Around ten billion years ago in a galaxy about 17 billion lightyears away, two black holes spiraled into each other and sent ripples in spacetime—that is, gravitational waves (GWs), barrelling through the universe. On May 21, 2019, the LIGO Scientific Collaboration (LSC) and Virgo Collaboration, together referred to as (LVC), detected the now-incredibly faint echoes of this cataclysmic event and, on September 2 this year, announced to the public the first-ever direct observation of an elusive intermediate-mass black hole, named GW190521.
Filling in the gaps
Black holes, in brief, are regions of the universe where spacetime curvature is so extreme that not even light emitted at the event horizon—the boundary between the inside and outside of a black hole—can escape. Typically formed during the deaths of large stars, black holes cram an immense amount of matter into an incredibly small space. One with the same mass as the Sun (one solar mass or Msun), would barely be six kilometers in diameter.
Black holes formed directly from the collapse of a large star are called stellar black holes and are one of the three main types of black holes that astronomers study. The two other types that populate the universe are elusive intermediate-mass black holes (IMBHs), with masses greater than 100 Msun, and supermassive black holes, with masses up to millions of solar masses or more.
Dr. Jay Weinstein, a Physics Professor at the California Institute of Technology and chair of the LSC team, which wrote one of the two published papers on GW190521, tells The LaSallian, “This particular detection stands out because it is the most massive ‘stellar mass’ black hole system we’ve ever seen.” Previously, he adds, the black holes astronomers were able to observe were either below 50 Msun or, in the case of supermassive black holes, millions to billions of solar masses. He expounds that until the LVC’s detection of GW190521, a black hole of greater than 100 solar masses was hypothetical.
In the binary black hole system that merged, one black hole was 85 [+21 -14] Msun—meaning that the range of possible masses was as high as 106 Msun or as low as 71 Msun—while the other was 66 [+17 -18] Msun, yielding a remnant IMBH of 142 [+28 −16] Msun. Weinstein emphasizes the importance of including the large measurement uncertainties when reporting the masses of the black holes. “If you say, ‘The masses are 85 and 66 solar masses’, that’s a meaningless, wrong statement. You must include the measurement uncertainty for it to be a meaningful statement,” he reasons.
When asked about why the masses of the black holes do not add up, Weinstein responds that the eight solar masses’ worth of material didn’t simply vanish. “Where did it go? It was radiated away in GWs! The merger converted black hole mass to GW energy by E = mc2,” he chimes. This, he points out, might be the most energy released at a single instant that astronomers have ever observed—or at least since the Big Bang itself.
Whispers in spacetime
Despite the enormity of the energy that these GWs initially carried, much had already been dissipated once they came ashore. Weinstein remarks that the interferometers used by LIGO and Virgo are “arguably the most sensitive measuring devices ever built.” These interferometers, which are the instruments used by the LVC, even have to filter out vibrations from things like cars and footsteps—though he assures that the astronomers at LIGO are “more than 99 percent confident that it is not a ‘terrestrial disturbance’ but is instead a GW from the cosmos.”
The instrumentation inside the interferometers is certainly complicated. Each one contains two 40-kilogram mirrors arranged in such a way that a laser beam traveling through the two four-kilometer arms cancels itself out. When GWs pass through, the distances between the mirrors shift slightly and the beam fails to cancel out. LIGO and Virgo can detect a change in the distance between these mirrors of “less than 10-19 meters—one-ten-thousandth of the diameter of a proton,” Weinstein clarifies.
A nascent field
On the importance of this discovery for such a young field, he comments, “Indeed, GW astronomy is just being born, and it is ‘discovery science.’ Whenever we observe something that is nothing like anything humans have ever observed before, it is a major milestone for science.” Weinstein cites previous examples such as GW150914, the first observation of a binary black hole merger, and GW170817, the first observation of a binary neutron star merger.
He relays that the work being done at the LVC “tells us more and more about the fundamental properties of GWs, strong gravity, and General Relativity; the properties of astrophysical black holes in the universe; and how black holes and binary black holes form in nature.” In the end, he notes that GW astronomy gives us tremendous insight into the lives of massive stars, “which are the sources of the heavy elements that make up planets and people. We are learning about our origins.”