Astrophysicists Unveil Glut of Gravitational-wave Detections

The latest catalog of cosmic collisions has been released by Gravitational Wave observatories. This brings their total to 90. This new batch of 35 events includes the discovery of the lightest neutron star, and two collisions that involve large black holes.
These detections were made at the Laser Interferometer Gravitational-Wave Observatory sites (LIGO) in Louisiana and Washington State. These detections were made over 21 weeks, with an average detection rate at one event per 4.2 days. The collaboration has grown to include the KAGRA detector from Japan, which began making observations in February 2020. In a paper published on arXiv's preprint repository, the LIGO-Virgo–KAGRA collaboration presents its results.

Gravitational waves, which are ripples in space-time caused by large masses accelerating, are ripples in the fabric. LIGO-Virgo reported earlier detections. The latest ones can be attributed to dense stellar remnants merging into one another, similar to the LIGO-Virgo detections. Most of them, including the historic LIGO detection of 2015, involved pairs of black hole, although in a few cases, one or both were neutron stars.

Initial data released by the collaboration focused on high-confidence detections. However, the latest catalogue, which was released in October 2020, includes detections with better than even chances of being gravitational waves. According to the team, around 10-15% of all the new candidates in the catalog are false alarms. These were "caused" by instrumental noise fluctuations.

Monster mash

Researchers can determine the details of many features of the objects involved by analyzing the frequency and shape of the waves created by mergers. This includes their mass and distance from Earth. The distance of the 35 most recent events ranged from approximately 245 million to over 2.2 billion parsecs (800m to more than 7 miliarde light years) apart.

They also include some monsters. Two events involved black holes whose masses were more than 60 times those of the Sun. These black holes are problematic for astrophysicists because they exist. Black holes typically form when a massive star collapses at the end its life. However, prevailing theories suggest that dying stars will explode and not collapse. This should result in a lack of black holes within the range of 65-120 solar mass.

The merger that LIGO/Virgo picked up 21 May 2019 was revealed in last year’s catalogue. It involved a black hole with 85 solar masses. It seems less likely that the first one was a fluke, now that the team has identified two additional outlier events.

Alessandra Buonanno is a LIGO physicist at Max Planck Institute for Gravitational Physics, Potsdam. She suggests that these black holes could have formed as a result a prior merger rather than the collapse a single star. She says, "What we're seeing could be a second generation binary."

On 19 December 2019, the observatories saw another interesting event. It involved a black hole with a mass 30 times greater than the Sun, swallowing a small neutron star. It is just 1.17 solar masses and one of the smallest known neutron stars. Buonanno warns however that it was one of the lower confidence detections and could be background noise, rather than a real event.

Recognition of patterns

Daniel Holz, a LIGO scientist at the University of Chicago, Illinois, said that researchers are now able to discern patterns in the abundance of black-hole mergings. Most striking is the fact that black-hole merging tends to occur more frequently in galaxies farther from us in time and space.

It is clear that black-hole mergings are becoming less common as the Universe matures. Holz states that this is in line with theoretical expectations. "There were more stars created in the Universe earlier, so it is normal to expect there would be more black hole creations and thus more black-hole mergings."

LIGO and Virgo were shut down on the 27th of March 2020, earlier than expected due to the COVID-19 pandemic. They have also been undergoing major upgrades. They will reopen in the late 2022 to allow for more observations. This, along with KAGRA, could potentially double the bounty. Once they have hundreds of events to compare they expect to be able see trends that can point out the origins and the history of binary systems involved in the mergers as well as the history the Universe. Holz says, "When you get to the end of it, it's going be amazing."

Astronomers all over the globe are still hopeful of a repeat of August 2017's merger of two neutron star nuclei. This is the only known gravitational-wave event that conventional observatories have seen.

This article was published with permission on November 9, 2021.