How do these Black Holes exist? On May 21 of 2019, the LIGO observatory facilities in Washington and Louisiana, and the VIRGO facility in Italy detected a few tiny wobbles in the fabric of space-time, enough to wiggle the position of highly sensitive optics and generate a detectable signal. This was the passage of a gravitational wave, one of now several dozen potential candidates discovered by these observatories since they came fully on line a few years ago. Out of only a handful of confirmed gravitational waves, this one has some unique properties.
Gravitational waves are produced when something really big happens in the universe. The ones humans have detected so far involve the mergers of black holes and neutron stars; as these massive objects approach each other, they convert some of their mass into energy that bends spacetime, and that energy propagates outwards as a gravitational wave. As a gravitational wave passes, the detectors in the LIGO facilities wobble ever so slightly, creating a measureable signal that can be recorded as a waveform. Scientists have produced a number of models for the shape of the wave as it passes Earth and can compare the measured waveforms to simulated waveforms that estimate the distance to the wave source, the masses of the objects involved in creating it, and some properties of their orbit and spin. Although there are some other possibilities, the best fit for this gravitational wave was a merger between 2 black holes, one of which had a mass 65 times that of our sun and another which had a mass 85 times the mass of our sun. These masses are particularly interesting in that at least one of those black holes…theoretically shouldn’t exist. Black holes are formed when large stars, several times the mass of our sun, collapse. Scientists have located candidate black holes that are only a few times the mass of our sun, and they have located massive to supermassive black holes with masses greater than 150x the mass of our sun, but there is a gap in-between, for intermediate mass black holes. While black holes can have any masses, there is a problem in creating intermediate mass black holes. When a star is collapsing inwards, the pull of gas inwards causes the material to heat up. In smaller stars, as this is happening the core of the star produces gamma rays that push back against the force of gravity, limiting the flow of gas into the core. It turns out, these gamma rays actually protect the black hole as it is in the process of forming from another violent effect. If the gas in the star is pulled in too much, it heats up and converts those gamma rays into electron/positron pairs. When this happens, the core loses its support, and gas comes rushing in so fast that it triggers a re-ignition of fusion in the star. This new fusion releases energy that either blasts away some of the star or, alternatively, rips the entire collapsing star apart. If a star has high enough mass, then as it collapses all the gas just reaches the state where it becomes a black hole and it converts directly to one during a supernova. But this “pair instability” problem of creating electron/positron pairs and removing the gamma rays that keep the forming black hole in balance predicts that there is a range of masses we shouldn’t see in black holes. The 85 solar mass black hole falls right in this range. According to the current models, it can’t be produced by a supernova; the star should have ripped itself apart rather than creating that black hole. So, how did this black hole form? Well, one possibility is that our understanding of the physics is slightly wrong. Describing the pair-instability mass range requires a detailed understanding of how the exact isotopes that flare up in a collapsing star behave under extreme conditions, and this black hole could be telling us that we don’t have those numbers quite right. Alternatively, the other way to make this black hole would be to combine 2 smaller black holes to make this one; in this case, we would be observing at least the second black hole merger in this object’s history. There is some support for this model; the orbits and spin suggested by the gravitational wave suggest that this black hole may have had a particularly rapid spin, possibly relating to a previous black hole merger. Furthermore, an observatory in California detected a gamma ray flare occurring 35 days after the initial gravitational wave from the same direction in the sky; this flare could have been produced by dust heated from the energy of the black hole merger. If that’s the case, it suggests these black holes are in a dusty molecular cloud, a region of high star formation where multiple black hole mergers would be more likely. Overall, this gravitational waves shows us the merger between a 65 solar mass and an 85 solar mass black hole, with 8 solar masses being converted into energy to leave behind a 142 solar mass black hole. At the very least, this observation seems to confirm the presence of intermediate mass black holes, the size that previous sky surveys have yet to uncover. -JBB Image credit and press release: https://news.mit.edu/2020/ligo-virgo-gravitational-wave-0902 Original papers: https://physics.aps.org/articles/v13/111 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.101102 https://iopscience.iop.org/article/10.3847/2041-8213/aba493
