I see, hear, and read about black holes and their consumption power often. They grow as they consume. Eventually, as they keep growing and consuming, wouldn’t they combine and continue the process until they become so large as to devour everything in existence?
In: Physics
Black holes are somewhat self-limiting. As matter falls in, it contributes to the mass, yes. However, angular momentum is always conserved – meaning, stuff is orbiting around the black hole, and the stuff itself is spinning. That rotational energy becomes part of the black hole system, making it spin faster. As it does, it twists and pulls space, too, which drags everything around the black hole faster.
Orbit distance is a function of the speed. If something orbits faster, its orbit will be higher. As the black hole spins faster and faster, the stuff around it gets flung *outward*. Eventually, the orbits will be too fast and too high, and stuff will no longer fall into the black hole.
Moreover, when stuff falls into the black hole, it smashes and grinds against all the stuff swirling around. This is what makes the accretion disc, the bright glowing rings around the black hole. The accretion disks of a supermassive black hole are among the brightest objects in the universe, often outshining every other star in the galaxy around the black hole *combined*. Light has momentum and all that light blasting outwards creates a pressure that pushes against the matter being drawn in. Again, at some point the pressure from the light will reach equilibrium with the gravity pulling inwards (as long as it’s beyond the event horizon), and no more stuff can fall in.
Between these two things, and the evaporation of black holes due to Hawking radiation, black holes all stop “feeding” and eventually completely disappear. Although, the “disappearing” part takes a very very long time – like, 10^67 years for a black hole only the size of the Sun. A supermassive black hole will take much much longer than that, something along the lines of 10^100 years.
The three possible ends of the universe are:
Big Crunch – In this scenario, the expansion of the universe slows down before it can overcome gravity. The collective gravity of all the matter pulls inwards until everything slows down and then reverses, falling inwards towards itself. The universe collapses into another Big Bang.
Big Rip – In this scenario, expansion continues to accelerate until it outpaces not only the gravity of superclusters, and then clusters, and then local groups…and so on, until space is expanding so fast that even solar systems are flung apart. Then, space will expand fast enough that the gravity of stars and planets holding themselves together fails, and they fall apart. Then, expansion goes so fast that it overcomes the forces holding molecules and atoms together, and the entire universe falls apart at the most fundamental level, faster than light.
Heat Death – In this scenario, expansion doesn’t keep accelerating, but it’s still too fast for gravity to pull inwards to the Big Crunch. Over an incomprehensibly long timeline, every star runs out of fuel until only black holes are left, and then those evaporate, too, until literally nothing is left except lone protons and photons too spread out to ever meet.
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