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Question about expanding black holes

  1. Aug 18, 2004 #1
    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?
  2. jcsd
  3. Aug 19, 2004 #2
    doesn't a black hole lose mass as well. This would mean that the event horizon would have a limit to its size.
  4. Aug 19, 2004 #3


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    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.

    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.
  5. Aug 19, 2004 #4
    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.
  6. Aug 19, 2004 #5


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    Well.... perhaps not. All that matters to the orbiting bodies is the mass interior to their orbits, which hasn't changed.

    - Warren
  7. Aug 19, 2004 #6


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    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).
  8. Aug 19, 2004 #7


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    Actually, it works in reverse. The bigger the black hole, the 'cooler' it is. A density function thing. Low mass black holes, such as Hawkings primordial black holes, radiate furiously. The big ones walk down to the pasture and open the gate.
  9. Aug 20, 2004 #8


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    What you say is correct if the infalling mass was (in orbit) closer to the orbiting object in question. However, if it came from outside, it would be a different story.
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