Astronomers have detected the most massive pair of black holes to merge ever seen: one had 100 and the other 140 times the mass of the Sun, far heftier than any pair ever seen to merge before! This leaves us with a pretty big puzzle: where did these two big fellas come from?

By “merger” I mean literally two black holes eating each other to form a single, more massive black hole. This can occur when two black holes orbit each other. The binary system itself usually starts out as two massive stars orbiting each other. The stars individually explode after they run out of fuel, each creating a supernova. The outer layers blast away in the incredibly powerful event, but their cores collapse to form black holes.

The black holes can then orbit each other for billions of years. But as they do, they create gravitational waves, literally ripples in the fabric of spacetime. It’s a little bit like how something floating in water creates ripples as it bobs up and down — that’s not a great analogy, but it’s the general idea. One of Einstein’s last untested predictions of Relativity was that massive objects that are accelerating create these waves, but those waves are very hard to detect. They’re usually very low amplitude, and mushy. You need incredibly massive objects accelerating VERY hard to get detectable waves.

Enter black holes. As they orbit each other, they emit very low energy gravitational waves. This takes orbital energy away from the black holes, so their orbit very slowly contracts, bring the two closer together. But that means they orbit each other more rapidly, so they emit more waves, bringing them even closer together. Eventually, after many eons, they get so close that their orbital velocities increase to very nearly the speed of light. The gravitational waves they emit get very strong indeed, dropping the black holes toward each other ever faster. In the last few orbits before they merge, taking just a fraction of a second, they create a blast of waves that can be incredibly powerful. These waves march across the Universe, and flow over Earth.

When that happens, spacetime itself contracts and expands. The effect is small; over an object the size of Earth the stretching is only a few times the size of a proton! That’s very tiny. But it turns out it can be detected: using lasers to measure the distances between a set of mirrors located a kilometer or two apart, that rippling (which over that scale is about one-thousandth the size of a proton) creates a pattern in the reflected laser light that can be measured.

The first such gravitational wave observatory, LIGO (for Laser Interferometer Gravitational wave Observatory) detected its first black hole merger in 2015, finally confirming Einstein’s prediction. There are two such observatories in the US, to help triangulate the location of the merger on the sky. There are more now, including the European Virgo and Japanese KAGRA facilities. All three partner with each other, to increase the ability to detect and measure these signals.

That last gasp of waves the black holes emit can tell us a lot about them, including their masses, the mass of the resulting black hole after they merge, how rapidly the black holes are spinning, and more. 

On November 23, 2023, the three observatories detected a signal, designated GW231123. Unraveling the measurements, scientists found the two black hole components were more massive than any previously seen, 100 and 140 solar masses. The final black hole mass was about 225 times the mass of the Sun; the missing 15 solar masses were converted into gravitational waves during the merger. The most massive black hole ever seen after a merger previous to this one had a mass of about 140 solar masses.

So it’s a record! Cool! But it’s also weird. Why?

Because the biggest black hole you can get from a supernova is maybe a few dozen times the mass of the Sun. 100 and 140 solar masses are way above that limit.

So. How did these two form?

One way could be from lower mass black holes that merged before, building up two black holes that are much higher than the expected limit. However, that’s pretty hard to do. You need a lot of massive stars near each other, and they have to behave very well to form black holes that can merge multiple times. I suppose this might be more likely in a crowded environment, like in a globular star cluster, where there could be lots of black holes in the dense core of the cluster, and they could interact more often.

It’s also possible to get much heftier black holes back in the early days of the cosmos. The first stars that formed could have been far more massive than stars that exist today, and theoretically could form much larger black holes, even as big as the ones that merged to create GW231123. If that’s the case, we saw an event that was the final cry from two of the earliest stars to exist in the Universe. 

That’s still pretty theoretical though. For now it’s not clear how these two black holes came to be, but we’re still in the early stages of figuring this all out.

And I’ll leave you with this. I mentioned that the missing 15 solar masses were converted into gravitational waves. The physics behind that is complicated (duh) but the bottom line is that that mass was converted into energy via Einstein’s E = mc² equation. Plugging the mass into that and converting, the amount of energy emitted by this merger as gravitational waves was about 3 x 10⁴⁸ Joules, a number so huge it makes my teeth hurt (I’ll note that the number 10⁴⁸ is called a quattuordecillion, a new one on me). 

How much energy is that? Well, it would take the Sun about 200 trillion years to emit that much energy. It the same as all the energy every single star in the Milky Way galaxy emits in 2,000 years. Or — and this one is what really gives me chills — it’s comfortably more than the amount of energy every single star in the Universe combined emitted over the same time it took to blast out those gravitational waves. In terms of energy, this was the brightest event in the entire cosmos during that moment.

Weirdly, though, it emitted no light at all! All that energy was instead channeled into shaking the fabric of the Universe, and sending those waves out across billion s of light years, where they weakened so much by the time they got here they distorted our planets shape by the width of a subatomic particle… yet we could detect it.

Some people say science is boring. I really have no idea what they’re talking about.

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