# Focus Week on Primordial Black Holes

Asia/Tokyo
Lecture Hall (Kavli IPMU, Kashiwa, Japan)

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan
Description

Dates: December 2 - 6, 2019

Venue: Lecture Hall, Kavli IPMU, Kashiwa, Japan

Overview:

Primordial black holes were brought in sharper focus by recent progress in theoretical understanding, as well as observations in traditional and gravitational wave astronomy.  There is growing evidence that black holes exist, but their origin is not well understood.  Black holes of primordial origin could be an important component of dark matter in today’s universe, and they could be the key to a range of astrophysical puzzles.  The subject of primordial black holes has a strong overlap with scientific programs at Kavli IPMU, and it engages particle physicists, astrophysicists, and cosmologists to work together, building on synergy of the multidisciplinary institute.

Invited speakers:

Chris Byrnes   (U Sussex)
Bernard Carr  (Queen Mary Univ. of London)
Alexander Dolgov (Novosibirsk State U.)
George Fuller    (UCSD)
Cristiano Germani (ICCUB)
Ruth Gregory (Durham U)
Alexandros Kehagias (National Technical University of Athens)
Kazunori Kohri (KEK)
Savvas Koushiappas (Brown U)
Antonio Riotto  (U Geneva)
Volodymyr Takhistov (UCLA)
Edoardo Vitagliano (UCLA)

Organizing Committee:

Alexander Kusenko (UCLA/Kavli IPMU)
Misao Sasaki (Kavli IPMU)
Masahiro Kawasaki (ICRR/Kavli IPMU)

Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU),
the University of Tokyo, 5-1-5 Kashiwa-no-ha, Kashiwa City, Chiba 277-8583, Japan

Participants
• Alexander Dolgov
• Alexander Kusenko
• Alexandros Kehagias
• Anand Hegde
• ANAND S HEGDE ABLI
• ANTONIO RIOTTO
• Bernard Carr
• Christian Byrnes
• Chulmoon Yoo
• Cristiano Germani
• Edoardo Vitagliano
• George Fuller
• Hayato Motohashi
• Ilya Khrykin
• Kazunori Kohri
• Masahiro Kawasaki
• Metin Ata
• MINXI HE
• Misao Sasaki
• Nao Suzuki
• Naoya Kitajima
• Neil Barrie
• Ruth Gregory
• Sandipan Bhattacherjee
• Satoshi Shirai
• Savvas Koushiappas
• Sergey Ketov
• Shi Pi
• Shuichiro Yokoyama
• Sunao Sugiyama
• Takashi Torii
• Teruaki Suyama
• Tian Qiu
• Tom Melia
• Valeri Vardanyan
• Volodymyr Takhistov
• Wentao Luo
• Yi-Peng Wu
• Ying-li Zhang
• Yun-Long Zhang
Contact
• Monday, 2 December
• 13:00 13:45
Bernard Carr: PRIMORDIAL BLACK HOLES AS THE SOLUTION OF MANY COSMOLOGICAL CONUNDRA 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

Studies of primordial black holes (PBHs) have often focused on constraints on their abundance since this has interesting implications for cosmology even if they never formed. However, in recent years attention has turned to the possibility that they actually exist and solve various cosmological conundra. The most exciting possibility is that they provide the dark matter, although this is only feasible in a few mass windows. In particular, if they form at the QCD phase transition, the tiny collapse fraction required might naturally explain the cosmic photon-to-baryon ratio and the comparability of the PBH and baryon densities. Even if PBHs provide only a small fraction of the dark matter, they might still explain some of the OGLE and quasar microlensing events, the LIGO/Virgo gravitational wave events, the spatial coherence in the fluctuations of the source-subtracted cosmic infrared and soft X-ray backgrounds, some anomalies associated with Ultra Faint Dwarf galaxies, and the supermassive black holes in galactic nuclei. With a suitable extended mass spectrum, they might even explain all these anomalies. So an exciting new era in PBH research has began and various observations will probe this proposal in the near future.

