Primordial Black Hole Detection 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?"
 A: 
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.
A: 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.
LC
