Questions about Black Holes, Hawking Radiation, and Dark Matter

  • #1
ShadowKraz
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If this post should be in another forum, please advise so I can post there instead. I hope I got the right prefix. Thank you.

1) Is it possible for Hawking Radiation to be dark matter, at least in part? Do the equations rule out this possibility?
2) Are Black Holes actually spinning or is our perception that they do due to our relative position as observers within the medium their gravity is distorting?

I'm obviously not a physicist, but a very interested person who has been following the discoveries and breakthroughs with some comprehension since the late 70s. I know my thinking has flaws, I just don't know enough to know quite where and appreciate being corrected so that my thinking becomes more accurate.
 
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  • #2
Hawking radiation is utterly negligible. At this time, black holes absorb more energy from infalling Cosmic Microwave Background radiation than they radiate via Hawking radiation, and the CMB is itself pretty much negligible in mass terms

Black holes have angular momentum which you can (in principle) tap as an energy source, like a flywheel. Whether that means they are "actually spinning" or not depends on what you mean by "actually".
 
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  • #3
Ibix said:
Whether that means they are "actually spinning" or not depends on what you mean by "actually".
As a famous man once said "It depends on what your definition of 'is' is."

As far as the first part, not only is hawking radiation negligible, but its also ordinary thermal radiation - not anything dark.
 
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  • #4
Ibix said:
Hawking radiation is utterly negligible. At this time, black holes absorb more energy from infalling Cosmic Microwave Background radiation than they radiate via Hawking radiation, and the CMB is itself pretty much negligible in mass terms

Black holes have angular momentum which you can (in principle) tap as an energy source, like a flywheel. Whether that means they are "actually spinning" or not depends on what you mean by "actually".
It's all relative... lol.

From our perspective, a black hole is spinning, but... well, maybe it isn't. hmmm... ok, I think it may be analogous with how we viewed the cosmos many centuries ago; the Earth is stationary and everything, the Sun, Moon, planets, stars, etc., is revolving around it. That was our perception based upon our own perspective but once Copernicus really started thinking about the inconsistencies of observed motions for the other celestial bodies, he formulated the heliocentric theory which was closer to the reality, although still not quite right. No, I'm not putting myself in Copernicus' shoes, so to speak.

What I'm thinking is that from the perspective of the black hole itself, it isn't spinning but the Universe is spinning around it. How do we know for certain it has angular momentum, from the conservation principle? Is it possible that what conserves it is not the black hole itself but the event horizon or the spacetime around it, thus making it appear as though the black hole has angular momentum? Or is this just another 'ha, ha, I just came up with silliness' moment? You can be honest, I can take it.
 
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  • #5
Vanadium 50 said:
As a famous man once said "It depends on what your definition of 'is' is."

As far as the first part, not only is hawking radiation negligible, but its also ordinary thermal radiation - not anything dark.
As for the Hawking Radiation, how certain are we that it is just ordinary thermal radiation?
And... well, I understand that at any given time the amount of Hawking Radiation released is negligible, but its effects would add up over the long span of the Universe, yes?
Thank you for your patience.
 
  • #6
ShadowKraz said:
As for the Hawking Radiation, how certain are we that it is just ordinary thermal radiation?
And... well, I understand that at any given time the amount of Hawking Radiation released is negligible, but its effects would add up over the long span of the Universe, yes?
Thank you for your patience.
Hawking radiation is mostly photons and it "adds up" only in the same way that photons "add up" due to getting emitted all the time by stars except that the amount of Hawking Radiation is essentially zero relative to photons emitted by stars. SO ... give it up and move on --- Hawking Radiation is not dark matter.
 
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  • #7
ShadowKraz said:
What I'm thinking
...sounds a lot like a personal theory. These are not allowed at PF.

ShadowKraz said:
As for the Hawking Radiation, how certain are we that it is just ordinary thermal radiation?
It is true by definition. If it were non-thermal, it would be something else.
 
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  • #8
ShadowKraz said:
Or is this just another 'ha, ha, I just came up with silliness' moment?
It's just stringing words together, I'm afraid. You cannot paint a mark on a black hole and see if it's spinning. All you can do is observe that it has angular momentum and that you can change that value.
ShadowKraz said:
As for the Hawking Radiation, how certain are we that it is just ordinary thermal radiation?
There's no theory describing anything except thermal radiation coming out. So you need a new theory that describes black holes emitting something that isn't thermal radiation and doing it at such a rate that the total emission is many times the mass of the black holes, without the black holes vanishing. It sounds like a non-starter to me.
ShadowKraz said:
its effects would add up over the long span of the Universe, yes?
No. The total emission of Hawking radiation across the whole galaxy is negligible next to the emission of one star.
 
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  • #9
Ibix said:
next to the emission of one star.
Or the energy use of one ant.
 
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  • #10
Vanadium 50 said:
Or the energy use of one ant.
Ants can be pretty high energy. It's all the ant-y particles.

Seriously, having now looked at the maths in a bit more detail, I see there seem to be a lot of ##10^{-\text{something large}}## in the constants and the total emission is that very small number divided by the black hole mass squared. So I see your point.
 
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  • #11
One could even make the case the energy is negative, as the BHs absorb more than they emit. But in any event the OP's theory is off by so many orders of magnitude it doesn't really matter. 30? 90? Does it really matter?
 
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  • #12
Thank you, Vanadium 50, phinds, and Ibix for your patience and explanations. Now I can stop thinking along the wrong lines.
 
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