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.

But one caveat first: I’m writing this as a blog post and a simple note, and the numbers here haven’t been reviewed, and certainly haven’t been endorsed by the LIGO Scientific Collaboration, or indeed anyone else in any official position. I’ve also not included any information beyond what’s public. You’ll need to wait a bit longer for that!

Published by the LIGO-Virgo-KAGRA Collaborations (O1-O3)

The normal number used in papers for the number detected thus far is the one derived from GWTC-3, the LVK’s flagship results paper which covers the first three observing runs.

That contains 83 binary black hole (BBH) signals, 6 neutron star black hole (NSBH) signals, and 2 binary neutron star (BNS) signals, for a total of 91 events. However it only claims that 90 of these are signals; one of them just misses out on the significance criteria for the catalogue (GW200105_162426, which is one of the NSBH events in O3b).

So the headline here:

The LVK has published 90 confident gravitational wave detections in the first three observing runs.

A breakdown of where these come from is in the table below.

Published by other authors

Gravitational-wave detector data is eventually made publicly available, so anyone is able to search for signals in it. And they have.

In January 2025 I put together a paper which reviews all of the events claimed in the literature.

In summary there are 61 further events claimed across the literature. I was able to reproduce results for 57 of these. There are some notes in my paper about how some of these “community” events have been received, but if they’re all taken as being real:

In total 151 events have been claimed across the literature.

This would extend to 152 events if GW200105_162426 is included.

I expect there’s a high probability that this number will increase as more machine learning algorithms are applied, and more exotic regions of parameter space are explored by search algorithms on archival data.

So, for the first three observing runs (2015-2020) there are between 90 and 152 claimed events, depending on how you wish to select events.

Published by the LVK (O4)

We’re not expecting to see final results from any part of the fourth observing runs, which are currently ongoing, until summer 2025. However, one event, GW230529, was announced in June 2024.

This was an NSBH event, so brings the total NSBH count to 6(+1, -0), the total LVK number to 91(+1,-0), and the total all-literature number to 152, or 153 including GW200105_162426.

To date (2025-01-24) 91 events have been confidently detected by the LVK and 152 have been claimed throughout the literature.

Public announcements by the LVK in O4

We can now go deeper into the realms of speculation, and look at the number of publicly announced triggers in O4 from the collaborations.

In O4a there were 81 alerts which were not ultimately retracted. One of these was for GW230529, so that gives an additional 80 events. We don’t disclose enough information to confidently sub-categorise these by event type until the publication of catalogues.

In O3b, which finishes later this week (and will be immediately followed by O3c), there have been 105 further alerts which have not been retracted. (Correct as of 2025-01-24).

The LVK have issued alerts for 186 events in O4, 185 of which are not contained elsewhere in the literature.

The LVK has announced 276 gravitational-wave signals; 91 have been published as confident detections, 185 more have only been issued as alerts.

So, In total 337 gravitational-wave signals have been claimed across the literature, though not all are published or claimed as detections.

So what’s the answer?

The “official” LVK answer is 91 events, which is also the smallest possible number. The largest, but most unreliable, is 337, since it’s made by including lots of alerts which may not end-up being claimed as detections.

Footnotes

1. GWTC-2.1 should be considered the definitive list of events from O3a, and provides an update relative to GWTC-2, having used the final calibration of the strain data and improved denoising techniques which were later used for GWTC-3. GWTC-2.1 presented 8 additional BBH events compared to GWTC-2.
2. I have included GW190814 in the list of NSBH events, but the precise nature of the lower mass object (the “grey hole”) is unclear. GW190426_152155 fits more comfortably into the range for an NSBH event, however.
3. A total of four events in O3b could be NSBH events; the fourth event is GW200105162426, which has p astro=0.36, but stands as a clear outlier from the noise distribution, so is not strictly contained within the 90 events which are generally claimed.