I recently had a new paper accepted for publication in Physical Review D. I’m also trying to write summaries of my papers, and papers which I’ve been closely involved with, here on my blog.

This paper, “A Precessing Numerical Relativity Waveform Surrogate Model for Binary Black Holes: A Gaussian Process Regression Approach” is the published version of the final chapter of my PhD thesis (which is something else I’ve yet to write a blog post summarising). The work which is reported in the paper was carried-out over the last four years, and along the way I learned a lot about how the unnderlying statistical processes work, mostly through making lots of mistakes, and iterating. Hopefully my next paper will take a little less long to reach publication!

# Colliding black holes

Gravitational waves, like the ones which were discovered way back in September 2015 (in fact, on the day that I started my PhD!) can be created by two black holes colliding with each other, and then merging into one larger black hole, while losing energy in the form of gravitational waves.

The amount of energy which is released during the final stages of the coalescence of the two objects is enormous: in fact, they’re the most energetic explosions we’ve ever observed, and can produce more power than the entire observable universe for the short duration of the merger. However, the events which we’ve observed have also been extremely distant, and so the strength of the signal is tiny by the time the gravitational waves from the event reach the Earth. In order to detect these weak signals we use a data analysis technique which compares the signal measured by detectors like LIGO to the signals we expect from our knowledge of the physics of black holes.

The signal which is produced by two black holes colliding follows a common pattern, which is divided into three phases.

- The inspiral
- The two black holes start their collision course by orbitting each other.
- Over time the orbit decays, and the black holes get closer to each other over time.
- Once the black holes are close enough that the gravitational waves are strong enough to be detected by the detectors which exist at the moment they’re only a few orbits away from colliding.
- The merger
- After a final few orbits the two black holes touch, and combine.
- Understanding the physics of this part of the process is difficult, and requires long and expensive computer simulations to determine.
- The ringdown
- After the two black holes have combined into a single larger black hole there’s still lots of energy which needs to be radiated.
- The black hole rings like a bell, producing a decaying pattern of gravitational wave energy.

The theory which allows us to understand