• 13:50 14:35
Masahiro Takada: Constraining PBH with microlensing 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

I here describe how microlensing is powerful to constrain the abundance of PBH.

• 14:40 15:00
Shi Pi: Universal infrared scaling of induced gravitational waves 20m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

I will talk about the general infrared behavior of the power spectrum of a stochastic gravitational wave background $\Omega_{GW}$ produced by stress tensor with bilinear structure, focusing on the gravitational waves induced by scalar perturbations. If the source term is bounded in both frequency and time and reenters the Hubble horizon in a radiation-dominated stage, we have a universal $k^3$-scaling for $k$ smaller than all the physical scales associated with the source, like the peak frequency, peak width, and time duration, etc. I will also talk about possible violations of these conditions and their physical implications.

• 15:00 16:00
IPMU Tea Time 1h 3rd Floor, Fujiwara Hall

#### 3rd Floor, Fujiwara Hall

• Tuesday, 3 December
• 10:00 10:45
Ruth Gregory: Primordial Black Holes and Higgs Vacuum Decay 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

The recent discovery of the Higgs and its mass suggests that our vacuum may by metastable. I will review how we calculate the probability of vacuum decay and discuss how black holes alter this rate. This leads to a constraint either on primordial black hole formation or the Higgs potential.

• 10:50 11:10
Coffee break 20m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan
• 11:10 11:55
Alexander Kusenko: PBH production via scalar field fragmentation 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

I will discuss a novel class of scenarios for PBH formation, which do not place any particular requirements on inflation. A scalar field with a relatively small mass can develop a large VEV during inflation. Relaxation of such a field after inflation can result in fragmentation via a well-known instability, with formation of matter in the form of scalar lumps or Q-balls. If this kind of matter comes to dominate the energy density temporarily, until the lumps decay, then PBH can form from the relatively large fluctuations associated with the small number of "particles" per volume.

• 12:00 12:10
Group Photo 10m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan
• 12:10 13:00
Lunch 50m
• 13:00 13:45
Volodymyr Takhistov：Gaining Insight into PBH Dark Matter with Compact Stars 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

Recent re-evaluations of PBH constraints show that the parameter space window for small PBHs to constitute all of the DM is significantly larger than previously thought. I will discuss how novel signals from tiny PBHs interacting with compact stars can not only lead to new insights into this difficult to probe parameter region of dark matter, but also contribute to resolution of long-standing astronomical puzzles such as the origin of gold and other heavy elements.

• 13:50 14:35
George Fuller: Supermassive Stars and Black Hole Seeds: A narrow mass range with triple trouble; Dark Sector production of supermassive black hole seeds 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

I discuss two topics: (1) A narrow range of masses for primordial metallicity supermassive stars exhibit a coincidence of three factors that may or may not set them up for explosion after the general relativistic instability initiates their collapse; and (2) Late vacuum phase transitions and a variety of other dark sector beyond-standard-model physics may generate fluctuations which give rise to black holes

• 14:40 15:00
Hayato Motohashi: Constant roll and primordial black holes 20m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

Constant-roll inflation is an interesting phenomenological class of inflationary models in which the assumption of inflaton slow-roll is replaced by more general constant-roll condition, and the second slow-roll parameter is not necessarily negligible. The constant-roll inflation with small positive value of the constant-roll parameter has been known to produce a slightly red-tilted curvature power spectrum compatible with the current observational constraints. In this work, we shed light on the constant-roll inflation with the constant-roll parameter with the range $-3/2 < \beta < 0$, which allows for a constant-roll attractor stage generating a blue-tilted curvature power spectrum without superhorizon growth, and investigate its application to production of primordial black holes. References: [1] H. Motohashi, A. A. Starobinsky, J. Yokoyama, JCAP 1509 (2015) 09, 018, [arXiv:1411.5021]. [2] H. Motohashi, S. Mukohyama, M. Oliosi, [arXiv:1910.13235].

