John's new Presentation

Primordial Black Holes and Dark Matter? John Miller (Oxford) Collaborators: Ilia Musco (Oslo) Antonella Garzilli (SISSA) Contents of the Universe: Evidence from the CMB, supernovae, gravitational lensing, nucleosynthesis and motion of stars in galaxies ~ 73% in dark energy ~ 23% in dark matter

~ 4% in atomic matter 2 What makes up the dark matter? - we know that its Non-Baryonic in the sense that it mustnt mess up cosmological nucleosynthesis - widely thought to be particles which are: - Weakly-Interacting (mainly gravity) & Cold(??) - main candidate: supersymmetric particles But could it be Primordial Black Holes (PBHs) formed

< 1 min after the Big Bang?? up in - standard matter and radiation locked 3 PBHs before Constraints from microlensing, etc: Allowed mass range for PBHs as significant dark matter - 1017 1026 g (rs ~ 100 fm 10-2 cm) 4

Could we detect the allowed range by interactions with stars? - PBHs moving in the Galaxy with ~ virial velocity (2 x 107 cm/s) could collide with stars - get interaction with stellar matter via dynamical friction - brightening of star - asteroseismic disturbance - possible ignition of nuclear reactions How could so many PBHs with masses in this range be formed? 5 Standard picture for the formation of cosmic

structures - originated as small quantum fluctuations then inflated onto supra-horizon scales - subsequently come back inside the horizon again as the Universe continues to expand; they can then collapse - start on the supra-horizon scale as a mixture of growing and decaying modes in a linear regime - but the decaying modes soon become small, leaving just the growing modes

- these are special types of perturbation 6 Large-scale structure: - comes from perturbations re-entering the horizon in the matter-dominated era In the radiation-dominated era: - just 2 possibilities for re-entering perturbations: to - those above a critical amplitude collapse form black holes

- smaller ones disperse into the background - PBHs formed from growing-modes follow a scaling law 7 Scaling law for PBHs: What you get in PBHs depends on P() - probably need help from a phase transition to temporarily soften the equation of state - mechanism might work but it looks difficult to get the right number of PBHs 8

Conclusions is - Having microscopic PBHs as the dark matter not ruled out by present observations - Finding direct evidence for them is hard but not impossible - There is a plausible mechanism to form them but it looks difficult to get the right number - seems to need a continuous phase transition

and the fluctuation spectral index n to increase suitably at small scales - Bottom line: Having PBHs as the dark matter is a very long shot, but is not ruled out and probably deserves further study 9