Gravitational waves reveal ‘stellar graveyard’ packed with neutron star and black hole mergers

Using gravitational waves, tiny ripples in space-time first predicted by Albert Einstein back in 1915, astronomers have discovered that a “stellar graveyard” is packed with mergers between extreme stellar remnants like black holes and neutron stars, created when massive stars die in supernova explosions.

Evidence of these mergers also came in the form of the most massive binary black holes “heard” in this domain of gravitational waves to date.

The newly analyzed data — collected by the gravitational wave detectors LIGO (Laser Interferometer Gravitational-Wave Observatory), Virgo and KAGRA (Kamioka Gravitational Wave Detector) — doubles the number of known “mixed mergers” between black holes and neutron stars, from 1 to 2. In total, 128 new mergers of various types were “heard” during the fourth operating run of LIGO, Virgo, and KAGRA between May 2023 and January 2024, the first nine months of its 18-month 4th operating run (O4).

“This new update really highlights the capabilities of both the international network of gravitational-wave detectors and the analysis techniques which have been developed to dig very faint signals out of the data,” team leader Daniel Williams, a researcher at the Institute for Gravitational Research (IGR) at the University of Glasgow in Scotland, said in a statement.

“What we’ve observed in the first part of the two-year-long fourth observing run has broadened our understanding of the cosmic graveyard: we’ve seen the heaviest black holes yet,” Williams added.

The new research could help scientists better understand the stellar cycle of life and death that births black holes and neutron stars, and could also shed light on the process that sees black holes increase in size by colliding and merging.

“In a similar way to how a paleontologist can learn about long-extinct dinosaurs by looking at their fossilized bones, we can learn about stars by looking at their black hole or neutron star remains,” said team member Christopher Berry, also of the IGR.

“The biggest stars live the shortest lives, so they can be hard to study in other ways. Stars live their lives in many different environments. Some form in dense stellar environments like nuclear star clusters, where millions of stars are in close proximity,” Berry added. “Here, we might expect that following a binary black hole merger, the remnant black hole could find a new partner and merge again, forming an even bigger black hole.”

Berry said that, with GWTC-4.0 (Version 4.0 of the Gravitational-Wave Transient Catalog), LIGO-Virgo-KAGRA scientists have seen telltale hints that some of the sources could come from black holes that are themselves the result of previous mergers.

“Teasing out the black holes formed from collapsing stars from those formed from previous mergers will tell us about how stars live their lives, and where they live their lives across the universe,” Berry continued.

Not only could this research paint a more complete picture of the life and death of stars that are at least eight times as massive as the sun, but it could also help better understand the speed at which the universe is expanding.

“The universe is expanding, and the speed at which it is doing so is known as the Hubble Constant. A unique feature of black hole mergers is that we can tell how far away they were directly from our observation,” said team member and IGR researcher Rachel Gray. “This means that each merger we detect gives some information about the universe’s expansion rate.

“By combining this information from many mergers, we can improve our measurement of the Hubble Constant, helping to answer one of the big unanswered questions of modern astronomy: exactly how fast is the universe expanding?”

The new data set contains a gravitational signal called GW230814, which is the loudest detected by these instruments to date. Detections like these are also the perfect way to test Einstein’s 1915 theory of gravity, general relativity, in which gravitational waves were first postulated.

“The louder the signal, the more precise our measurements of any potential deviations,” IGR scientist and team member John Veitch said. “So far, Einstein has passed every test, but we will keep looking closer! For these types of analysis, it is very important to have observations from multiple gravitational-wave detectors, so you can cross-reference the signal in both.”

All this has been made possible by upgrades to LIGO, Virgo, and KAGRA performed from 2020 onward that have boosted the sensitivity of these gravitational wave detectors, based in the U.S., Italy and Japan, respectively.

“During the fourth observing run, the detectors have routinely been able to make measurements more than 25% more sensitive than in the previous observing run,” IGR researcher Andrew Spencer said. “This allows us to observe a much larger fraction of the universe.”

One thing that is absent here are the flashes of light that should have accompanied the two observed mixed mergers between black holes and neutron stars, represented by the gravitational wave signals GW230529 and GW230518.

“This time around, we didn’t see anything except for gravitational waves from these mergers, but exciting new telescopes such as the Vera Rubin Telescope mean that making coincident detections of gravitational waves and light is becoming much more likely,” Williams concluded.

The team’s research is available as a preprint on the paper repository site arXiv.