New Black-Hole-Merger Detections Hint at Exotic Astrophysical Scenarios

 

Black Hole Merger

 

An exciting new era in astronomy began billions of years ago with a collision between two black holes that sent gravitational waves rippling through the universe. In 2019, signals from these waves were detected at the gravitational wave observatory LIGO and the detector Virgo. Fast forward to 2022, Princeton researchers announced the discovery of 10 new black hole mergers hiding in the data from LIGO and Virgo  gravitational wave detectors

Princeton University has announced that an international group of astrophysicists re-examined the LIGO-VIRGO data and found 10 additional black hole mergers, all outside the detection threshold of the original analysis. These newly discovered  mergers hint at exotic astrophysical scenarios that, for now, are only possible to study using gravitational waves. For example, a black hole in one of the newly identified mergers was spinning backwards with respect to its orbit, suggesting the binary black hole was dynamically captured within a dense stellar environment such as a globular cluster or nuclear star cluster near the center of a galaxy. 

Binary Black Hole Mergers

In the last seven years, scientists at the LIGO-Virgo collaboration have detected at least 90 gravitational wave signals –perturbations in the fabric of spacetime that race outwards from cataclysmic events like the merger of binary black holes. In observations from the first half of the most recent experimental run, which continued for six months in 2019, the collaboration reported receiving signals from 44 binary black hole mergers.

Buried in the Data.

 

 

Expanding the search, an international group of astrophysicists re-examined the data and found 10 additional black hole mergers, all outside the detection threshold of the original analysis. 

Discovery of a System Never Seen Before

“The new detections include GW190910_012619: the first system to have confidently measured negative effective spin when parameters are inferred under the prior distribution describing the black hole spins in the source channel that produced such systems,” astrophysicist, Seth Olsen wrote in an email to The Daily Galaxy. Olsen is a Ph.D. candidate in the Princeton University Physics Department. He studies gravitational wave data analysis with Professor Matias Zaldarriaga at the Institute for Advanced Study.

“A negative effective spin is to have one or both of the black holes (and if the mass ratio is very asymmetric, it must be the heavier one) spinning about its own axis in the opposite direction of the orbital rotation,” Olsen explained in his email. “This is a type of system that we have not seen before, although we may have seen others from the same “dynamical capture channel” that produces negative effective spin systems, and it is the type which is most difficult to produce from any other formation channel. Therefore it is the event we are most confident comes from this interesting subpopulation.”

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“We also help fill in the so-called upper mass gap and lower mass gap in the black hole mass spectrum by adding events to these regions with few previous detections,” Olsen notes in his email. “The upper mass gap is a range of black hole masses to which a star cannot collapse according to nuclear physics models, meaning black holes in this mass range may be remnants of previous mergers. The lower mass gap is the mass range of the lightest stellar black holes, which are harder to detect but have been found in both gravitational wave and electromagnetic observations over the past few years.”

“Finally,” Olsen ends his email, “ we find a number of very distant events (redshift ~1), which can tell us more about the cosmological history of the binary black hole population.”

 

“With gravitational waves, we’re now starting to observe the wide variety of black holes that have merged over the last few billion years,” said Olsen. “Every observation contributes to our understanding of how black holes form and evolve, he said, and the key to recognizing them is to find efficient ways to separate the signals from the noise.”

Notably, the observations included phenomena from both high- and low-mass black holes, filling in predicted gaps in the black hole mass spectrum where few sources have been detected. Most nuclear physics models suggest that stars can’t collapse to black holes with masses between about 50 and 150 times the mass of the sun. 

“When we find a black hole in this mass range, it tells us there’s more to the story of how the system formed,” Olsen said, “since there is a good chance that an upper mass gap black hole is the product of a previous merger.”

Nuclear physics models also suggest that stars with less than twice the mass of the sun become neutron stars rather than black holes, but almost all observed black holes have been more than five times the mass of the sun. Stellar evolution models predict that a progenitor with less than about 25 times the mass of the sun will explode as a supernova and leave behind a 2 solar-mass neutron star remnant whereas a more massive progenitor collapses directly into a black hole of at least 5 solar masses. Observations of low-mass mergers can help bridge the gap between neutron stars and the lightest-known black holes.

For both the upper and lower mass gaps, a small number of black holes had already been detected, but the new findings show that these types of systems are more common than we thought, Olsen said.

Separating True Signals from Background Noise

Identifying events like black hole mergers requires a strategy that can distinguish meaningful signals from background noise in observational data. It’s not unlike smartphone apps that can analyze music — even if it’s played in a noisy public place — and identify the song that’s being played. Just as such an app compares the music to a database of templates, or the frequency signals of known songs, a program for finding gravitational waves compares the observational data to a catalog of known events, like black hole mergers.

To find the 10 additional events, Olsen and his collaborators analyzed the data from LIGO and Virgo using the “IAS pipeline,” a method first developed at the Institute for Advanced Study and spearheaded by Matias Zaldarriaga, an IAS astrophysicist who is also a visiting lecturer with the rank of professor at Princeton University.

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Institute-for-Advanced-Study Data Pipeline

The IAS pipeline differs in two important ways from the approach used by the LIGO and Virgo teams. First, it incorporates advanced data analysis and numerical techniques to improve the signal processing and computational efficiency. Second, it uses a statistical methodology that sacrifices some sensitivity to the most common sources in order to gain sensitivity to the sources that the traditional approaches are most likely to miss, such as rapidly spinning black holes.

 

 

Previously, Zaldarriaga and his team have used the IAS pipeline to analyze data from earlier runs of the LIGO-Virgo collaboration, and similarly identified black hole mergers that were missed in the first-run analysis. It’s not computationally feasible to simulate the entire universe, Olsen says, or even the staggeringly wide range of ways in which black holes might form. But tools like the IAS pipeline, he said, “can lay the foundation for even more accurate models in the future.”

Other collaborators on the analysis include Tejaswi Venumadhav at the University of California-Santa Barbara and the Tata Institute of Fundamental Research, Jonathan Mushkin and Barak Zackay at Weizmann Institute of Science, and Javier Roulet at the University of California-Santa Barbara.

Image credit: Merging black holes, Shutterstock License

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Seth Olsen  and Princeton University

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