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There's ample direct evidence for the existence of galactic and stellar mass black holes. However, there is no such direct evidence of primordial black holes, those formed after the Big Bang. A recent paper [http://arxiv.org/abs/1106.0011] describes oscillations the Sun might undergo if it encountered a primordial black hole. The theory that primordial black holes exist hasn't had experimental backing. Why should one believe they actually exist? Is this science or "guesstimating?"

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Sorry if my disbelief is so open. –  Michael Luciuk Jun 3 '11 at 14:52
Since my early reading of Stephen Hawking books I've been fascinated by the idea that interstellar space is littered with mountain-sized black holes. I'm interested to hear some updates on the current state of thinking about these. –  AlanSE Jun 3 '11 at 23:27
The title is about formation, but the question is about detection. Maybe you could change the title? –  Ben Crowell Aug 9 '11 at 14:22
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2 Answers

Is this science or "guesstimating?"

It's theory.

  • General relativity allows for black holes at essentially any mass.

    And the theory is strongly supported on several fronts, including the apparent observation of large and huge compact object which fit the bill a black holes.

  • Big Bang cosmology allows for the conditions necessary to produce "small" ones.

    Again the theory has some experimental evidence to back it up, though in this case some of the fine detail needed to be convinced that primordial black hole did or did not form is lacking.

Why do you imagine that every reasonable theory will have or not have experimental justification right away? Some experiments or observations are hard or just take a lot of time.

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I really don't believe that every possible theory needs instant justification to be correct. Perhaps observational evidence for primordial BHs is too faint to be detected. However, to be fair, GR is almost a century old, and the Big Bang over half that. If they existed in quantity one might expect the heavens to be lit up with x-ray/gamma radiation. –  Michael Luciuk Jun 3 '11 at 16:17
It's possible to determine the mass of primordial BHs that would evaporate via Hawking radiation over the past and present. Could the paucity of radiation give hints on the mechanism of their theoretical formation? –  Michael Luciuk Jun 3 '11 at 16:31
@Michael: It can put a upper bound on the density now and from that contribute to understanding the very early universe. But you'd have to ask a cosmologist what the state of these bounds are. Last I heard it wasn't very strong. –  dmckee Jun 3 '11 at 17:57
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The question might be asked as to why the universe did not produce a gas of black holes? The universe started out at a very low entropy, and black holes have entropy proportional to their horizon areas. So there could not have been a lot of primordial black holes in the early universe.

The lifetime of a black hole is $$ t~=~\frac{5120\pi G^2M^3}{\hbar c^4}. $$ A black hole generated at the start of the universe and a mass of $1.7\times 10^{11}$ kg would Hawking evaporate about now. The final explosion would involve the quick conversion of mass into energy, where $10^5$ kg would be converted into energy in around a second. This is a considerable explosion, but it is less than conversion rate of matter by a star. So this is not as easy as looking at supernovae if primordial black holes are very distant. Also the energy would largely be in the gamma ray domain. The energy output would be a chirp function in its frequency increase and energy. The primordial black hole needs to explode in our local neighborhood. The actual dynamics of the explosion needs further examination, which I am sure is in the literature.

The Chandra satellite could pick up primordial black hole explosion, but so far nothing like this has been found. The lack of such detection gives weight to the estimate that the universe had a very low entropy at the start.


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Chandra has been active for a dozen years without detecting a primordial BH radiation chirp signal. I'm curious what the mass distribution of these theoretical items was expected to be in their BB formation. Is it possible that the creation probability of less massive BHs, say less than 1.7E11 kg, was much smaller than for more massive BHs? –  Michael Luciuk Jun 4 '11 at 3:22
Lawrence, Out of topic, but have you noticed that you lost your reputation once again? tsk tsk :) –  anna v Jun 4 '11 at 3:51
""The primordial black hole needs to explode in our local neighborhood."" A black hole living somewhere in a galaxy, has a lot of chances to grow sucking in interstellar material, hasn't it? –  Georg Jun 4 '11 at 8:53
In terms of entropy, the statement I've heard is that the most likely state for the early universe is one dominated by gravitational waves (like the mixmaster universe), not by black holes. I don't know this based on any personal understanding of the calculations. But if we were going to maximize entropy by having black holes, we would certainly do that by having big ones, not little ones. –  Ben Crowell Aug 9 '11 at 14:12
There are some theories that gamma ray bursts are related to primordial black holes, particularly the "short" type of gamma ray bursts. adsabs.harvard.edu/abs/1996NuPhA.610..500C –  Benjamin Horowitz Aug 10 '11 at 3:21
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