# Question about expanding black holes

## Main Question or Discussion Point

Somewhere on the web I read that as new matter is captured by a black hole, the event horizon expands. But as it expands it does not engulf the matter that previously accumulated close to the horizon. Instead all that matter is carried outward along with the expanding event horizon. But "carried outward" doesn't mean the matter backs up. The distance of that matter from distant objects remains the same.

Is this true? If so, is it only the matter around the horizon that maintains a constant distance from distant objects? Or does the event horizon itself maintain a constant distance from distant objects?

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doesn't a black hole lose mass as well. This would mean that the event horizon would have a limit to its size.

mathman
When a black hole expands (by stuff falling in), its gravitational field is increased. Things nearby are presumably in some sort of orbit. What happens depends on the dynamics. The orbit would change, and it may fall in.

doesn't a black hole lose mass as well. This would mean that the event horizon would have a limit to its size.
The bigger the black hole, the slower the mass loss. There is no size limit. There are estimates for gigantic black holes at galactic centers. These have masses of millions to billions of solar mass.

isnt there a point where the rate of rediation emitted from a black hole is greater than the rate consumed? Wouldent this be a limit on the maximum size of a black hole.

chroot
Staff Emeritus
Gold Member
mathman said:
When a black hole expands (by stuff falling in), its gravitational field is increased. Things nearby are presumably in some sort of orbit. What happens depends on the dynamics. The orbit would change, and it may fall in.
Well.... perhaps not. All that matters to the orbiting bodies is the mass interior to their orbits, which hasn't changed.

- Warren

russ_watters
Mentor
isnt there a point where the rate of rediation emitted from a black hole is greater than the rate consumed? Wouldent this be a limit on the maximum size of a black hole.
No, since the rate increases as the size decreases. It really depends on the black hole (or rather, how much matter is near it for it to consume).

Chronos