. This is an idea that has gained more and more traction since we’ve been searching for dark matter for a long time and nothing has turned up yet. I wouldn’t go so far as to speculate that there is dark matter ice cream, but there could be dark matter versions of galaxies.
When you picture a galaxy, you probably think of something that’s a disk-shaped conglomeration of stars. The disk-y shape happens because galaxies start out hot and hot gas is hard to compress, but the baryons can cool down (like a cup of tea) which makes it possible for them to be compressed into a disk. The reason why it doesn’t get compressed further is because of conservation of angular momentum which stabilizes the disk and allows for the evolution of life (and hence the invention of ice cream.) In the inert-dark-matter-cloud model, every galaxy lives inside a huge dark matter halo (a blob of dark matter.) But in a more complicated dark matter world, there could also be a dark matter disk. This could happen in many descriptions of dark matter— the only thing that is required is some way for the dark matter to cool down. For example, dark matter could cool down if there is a dark matter version of radiation.
If there is a dark matter disk in our Milky Way, it would leave its signature on the baryons, since after all the two worlds interact via gravity at the very minimum (otherwise we would have never discovered dark matter!) One of the wildest ideas has been that a disk of dark matter could have caused the extinction of the dinosaurs because its gravitational pull would periodically disrupt the orbits of comets and send them crashing to earth. The presence of a dark matter disk can also change how we search for it in the sky and in experiments on Earth. It would be amazing if a dark matter disk were discovered because it would mean there is a “hidden dark matter sector,” which could have untold implications for physics and (much more speculatively) maybe even applications for human use.
This is where we came into the picture, wielding the shiniest new dataset from the Gaia spacecraft. Gaia is equipped with a lot of amazing technology which allows for extremely good resolution of the positions and speeds of stars. When I say “extremely good resolution,” picture having the ability to view a quarter on the surface of the moon from your living room (minus the presence of Earth’s turbulent atmosphere which spoils the view... but getting rid of that atmosphere spoils the ability to breathe, making your living room much less comfy… that’s why Gaia had to go to space!) Gaia is using its extremely good resolution to map out the positions and speeds of one billion stars, making it the most complete map of the local Milky Way.