> Primordial black hole would have formed before big bang nucleosynthesis and so would not contribute to the relevant amount of baryonic matter.
What would they have been made up? Do quarks have mass, and there was enough of them? Or does energy through E=MC^2 mean that enough pure energy density can cause a black hole?
>What would they have been made up? Do quarks have mass, and there was enough of them? Or does energy through E=MC^2 mean that enough pure energy density can cause a black hole?
I'm not sure exactly when primordial blackholes would have formed but I think it would have been sometime after inflation, though perhaps before baryogenesis. Here's the reasoning: I think it would be after the GUT epoch so gravity would have splintered off from the other forces and I think it would have been after inflation started so that inflation had a chance to exaggerate the scale of quantum fluctuations to create the necessary size & scale of density fluctuations for blackholes to form.
Energy alone is enough to form a blackhole. However, it's not the energy density per se. That's sort of a necessary but not sufficient condition since you also need the surrounding spacetime to be at a low enough density relative to the region where you are expecting a blackhole to form.
This, incidentally is a common source of confusion about the big bang. "Why didn't it just form a blackhole?" The answer is because ALL of the spacetime was at the same density.
I could say that a small sphere is denser than a large sphere because the distances between points are smaller on average, but that requires the spheres to be embedded in something with a distance metric(Euclidean 3-dimensional space).
What is the thing that spacetime is embedded in that provides a pointwise distance metric?
This may help: Given a spacetime manifold with metric and no matter that is flat an infinity, you can mathematically define the amount of curvature in it, which is directly related to how much energy in conventional forms (say, kinetic) that could be transferred to matter. Just as two merging black holes can emit gravitational waves, which have energy in this sense and escape to infinity, gravitational waves coming in from infinity can collapse into a black hole where no black hole previously existed.
This can be (kinda) explained via the notion of space-time expansion. (This is an intrinsic phenomenon, having nothing to do with objects moving through space.) As the space-time expands, its density becomes lower. The (relative) change in density can be measured indirectly by measuring (the effects of) the expansion.
A technical explanation of the expansion involves the inflaton field.
(There's also energy density of the gravitational field.)
