Okay, I'm sorry if this is an enormously stupid question. I just don't have an answer.
So, from reading the "Black Hole FAQ" by Ted Bunn, as well as posts on this forum, I've gathered that:
1. Gravity strength is determined by an object's mass concentration.
2. Black holes are so small and massive that nothing can escape their gravity.
3. Black holes will swallow anything within a certain radius.
4. The radius is determined by the mass concentration of the black hole's singularity.
So that leads me to assume the following:
1. Objects "consumed" by a black hole get added to the singularity's mass.
2. The increased mass thereby increases the black hole's gravity.
3. The increased gravity increases the black hole's Schwarzschild radius.
So my question is:
If the radius continues to increase, would they eventually become so large as to consume the entire universe?
Stray comets/asteroids/dust keep getting sucked into any black hole, increasing the Schwarzschild radius enough to reach a planet, rinse and repeat until sooner or later they just become so large that they overlap each other, multiplying their effect, etc.?
Possible answers that I've come up with are:
1. No. Space is so damn big that even a huge asteroid wouldn't increase the Schwarzschild radius enough to reach planets. And even then, they're just too isolated. Space is huge, man.
2. No, one of my assumptions about gravity/black holes is just plain wrong.
3. Yes. Given enough time (how about eternity), the black holes would gather enough mass to swallow all matter in the universe.
Thanks for your wisdom.