Primordial black hole scenario

for the gravitational-wave event GW150914

Refs. Misao Sasaki, Teruaki Suyama, Takahiro Tanaka, Shuichiro Yokoyama, PRL 117(2016) 061101

1. Introduction

2. Primordial black hole scenario 3. Result

No.2

Teruaki Suyama (RESCEU, University of Tokyo)

Discovery of BH binary by LIGO

(LIGO)

GWs from the merger of a BH binary was detected for the first time in Sep 14, 2015 (GW150914).

BH mass : 36 , 29Source redshift : 0.09Event rate : 2-53 (0.6-12) /Gpc^3/yr

We now know that there are numerous BH binaries in the Universe.

The event GW150914 was a bit unexpected.Before LIGO, there had been no observational indicationof such heavy BHs.

Natural questions

What is the origin of such heavy BHs?

What is the mechanism of the binary formation?

We proposed the detected GW was caused by the merger oftwo primordial black holes (PBHs).

PBH : BH produced just after the Big-Bang. (0.1 ms for 30 )

Requirements for any successful scenario

1. Formation of about 30 BHs.

2. Formation of about 30 BH bianries.

3. Event rate consistent with LIGO observation.

In the PBH scenario, these requirements can be naturally satisfied.

Requirements for success

1. Formation of about 30 BHs. One of the popular mechanism of the PBH formation is the direct gravitational collaplse of large primordial density perturbation.

There are inflation models that predict such primordial perturbation for the formation of PBHs.

-10

-8

-6

-4

-2

0

10-4 10-3 10-2 10-1 100 101 102 103 104 105 106

log 1

0 P

k [Mpc-1]

Power spectrum of primordial perturbationpredicted in a particular type of multiple-field inflation model. PBHs are formed out of perturbations with their scale at the spike.

(Kawasaki, Kusenko, Yanagida, 2012)

2. Formation of about 30 BH binaries.

There is simple physical mechanism proposed by a paper (ApJ 487, L139-142, 1997) by T.Nakamura, M.Sasaki, T.Tanaka and K.Thorne (see the picture below).

In this work, I simply assume that PBHs all having 30 are produced in the early Universe instead of assuming particular inflation model.

For simplicity, we assume initial PBHs distributed randomly in space.

There is a small but non-vanishing probability that two neighboring BHs happen to be much closer than the mean distance. Such BHs in a pair defeats the cosmic expansion and attract each other by their gravity. Due to the distant third BH, BHs in a pair receives tidal force during their infall motion. As a result, direct collision is circumvented and the BHs in a pair forms a binary.

3. Event rate consistent with LIGO observation. PBH number density is left as a free parameter in this scenario.

Thus, the problem becomes to clarify if there is a parameter region that can explain the event rate by LIGO without manifest contradiction with other constraints.

PBH merger rate

LIGO

CMB constraint

We found that the event rate in the PBH scenario becomes consistent with the one given by LIGO if PBHs constitute about thousandth of whole dark matter.This result also excludes a possibility that 30 solar-mass PBHs are whole dark matter. This is a first time to exclude PBH dark matter in this mass range only by the use of the GW observations.There is uncertainty in the CMB constraint that is diffcult to quantify.Next thing to do is to investigate how to test the PBH scenario.