Primordial Black Holes and Dark Matter?

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Primordial Black Holes and Dark Matter?

• John Miller
• (Oxford)

Collaborators Ilia Musco (Oslo)
Antonella Garzilli (SISSA)
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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
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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 lt 1
min after the Big Bang?? - standard matter
and radiation locked up in PBHs before
nucleosynthesis
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• Constraints from microlensing, etc

Allowed mass range for PBHs as significant dark
matter - 1017 1026 g (rs 100 fm 10-2 cm)
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• 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?

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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
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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 - those above a critical
amplitude collapse to form black holes -
smaller ones disperse into the background - PBHs
formed from growing-modes follow a scaling
law
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Scaling law for PBHs
What you get in PBHs depends on P(d) - 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
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Conclusions - Having microscopic PBHs as the
dark matter is 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
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Growing-mode perturbations - have a particular
combination of density and velocity
perturbation which makes them hold together
as they evolve - define density and velocity
perturbations de (e eb)/eb du (U
Ub)/Ub - in the linear regime during the
radiation-dominated era du - (de /4) for
a growing mode - typically have an over-density
surrounded by a compensating under-density
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The wind and the void