A fourth detection of gravitational waves

On 27 September 2017, astronomers announced the fourth detection of gravitational waves. This was the first time the Italian-based VIRGO observatory has detected gravitational waves; the same waves, of course, were picked up by the two LIGO observatories.

The VIRGO observatory in Italy consists of two 3km-long arms, which together form an interferometer. (Credit: VIRGO Collaboration)

We are now entering the era of gravitational wave astronomy. When the number of observatories detecting a gravitational wave signal increases from two to three, scientists can glean much more information about the source. For example, they can gain information about the polarization of the wave. (It looks as if the general relativistic prediction about the number of polarizations is correct; Einstein wins again.) And, as the image below demonstrates, they can localise the source much more accurately. (Astronomers pointed 25 optical telescopes in the direction suggested by the LIGO/VIRGO discovery, but saw nothing – as was to be expected if the source of the gravitational waves was the collision of a pair of inspiralling black holes.)

When VIRGO works in tandem with the two LIGO observatories, astronomers can localise a source with much more accuracy. (Credit: VIRGO Collaboration)

So what was the source of this gravitational wave event? It seems to have been the merger of a pair of black holes, with mass 31 and 25 times that of the Sun, about 1.8 billion light years away in the constellation of Eridanus. The merger created a single black hole with a mass 53 times that of the Sun; thus three solar masses was radiated away in gravitational waves. That’s a huge amount of energy. LIGO and VIRGO caught a tiny fraction of it.

We now have data from four events. That’s too few to start drawing conclusions, but these four events are interesting. They all come from the merger of two quite large-mass black holes, as the table below shows:

Event Mass of BH1 (solar masses) Mass of BH2 (solar masses)
GW 150914 36 29
GW 151226 14 8
GW 170104 31 20
GW 170814 31.5 25

As I made clear in my book New Eyes on the Universe, I was sure that gravitational waves from merging black holes would soon be found. LIGO found them slightly more quickly than I anticipated, but I wasn’t surprised by the announcement. But I am surprised at the masses of the black holes that are involved: these seem to me to be on the high side. As more events are discovered in the coming years, it will be interesting to see whether these four events are typical.