It’s an exciting time to be a cosmologist: there’s a new thing to measure. For physicists (like yours truly) it’s a bit like a new toy; we just can’t wait to find out about all the things it can do! In a paper released this week, my collaborators and I studied what we can learn about Dark Matter using gravitational waves from a neutron star merger. Let me tell you what we discovered in this post.
What do we know about gravitational waves?
Tiny gravitational waves arise every time anything with mass or energy accelerates in an orbit through space, but they only become observable (with very large, high precision instruments called “laser interferometers”) when the objects are very massive and the orbit is relatively short. Since last year, we have been able to measure gravitational waves from a pair of black holes spiralling into each, other and eventually merging. This August, the LIGO/VIRGO experiments detected a pair of neutron stars undergoing the same fate.
What do we know about Dark Matter?
- It interacts via gravity. Really, that is just another way of saying it is there, as Dark Matter was “discovered” from the unexplained gravitational attraction after all the luminous matter was accounted for;
- It doesn’t seem to interact through any other force we know of, or we would have found it.
That’s it! Among the things we do not know, for example, are how many types of Dark Matter particles there are, how massive they are, and, of particular interest here, if there are any “dark forces”.
Can gravitational waves teach us about Dark Matter?
Maybe! In our paper this week, we showed how the gravitational wave would change if there was an extra repulsive force between the neutron stars, because a small fraction of the stars was made out of Dark Matter. This repulsive force would compete with gravity at the end of the merger, slowing down the process. The beginning of the merger would also change, as the neutron stars would emit extra energy in the form of Dark Radiation.
If such changes would indeed be measured in future gravitational wave observations, we could start to measure the strength of the dark force, and even the masses of some of the Dark Matter particles! Stay tuned!
Image credit: R. Hurt/Caltech-JPL