Astronomers observed 39 cosmic events that release gravitational waves over a six-month period in 2019 – a rate of more than once per week. The bounty, described in a series of articles published October 28, demonstrates how observatories detected these ripples – often generated by the fusion of two black holes – has increased significantly counting their sensitivity since the first determination was made in 2015. The growing data set is helping astronomers map the frequency of such events that have occurred in the history of the Universe.
Gravitational waves are ripples in space-time texture that are liberated by accelerating masses, especially when two massive objects twisted together and fused. Their detailed properties provide many tests of Albert Einstein’s general theory of relativity, including some of the strongest evidence to date for the existence of black holes. And through gravitational waves, astronomers have come up with a new way of observing the universe, alongside electromagnetic waves and cosmic rays.
The latest release describes the events observed in the third half of the Laser Interferometer Gravitational Wave Observatory (LIGO) observation – a pair of twin detectors based in Hanford, Washington and Livingston, Louisiana – and its European counterpart Virgo, near Pisa, Italy. This is the collaboration’s second event catalog, following the one published in December 2018 depicting their first 11 findings. In total, the network of observations currently observed 50 gravitational wave events (see ‘Cosmic Collisions’).
Most events are fusion of two black holes. The detectors have also detected some collisions between two neutron stars and at least one merger of a neutron star and a black hole. Mergers involving neutron stars are particularly interesting for astrophysicists as they are expected to release both ordinary light as well as gravitational waves, which was confirmed in one incident. mergers of neutron stars are seen in August 2017. Some of the most spectacular events in the catalog are already there. have been described in the papers. These include the largest-ever black hole merger and the most ‘skew’ – in which two very massive black holes collided.
One surprising finding is the volume of black holes involved in mergers. Astrophysicists expect a clear cut, no black hole 45 times heavier than the Sun. “Now we see that it’s not too sharp,” said Maya Fishbach, a LIGO researcher at Northwestern University in Evanston, Illinois. The catalog includes three events of greater mass, including one announced in September with a black hole with mass 85 solar masses.
The abundance of data now allows the LIGO-Virgo researchers to approximate the rate at which black hole mergers occur in an average galaxy. That ratio appears to have peaked about eight billion years ago, after the period when stars formed – and some later turned black holes – at an exceptionally high rate, Fishbach said.
The catalog also provides information on how black holes rotate, holding the key to understanding how objects orbit each other before they merge. It shows that, in some binary systems, two black holes have a skewed axis of rotation, which means they form separately. But many other binary codes appear to have an almost aligned spindle, which is what astrophysicists would expect when two black holes begin their lives as a binary star system. Fishbach says that the two schools of thought in astrophysics support one of the two scenarios, but for now they both seem to be correct.