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 Originally Posted by BananaStand
No. Does not compute. Either the milky way is on its way into that black hole, or it isn't.
Black holes don't "suck things up" any differently than any other massive body. I blame Hollywood for this one.
If something is in orbit, it will remain in orbit unless / until an outside influence affects that relation.
The pull exerted by gravity doesn't necessarily pull things into collision courses.
A galaxy isn't a rigid object. It's more like a gas, if we think of the stars as particles.
In order for something which is in orbit about the black hole to alter its course, it must change its energy.
Conservation of energy states that in order to change its energy, it must cause an equal and opposite change in energy to something else (the system around it).
If it is not interacting with anything nearby (the prevalent case for most stars not near the galactic nucleus), then it cannot change its energy, and the black hole can't "suck it in."
In order for something to be "sucked in" it has to deliver some of its energy to another body or bodies in the system.
So in order for something to be sent on a "sucked up" collision course, something else had to be sent the other direction.
So your question is incorrect. It assumes that ALL of the Milky Way will be consumed by the black hole or NONE of it. The actual answer is in between. Some of the Milky way will end up as Sagittarius A*, some of it will be flung into intergalactic space.
Originally Posted by BananaStand
Assuming that it is, at some point, going to be consumed by the black hole, then wouldn't that make the black hole bigger, denser, and with a stronger gravitational pull? In other words, it will be able to consume things from further away. And so on, and so on.
As black holes consume matter, they grow in size and the gravitational effect on spacetime increases.
Yes, but this rate of expansion is slow, because most of space does not contain anything to consume... hence the name, I suppose.
Originally Posted by BananaStand
So what's stopping these black holes from eventually consuming the entire universe?
Some of the stuff in the universe is moving away from the black holes at a rate faster than their escape velocity. The growing gravitational influence of the black holes is not fast enough to counter this.
When we look at the longest time scales and play the guessing game about what will happen, there is an extreme time scale at which all the stars have burned out, all the stellar remnants have burned out, and the only things left are black holes and a smattering of hydrogen atoms in the diffuse "vacuum" of space.
Eventually, even the black holes evaporate and the universe is left a diffuse gas of particles, each separated by their closest neighbor by distances too vast to overcome their relative velocities to ever stop them and pull them back into a bound state.
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