Primordial Black Holes and Dark Matter? John Miller (Oxford) Collaborators: Ilia Musco (Oslo) Antonella Garzilli (SISSA)
Jan 03, 2016
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 it’s “Non-Baryonic” in the sense that it mustn’t 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??
- “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(δ)
- 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