Gravitational Waves Could Uncover Missing-Link Black Holes

By Ryan F. Mandelbaum on at

Scientists hope that the future of gravitational wave detection will allow them to directly observe a mysterious kind of black hole.

Gravitational wave detectors have seen direct evidence of black holes with roughly the mass of giant stars, while the Event Horizon Telescope produced an image of a supermassive black hole billions of times the mass of our Sun. But in the middle are intermediate-mass black holes, or IMBHs, which weigh between 100 and 100,000 times the mass of the Sun and have yet to be directly observed. Researchers hope that their new mathematical work will “pave the way” for future research into these black holes using gravitational wave detectors, according to the paper published today in Nature Astronomy.

A trio of gravitational wave detectors on Earth – the two LIGO detectors in the U.S. and Virgo in Italy – have begun observing the ripples in space-time itself, called gravitational waves, that result from black holes spiraling around one another and then merging. The largest black hole yet observed from such a merger was 80 times the mass of the Sun. But heavier black holes would produce a signal with a frequency too low for today’s observatories to hear. Intermediate-mass black holes might sit in the midst of collections of stars called globular clusters, or they could be the precursors to larger black holes in galactic centres. It’s hard to say.

“I’ve studied black holes for a very long time, and when I started out, they almost seemed more like a myth”

Researchers led by Vanderbilt’s Karan Jani described how a combination of upcoming gravitational wave experiments could be used to spot pairs of intermediate-mass black holes. A future space-based gravitational wave observatory called LISA would observe the black holes spiralling into one another at the lowest frequencies. The loudest piece of the detection, the merger itself, would then show up in more sensitive ground-based experiments. To boost the sensitivity of what we can observe from Earth, scientists are working on an improvement to LIGO called Voyager, as well as two new detectors called the Einstein Telescope under study in Europe and the Cosmic Explorer planned for “seismically quiet” land in the U.S.

The team compared different combinations of black hole masses and distances, which experiments would be able to see them, and what the signal would look like. For pairs of intermediate-mass black holes 1,000 to 3,000 times the mass of the Sun, LISA plus the Einstein Telescope might see such mergers originating billions of light-years away (with less distance for heavier combinations, or combinations where one black hole is substantially larger than the other). The most sensitive possible combination of instruments would be LISA plus the Einstein Telescope, though LISA plus Voyager could still lead to observations for intermediate black holes smaller than 2,000 times the mass of the Sun, according to the paper.

In other words, if intermediate black holes exist and if they merge, proposed gravitational wave experiments should be able to find them, and this paper suggests what their signals might look like. Plus, there’s already indirect evidence that intermediate-mass black holes exist, in the form of radio waves from a source called CO-0.40-0.22.

It’s exciting work. “This paper is unique because it’s the first I’ve seen to calculate IMBH waveforms in such a variety of ways. Our community has known that, in general, LISA plus Voyager can detect such binaries, but this paper has done the detailed calculations and provided the equations for doing so,” Jillian Bellovary, assistant professor at Queensborough Community College in New York who studies black holes, told Gizmodo in an email. “These calculations will help our community interpret the signals from gravitational wave detectors; the more calculations of waveforms we have, the better we can understand what the detectors are telling us.”

But this work is mainly data-driven modelling, and we still don’t know how common intermediate-mass black holes are in the universe. “It’s possible they will not happen at all during the LISA mission lifetime (four to 10 years), if IMBHs are really rare,” Bellovary said. “There’s no way to know until the detectors go online.” That will require actually funding and building these new or upgraded experiments.

But one thing is clear: physicists are only scratching the surface of observational black hole science, and there’s a whole zoo of objects yet to uncover.

“I’ve studied black holes for a very long time, and when I started out, they almost seemed more like a myth to people than reality,” Deirdre Shoemaker, study author and Georgia Tech physics professor, told Gizmodo by phone. “Now they’re so ubiquitous – it’s fun to see all of the different ways that black holes behave in the universe.”

Featured image: ESA/Hubble