This image is much better than any amount of words to describe the subject of Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars and Life in the Cosmos by Caleb Scharf. In a synthesis of discoveries about the formation and growth of black holes, a subject of continued and avid study, the author shows how some of the physically smallest objects in the universe govern the development of stars and galaxies on the largest scales.
Black holes are indeed small. The Event Horizon radius is a simple function of mass: r = 2.95km/Ms, where Ms refers to solar masses. Physicists like to use radius. The rest of everybody thinks in diameter. So I'll primarily use diameter when writing of an object's size. If the Sun were squeezed into a black hole, its diameter would be 5.9 km, which is about 3.7 miles. That is the size of a small city.
Black holes come in two ranges of size, so far as we know. Stellar-mass black holes range from about 2 to 20 or so (probably not exceeding 100) solar masses. Their diameters thus range from 12 to 120 km, to perhaps 300 km. Galactic black holes are much, much heavier and larger. Those so far known range from about 1 million to 20 billion solar masses. The diameter of their event horizon is thus anywhere from 6 million km to 120 billion km. An average one, if that makes any sense, would be about the size of the Solar System, but weigh a billion times what the Sun does. On the scale of a galaxy, that is tiny!
It is not known how mass is distributed inside the event horizon of a black hole, but in a non-rotating black hole, it may indeed all be concentrated at the center, in a "singularity" of zero size. Physics really cannot describe it.
I have used the term "event horizon" a few times already. Its radius is also called the Schwarzchild Radius, the point from which the escape velocity is equal to the speed of light. Let's think about that. An object falling from far away toward a black hole would approach the speed of light as it approached the event horizon. As it crossed the event horizon, would its velocity really exceed the speed of light? I have read descriptions that state that space itself gets dragged in, so that the object falling is exceeding the speed of light relative to "outside", but not relative to the infalling space. Such descriptions imply that space is a kind of something, which makes no sense. Whatever is really happening near and within an event horizon, we cannot describe it with any known science.
Two paragraphs earlier I mentioned a non-rotating black hole. Actually, no such thing exists. Every object we have observed, from moons and planets and galaxies and clusters of galaxies, is rotating. A huge rotating star that collapses into a black hole when its fusion fuel runs out will keep rotating, and its RPM's will increase dramatically. Dr. Scharf writes that black holes probably start out rotating near their maximum rate, at which any matter just inside the event horizon will have a speed just below that of light. For stellar black holes, that can be a million times per second, while for galactic black holes, the rotation rate is in the range of one RPM.
Deep in the center of a rotating black hole, there is not a point singularity. Instead, there is a ring. Whether it is a torus or a ring of infinite thinness is not known, but this gets us away from having all matter fall to a dimensionless point. In fact, if rotational energy is extracted from a black hole, and its RPM's decrease, the ring shrinks, but the slower it goes, the harder it is to get more energy out, so I suspect there is no such thing as a truly non-rotating black hole. Kind of like a Heisenberg Uncertainty Principle for angular momentum! (The HUP explains why temperatures of absolute zero are unattainable. This is similar.)
There is one other characteristic of a rotating black hole. There are two horizons. The inner one is spherical, just like a non-rotating event horizon. The outer one is an ellipsoid, shaped like a doorknob, and when rotation speed is the maximum possible, its outer radius is twice the radius of the inner sphere. The ellipsoid is tangent to the sphere at the rotational poles. Between these two horizons, space and time get mixed up, and matter that falls in off-center can pass through this Ergosphere and back out again, with more energy than it had going in. This is how energy is extracted from a rotating black hole. I have yet to read an explanation of these two horizons in which their size was clearly explained, but I think I understood that the outer edge of the ellipsoid is at the Schwarzchild Radius, and the polar radius is smaller. At maximum RPM, the inner sphere has half the size of the event horizon of a non-rotating black hole. I think...someone correct me if I got it backwards!
The first half or more of the book explains all this in an engaging way. It is all leading up to the explanation of the jets and bubbles seen in the image above, which requires one more parameter: magnetism. Whichever size of black hole we are talking about, the matter than came together to form it had a magnetic field. All known planets and stars and galaxies have a magnetic field. It is part of the territory, so to speak, when you have rotating mass. You may have read that Mars has no magnetic field. It's field is weaker than Earth's, by a factor of a hundred or so, but it is definitely not zero. So when a black hole is formed, the magnetic field doesn't vanish. Its total extent remains the same, but in the vicinity of the horizon, its intensity grows dramatically. A rotating magnetic field sets things up for an enormous dynamo.
I don't pretend to know the physics of it, and even this author glides right by it, but when the spinning magnetic field gets spun up enough, it twists into a "tail" at each rotational pole. Any matter in the vicinity of the black hole will either orbit it, or fall inward. Most of this is likely to be gas and thin dust. The dynamics of this orbiting stuff and the "frame dragging" of the black hole's rotation (a mysterious effect of general relativity) will force most of it into a disk in the plane of the hole's equator. Friction within this disk will heat it to a few thousand degrees. As the innermost material gets inside the ergosphere, it gets sped up a lot, and some of it gets caught up in the twisted magnetic field and squirted out into the jet within the magnetic tail. Here, it can approach the speed of light, and temperatures of millions of degrees. This jet crashes through any gas and dust it finds surrounding the area, and blows a bubble in it. These big bubbles interfere with star formation, and also reduce the rate that stuff can reach the black hole.
What does that have to do with stellar or galactic evolution? Just about everything! A distant object that Dr. Scharf and his colleagues studied, called 4C41.17, showed that galactic black holes formed very early. Their jets and bubbles slowed down the formation of stars and guided the development of the earliest galaxies. Distant, and thus early, galaxies are seen to have active nuclei, and the ones for which we are looking down the throat of the jet we call quasars. The bottom line is that the activity of these huge black holes, tiny as they are compared to their host galaxies, regulates the rate of star formation. Without them, most of the Universe might have burned out by now, leaving just quintillions of dim red dwarfs to sputter away for the next few trillion years.
Now I'm going to leave the author a few things to say. This all is just what interested me most. There is just one thing left I wonder about. So-called dark matter is posited to react only to gravity. It seems to make up between 80% and 90% of the mass of the universe. Isn't it also being pulled into black holes of all sizes? Does it contribute to their mass and perhaps other properties?
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