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Thread: Are black holes cold?

  1. #1
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    Are black holes cold?

    For a black hole without an accretion disk, I'm thinking there's no radiation coming from it so should be reasonable to say the temperature of that black hole is approaching 0 Kelvin. Is that right?

    I guess there's a bit of Hawking radiation to account for, but that wouldn't be enough to appreciably raise the detected radiative temperature, would it?

  2. #2
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    Lightbulb Cold

    The temperature of a black hole due solely to Hawking radiation is roughly 6x10-8/M Kelvins, where M is the black hole mass in units of solar masses. So the more massive the black hole, the lower its temperature. Obviously any kind of an accretion disk at all would radiate overwhelmingly compared to the Hawking temperature, so in practice I am sure such a temperature cannot be measured.

    The lifetime of a black hole due solely to Hawking radiation is about 1071M3 seconds, where M is once again in solar masses. 1071 seconds is about 3.2x1063 years, so even a single solar mass black hole would take that long to evaporate, if all it did was sit there and Hawking radiate.

  3. #3
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    Quote Originally Posted by Tim Thompson View Post
    The temperature of a black hole due solely to Hawking radiation is roughly 6x10-8/M Kelvins, where M is the black hole mass in units of solar masses. So the more massive the black hole, the lower its temperature. Obviously any kind of an accretion disk at all would radiate overwhelmingly compared to the Hawking temperature, so in practice I am sure such a temperature cannot be measured.

    The lifetime of a black hole due solely to Hawking radiation is about 1071M3 seconds, where M is once again in solar masses. 1071 seconds is about 3.2x1063 years, so even a single solar mass black hole would take that long to evaporate, if all it did was sit there and Hawking radiate.
    Wow. That's a long time. Thought I suppose that's more appropriate behavior for a black hole than Hawking a leugy...

    But a point remains: Eventually, all black holes will radiate themselves away, thus, they're not the end state of all matter and energy in the universe.

  4. #4
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    But as the predicted life of the Universe is about 10^10 years, only micro BHs are likely to evaporate.
    JOhn

  5. #5
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    This calculator shows the temperature of black holes of all sizes.
    http://xaonon.dyndns.org/hawking/
    Small ones, with less mass than a large asteroid, are hot, while large ones are cold.

  6. #6
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    Cool

    Thanks all, particularly eburacum45 for the Hawking Radiation Calculator. I like the idea that small black holes would make an excellent power source for an advanced civilization.
    Last edited by Cheap Astronomy; 2008-Dec-27 at 02:29 AM.

  7. #7
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    Quote Originally Posted by JohnD View Post
    But as the predicted life of the Universe is about 10^10 years, only micro BHs are likely to evaporate.
    JOhn
    NASA's WMAP project estimates the current age of the universe to be 1.373 =/- 0.012 x 1010 years, i.e. 13.73 billion years old.

    Again, in a universe where expansion is accelerating, there's no Big Crunch, and black holes (the ultimate fate of most, if not all galaxies) will have all the time in the universe (cough) in which to radiate away.

  8. #8
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    Lightbulb Eternal Black Holes?

    Quote Originally Posted by mugaliens View Post
    But a point remains: Eventually, all black holes will radiate themselves away, ...
    Maybe not. Consider a universe that consists of nothing but black holes and their Hawking radiation. The Hawking radiation from other black holes will be falling into any given black hole, thus compensating for its own Hawking radiation losses. Unless the expansion of the universe continues to the extent that all other black holes fall outside the observable universe of any give black hole, then they will not be able to radiate themselves away. Certainly we are not now in any reasonable position to assert that we can confidently predict the expansion history of the universe for any indefinite, and possibly infinite time into the future.

  9. #9
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    wouldn't that imply that all matter in a black hole is an bose-einstein condensate which someone said was impossible? and further doesn't that imply that all matter will eventually end in this or a state that is unknown beyond that state?

  10. #10
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    I think they have MADE bose-einstein condensate in the laboratory.
    *checks*
    Yep.

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