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?
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.