How Many Gravitational Waves have we seen?

Last week I got an email from a colleague, asking a question which, on the face of it, should have been easy to answer.

How many gravitational waves have we detected?

However, the fact that they needed to ask helps to reveal that there is not, in fact, a straightforward answer.

This is hopefully a complete, and fairly concise, set of potential answers to the question, along with caveats as appropriate.

Going Beyond GWTC-3

Twinkle, twinkle, big explosion

Today marks a major moment in the development of a project I’ve been working on for some time: me and my co-authors have completed a paper on inferring the opening angles of gamma ray bursts by observing binary neutron star mergers and gamma ray bursts. What does that mean? Well, I guess the point of this post is to explain just that. It should be said, while you can download the paper now, it’s still a pre-print: that means it hasn’t been peer-reviewed yet, so there’s a chance it may contain some mistakes which we’ve not picked up on. So I guess you might argue it’s probably not quite completed.

Fire and gold in the belly of the beast

This is the second time that I’ve written a blog post under an embargo; the first time was a few months into my PhD, and it was about something which had happened just about the time I started. As a result, I’d not really had much to do with it, and I’d spent most of the time that other people were analysing data and writing papers finding my way into the collaboration. That wasn’t quite so true this time, as I found myself invovled with the public outreach effort for LIGO’s next big announcement. It turns out that it’s hard to condense a groundbreaking discovery, which took over 3000 people to make, into one A4 sheet of paper. I also learned all sorts of new things I never expected to during my PhD, like the niceties of colour theory. However, on with the story.

Einstein Was Right!

It’s been almost a day now since the world found out about GW150914, the 0.2 seconds of tiny distortions in spacetime which have kept around a thousand scientists entertained for half a year, and which seems to be close to breaking the internet today. As the hype starts to settle down, I think it’s time to reflect a bit on what happened yesterday. You can tell that today’s not a day when many people in the department are getting very much work done.

Finding a way to Space

Today’s another big day in the world of gravitational waves. ESA are expected to announce the results of their LISA Pathfinder mission in just over an hour. I’m going to attempt to write this blog post in at least two parts: one part (now) before the announcement, and one part during.

So why is this all important? Well, LISA Pathfinder is a technology demonstration mission. That means that it wasn’t capable of detecting gravitational waves. Instead it was assembled and launched to prove that it is possible to perform measurements at very high sensitivities in space: exactly what is needed to detect gravitational waves.

Making injections with Bilby

The last couple of days I’ve spent a fair amount of time trying organising a number of diversity-related things for our upcoming collaboration meeting, and doing more admin than I can pretend to have enjoyed, so today’s note’s going to be a fairly short one, covering some work I did while helping a student using Bilby, the LSC’s new inference library (named, pleasingly, after an animal).

This post is unashamedly technical. Sorry.

Riding the Wave

I don’t think I’ve ever written a blog post under an embargo before, but it’s a crisp, sunny Glasgow afternoon, and in 24 hours’ time I’ll be heading across to the main building of the university, for there is to be a press conference. But you probably already know that, since I’ll be publishing this post after it.

Officially, I started my PhD on 1 October 2015, but being keen, and excited to start into three-and-a-half years of intensive research, I’d started reading a few weeks before hand. On 11 September I applied for membership of the LSC, the scientific collaboration which runs the LIGO detectors. By around 9am on Monday 14 September 2015 it had been approved.

Was Einstein Right?

It’s about time I started talking a bit about the science of this week’s announcement. One of the more exciting aspects of detecting gravitational waves was that they would confirm the last major prediction of the theory of General Relativity—Einstein’s revolutionising theory of how gravity works.

Well, they did that. We saw gravitational waves, and they seem to travel at the speed of light, so so-far it’s looking pretty positive for Mr Einstein. But what if General Relativity (let’s call it GR from now on, I’ll misspell relativity if I need to type it too frequently) isn’t the whole story? What if some other theory makes almost all the same predictions as GR, but some more ones which, for example, let us combine gravity with quantum mechanics. (This is a big problem at the moment in physics. Theorists are convinced that it should be possible, but they don’t know how, so they’re very keen to find weaknesses in the current theories of each which can be used to find a way to join them up).

What did we learn in O3? (Part 1)

I started this post with the idea of summarising all of the new science which has resulted from observations made up to the end of the third observing run of the advanced LIGO and Virgo detectors. It turns out there’s rather a lot, so this looks like it’ll be the first of several! I’ve tried to include links to as many papers as possible.

Catalogues

The foundational analyses for the third observing run were reported in two catalogues, GWTC-2 (later updated by GWTC-2.1) and then GWTC-3. These catalogue papers summarise the analysis of ninety-one signals from the first three observing runs, with the bulk of new events coming from the third observing run. Ninety of these are regarded a confident gravitational wave detections, and one is a marginal candidate, lying on the threshold of what the collaborations deem a confident detection. These catalogues provide details of the properties of the astrophysical systems which created the detected gravitational waves, but don’t generally discuss what can be gleaned from this new knowledge.