Somewhat naive questions, as I know very little about astronomy. Do black holes "move?" How is it that they could merge if they're stationary, unless they're pulling each other in I guess? If black holes are indeed pulling in everything, does that mean the whole universe would eventually be one giant black hole?
Yes, like any other massive objects, black holes can have velocity and momentum. Two black holes, or a black hole and another object like a star, can orbit each other in a way that almost follows Newton's laws.
> How is it that they could merge if they're stationary, unless they're pulling each other in I guess?
This gets at what makes LIGO's findings interesting. Two black holes merge if they fall into each other's event horizons. But Newtonian gravity predicts that, in isolation, this would never happen; two orbiting black holes would just maintain their elliptical orbit forever. (I'm hand-waving here, because Newtonian gravity can't properly model black holes at all.)
The theory of general relativity predicts that the gravitational curvature of the space around the black holes contains energy, similar to the energy in the electric field around a charged particle. And intense changes in curvature can create waves in the curvature of space, which carry away kinetic energy from the black holes and cause them to spiral into each other. Under normal conditions these waves are so unimaginably tiny that they're unmeasurable, but during a black hole merger, they become intense enough to be (barely) detected from billions of light-years away. This is what LIGO detected, confirming a long-known theoretical prediction of GR.
> If black holes are indeed pulling in everything, does that mean the whole universe would eventually be one giant black hole?
Not necessarily. Everything in the universe attracts everything else gravitationally, but that doesn't mean any two objects will inevitably collide. If they have enough energy to move apart faster than their common escape velocity, they are not gravitationally bound and will continue separating forever.
If we scale time to one-trillion-quadrillion years into the future, isn't is possible that all mass in the universe eventually coalesces into a massive universal super black hole? Or, does the expansion of the universe outstrip that?
Black holes move in the same way as any other object of the same mass, for example a star.
When they merge they are typically orbiting each other already in a similar way to binary stars. When they orbit the system will lose energy to gravitational radiation causing the black holes to spiral inwards. (This happens for all orbits, including the Earth's). Eventually they merge sending out a tremendous amount of gravitational radiation.
The gravitional pull of a black hole is just as strong as another object of the same mass. The difference is that the mass is concentrated to a point in the center. This is what makes them impossible to escape from.
It was believed the the whole universe could maybe collapse back into one point (like a black hole). Now it is believed that the universe will keep expanding forever. Actually the universe seems to be expanding ever quicker due to dark energy. But no one knows what dark energy is.
Black holes have mass, just like any other object in the universe - like a star, or a planet, or the sun.
Consequently, they follow orbits just as any other mass would. In some cases, they're the local most massive object and any other masses move more in response. Other times they are near other black holes, and they orbit one another until they collide and merge.
What makes black holes different is their density. The mass a black hole has is confined in a point of zero height, width and length called a singularity. The consequences are as we know, not fully understood by current models of physics.
edit: spelling
They do indeed move. It's conceivable that a black hole of mass X could be observed orbiting a red super-giant star of mass 100X. I don't think this happens much though, but the universe is big so who knows.
I guess this was the biggest question answering piece to me. When I think of a black hole, I assume it has a gigantic mass, enough that it's always the most massive local object, and subsequently pulls in all other things. I didn't realize that might not be the case. As black holes "absorb" everything that "falls" into them, do they continue to build mass then?
Whether or not something is a black hole is dependent on its total mass and its radius. So anything can mathematically become a black hole if you compress it enough. The earth could be a black hole if it's total mass were compressed to something like 'less than the diameter of a grapefruit'. At that point space itself cannot contain the mass, physics breaks down and you get a singularity.
The moon would continue to orbit as it always did, since the moon's centre of mass is still exactly the same distance from the earth's centre of mass as it was before you pressed the 'compress button' on the north pole.
It takes incredible amounts of energy to cause this compression however. And this is why only the biggest stars become black holes. As the outward pressure of fusion diminishes because the hyrogen/helium/lithium/berylium/etc fuel runs out, the sheer gravitational pull of all that mass suddenly takes over and that inward momentum from all directions is enough to cause a singularity.
> So anything can mathematically become a black hole if you compress it enough.
Not necessarily. If the Schwarzschild radius of the resulting black hole is on the order of a Planck Length you can't really say whether or not such an object is a black hole anymore (depending on your interpretation of the Planck Length).
On my calculator the lower bound is [\frac{r c^2}{2G}] which is ~1.09e-9 kg (give or take a factor of two depending on whether you want to consider the diameter or radius). Which is small but not as small as you might think. I also believe there are some upper limits on black holes too (that come from the upper limits of stars).
It depends less on your interpretation of the Planck length and more on how quantum gravity actually works. We have some guesses and semi-supported theories (like Hawking radiation) but the jury is still out as to how micro black holes work, if they do in fact exist.
The sun is the most massive local object in the solar system, by a very large margin, but does not pull in all other things. Or, well, it does pull, obviously, but it turns out that objects under gravity end up on Keplerian orbits if they have sideways velocity to begin with.
> When I think of a black hole, I assume it has a gigantic mass, enough that it's always the most massive local object, and subsequently pulls in all other things.
The sun is not pulling in the earth. Earth orbits a common barycenter with the sun. That barycenter happens to be inside the sun, but the sun is also orbiting around it.
So a small object orbiting a bigger object is an approximation. Orbiting their common center of mass is a better approximation. This is more easily visible in the pluto-charon system.[0]
The same of course applies to black holes. Them being heavier than most other objects in their vincinity does not render them immobile any less than the sun being the most massive object in the solar system renders it immobile.
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