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