Isobel Romero-Shaw

This poster is about gravitational-wave Astrophysics. Gravitational waves are ripples in space-time cause by the acceleration of mass. So far, all detected gravitational waves have been the result of binary compact stellar remnants -- black holes and neutron stars -- spiralling around each other and smashing into each other, somewhere in distant space. We have detected gravitational waves using giant instruments called LIGO and Virgo, each a kind of antenna with kilometres-long arms, which time the passage of light down each arm and detect when the light takes a tiny fraction of a second more to travel down one arm than it does the other. This happens when a gravitational wave has passed through the Earth, minutely stretching and squeezing distances by fractions that are imperceptible to the human eye, but detectable to these ultra-sensitive instruments.

In the top left we have some key bullet points outlining that different binary black hole parameters can be inferred from the gravitational wave signals they emit. In this case, we are interested in the eccentricity: the squashed-ness of the squashed circle that the black holes trace around each other as they orbit.

Eccentricity measures the amount of squashed-ness, and can be between 0 and 1. We care about eccentricity because it can tell us whether the binary evolved in isolation, or whether the two black holes encountered each other later through dynamical interactions in dense regions of space.
When binaries evolve in isolation (so stick together for their whole lives as stars as well as their post-stellar-death time as black holes) they are expected to lose eccentricity faster than they fall towards each other, since circular orbits are more energy-efficient. In contrast, we expect roughly 1 in 20 dynamically-formed binary black holes to have eccentricity greater than 0.1 when measured at a gravitational-wave frequency of 10 Hz.
There is an illustration comparing a signal with signatures of eccentricity to one without, and pointing out that the eccentric signal is harder and therefore slower for the computer to simulate.

In the top right, our method for measuring eccentricity is described: we do "normal" Bayesian inference by comparing our data to "normal" non-eccentric signals, which allows us to converge to roughly the correct values for other binary parameters (e.g., mass, distance), before doing a "reweighing" process using our eccentric model to shift our values to the correct values for an eccentric binary. This way we don't have to generate so many eccentric signals, so it's faster. Further technical details about the reweighting procedure are given in the paper accompanying this poster: https://arxiv.org/abs/1909.05466

In the bottom left we show the results from our analysis of the ten binary black hole mergers detected by LIGO and Virgo in their first two observing runs. All have eccentricities less than 0.06, measured at a gravitational-wave frequency of 10 Hz. However, this is not the end of the road for the theory of dynamical formation: since we need 1 in 20, and these results are only for ten binary black holes, further analysis might yield more informative results! If we detect one eccentric binary in LIGO and Virgo's third observing run, it's likely that all of these binaries are formed dynamically. If not, it's likely that they formed in isolation. LIGO and Virgo have completed their third observing run, and have recorded about 40 public alerts of candidate gravitational-wave signals. We needed 20 detections, and now we have something like 50. So watch this space for new measurements, coming very soon!