Keeping up with an interest amongst VFO readers, we turn now to a discussion on globular clusters.
These dense concentrations of about a million stars all in rapid orbit about each other had been notoriously uninteresting, that is until recently.
It seems that globular clusters may be the most efficient place to form a closely-attached pair of black holes called a “binary black hole”. Let us see how this curious arrangement may have come about.
The standard lore had been that any black holes formed in a globular cluster would get thrown out on the grounds that they did not “mix” well with the other normal shining stars. The result would be a globular cluster devoid of black holes.
Recently, using computer simulations that assign more realistic masses to the black holes of 3 – 80 times the mass of the sun, multiple research groups are finding that dozens to hundreds of black holes are retained within the globular clusters.
And it gets even better. The concentration of stars plus black holes is much much higher in globular clusters than it is in the Milky Way and other galaxies. This means that there will be a correspondingly higher incidence of black holes encountering other black holes.
Most of the time, when a black hole approaches another black hole it will zoom right past it. One can invoke a “putting green” analogy here. For strong hits of a golf ball (black hole #1) towards the hole (black hole #2), the golf ball rolls right past the hole, and the same goes for weak hits. In fact, even when you think you applied just the right amount of force to the ball, there inevitably still are a few failures.
But if you work at it long enough, and/or if you have dozens to hundreds of friends all putting on
other greens at the same time, someone will hit the ball into the “gravitational” hole and become bound to it (until you pick the ball out of the hole).
Likewise, there is only a vanishingly small probably of a black hole becoming bound to another black hole. But there are dozens to hundreds of “putts” going on continuously for billions of years, occasionally there will be successes in making this holy grail of objects, the “binary black hole.”
This is important as it is when binary black holes infall into each other that “ripples” in spacetime are produced which we can and do detect from Earth. For next time: how can we go out and observationally detect a binary black hole in a globular cluster?
