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Spinning black holes may prefer to lean in sync

A simulation of a black hole merger.
Enlarge / A simulation of a black hole merger.

LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)

I was pretty excited when LIGO, the giant double-eared gravitational wave observatory in the US, detected the first gravitational waves. When Virgo came online, triangulating gravitational wave signals became possible, and gravitational wave astronomy became a reality.

Once the initial excitement of seeing individual events died away, it was only a matter of time and statistics before scientists started pulling new insights out of the data. A pair of new papers has looked at black hole merger statistics, and the papers’ results suggest that there might be something unusual in the distribution of black hole spins.

The revealing death spiral

Gravitational waves are the result of mass moving through space and time. The mass stretches space and time, causing a ripple effect, much like the bow wave from a boat moving through water. And, just like a bow wave, the heavier and faster the mass, the bigger the wave. Unlike water, space-time is very stiff, so it needs more than an ocean liner to create a noticeable gravitational wave.

This means that we can only observe gravitational waves from very heavy objects that are moving extremely fast. Pairs of black holes that are in the final moments of a death spiral and collision definitely fit this requirement. In the last few orbits, the two black holes have speeds that are a respectable fraction of the speed of light.

These events produce massive gravitational waves, which, by the time they have reached us, are ripples that stretch the distance between New York and Los Angeles by a few femtometers. For perspective, a hydrogen atom is on the order of 100,000 femtometers.

For decades, this was all a matter of theory. But we eventually managed to construct detectors that allowed us to test this theory. Understandably, the first few detections were met with great excitement. Now, we have catalogs of gravitational wave events that can be data-mined.

First you spin, then you orbit

When two black holes are orbiting each other, their orbit defines a plane, with the direction of orbit ending up either clockwise or anticlockwise in the plane. The two black holes can also be spinning, but that spin does not have to be in the plane or have the same direction as the orbital rotation. In fact, according to the researchers, there is no reason to think that black hole spin is anything other than random.

However, it has also been predicted that, in a binary system, the two black hole spins will be anti-aligned. For example, if one black hole is spinning clockwise at 90 degrees to the plane of orbit, the second will be spinning anticlockwise at 90 degrees to the plane of orbit.

If the black holes’ spins are out of alignment with their orbit, then the black hole spins will precess like spinning tops while also maintaining their anti-alignment. Under the right circumstances, this can also introduce a wobble in the plane of the orbit and the possibility of a kind of resonance, called a spin-orbit resonance. (This is when the spin orientation causes the orbit orientation to change, which then causes the spin orientation to change and so on.)

So far, these ideas have only been investigated in models, but now that we have a catalogue of black hole mergers, we can finally start looking for evidence.

A surprisingly small kick

The research team behind the new papers used a statistical model to try to back-propagate the observed gravitational waves to the possible spin orientations of black holes. The researchers assumed two possible distributions of spins: evenly distributed (no preferred orientation) and a peaked distribution (a preferred orientation).

The outcome in both cases showed that the anti-alignment of black hole spins was preferred. This confirms (although weakly) the model predictions that also lead to spin-orbit resonance. Even in the case that assumed black hole spin has no preferred orientation, the data favored an orientation of 45 degrees to the plane of the orbit.

That’s a bit of a surprise, given that, so far, no model has predicted a preferred spin orientation. In both cases, though, the number of mergers is still too low to allow for strong conclusions, so consider this very preliminary.

The researchers also investigated how the spin orientation would influence the final state of the merged black hole. The production of gravitational waves gives a kick (think Newton’s third law for black holes) to the post-merger black hole. Some of the models with spin-orbit resonances had predicted that this kick could exceed 5,000 km/s. This is the equivalent of the galaxy’s bouncer grabbing the black hole by the scruff of the neck and hurling it vigorously into the intergalactic void.

Even though the spin alignment predictions are rather weak, this still translates into a strong prediction for a well-defined kick of about 300 km/s. Apparently, only the most refined astronomical abodes will kick you out for merging while also spinning (globular clusters, in fact).

Of course, with more data, the observed peak in spin orientation could disappear. This is what makes the papers exciting. Unlike previous years (when there was no data) or even a few years ago (when everyone based their speculation off of a single event), we now have data. And we will get more data. Models are being refuted, refined, or confirmed, and this is a handy example of that in process.

Physical Review Letters, 2022, DOI: 10.1103/PhysRevLett.128.031101

Physical Review D, 2022, DOI: 10.1103/PhysRevD.104.103018 (About DOIs)

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