• 15:00 16:00
IPMU Tea Time 1h 3rd Floor, Fujiwara Hall

#### 3rd Floor, Fujiwara Hall

• 16:00 16:20
Teruaki Suyama: Clustering of primordial black holes with non-Gaussian initial fluctuations 20m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

We formulate the two-point correlation function of primordial black holes (PBHs) at their formation time, based on the functional integration approach which has often been used in the context of halo clustering. We find that PBH clustering on super-Hubble scales could never be induced in the case where the initial primordial fluctuations are Gaussian, while it can be enhanced by the so-called local-type trispectrum (four-point correlation function) of the primordial curvature perturbations.

• 16:25 16:45
Chulmoon Yoo: Abundance of primordial black holes with local non-Gaussianity in peak theory 20m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

We discuss the effect of local type non-Gaussianity on the abundance of primordial black holes (PBH) based on the peak theory. We provide the PBH formation criterion based on the so-called compaction function and use the peak theory statistics associated with the curvature perturbation with the local type non-Gaussianity. Providing a method to estimate the PBH abundance, we demonstrate the effects of non-Gaussianity. It is explicitly shown that the value of non-linear parameter |fNL| ∼ 1 induces a similar effect to a few factors of difference in the amplitude of the power spectrum.

• Wednesday, 4 December
• 10:00 10:45
Edoardo Vitagliano: Exploring Primordial Black Holes from Multiverse with Optical Telescopes 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

Primordial black holes (PBHs) have long been considered a viable candidate for the dark matter. While the abundance of PBHs with large masses has been constrained with a multitude of astrophysical observations, recent re-analyses of bounds on smaller PBHs have opened a significant window of the previously excluded parameter space for PBHs to constitute the dark matter (DM). In light of this, we revisit and generalize the treatment of PBHs generated by vacuum bubble nucleation during inflation that can constitute all of the DM. The resulting PBHs have a broad mass spectrum distribution with an extended tail that could be detected with optical surveys and that can naturally explain the candidate event in Subaru Hyper Suprime-Cam (HSC) data. Future observations of HSC and other optical surveys such as LSST will be able to not only probe the formation epoch of such black holes, but entirely rule out this generic formation mechanism as the dominant source of PBH dark matter.

• 10:50 11:10
Coffee break 20m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan
• 11:10 11:55
Kazunori Kohri: Formations and evolutions of PBHs in the matter-dominated Universe 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

Primordial Black Holes (PBHs) can be produced even in the early matter
dominated Universe due to collapses of regions which have large
curvature perturbation produced by inflation.

I will review the current status of formations and evolutions of PBHs
in the early matter dominated Universe. If time allows, I will also
mention cosmological and astrophysical constraints on PBHs with
introducing my own bounds on PBHs in terms of polarization of Cosmic
Microwave Background photons due to cosmological accretions onto PBHs
(arXiv:1707.04206 [astro-ph.CO]), cosmological/gamma-ray/cosmic-rays
bounds on evaporating PBHs (arXiv:0912.5297 [astro-ph.CO]), Higgs
phenomenology (arXiv:1708.02138 [hep-ph]), stochastic GWs
(arXiv:1903.05924 [astro-ph.CO], arXiv:1903.05924 [astro-ph.CO]), dark
matter (arXiv:1802.06785 [astro-ph.CO]), ultra-compact mini halo
formations (arXiv:1712.08820 [astro-ph.CO] and arXiv:1905.04477
[astro-ph.CO]), clustering (arXiv:1909.04053 [astro-ph.CO]) and so on.

• 12:00 13:00
Lunch 1h Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan
• 13:00 13:45
Chris Byrnes: Determining the origin of LIGO's merging black holes 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

Although there is a reasonably broad consensus that primordial black holes in the mass range detected by LIGO and Virgo cannot make up more than a small fraction of the dark matter, it remains possible that all of the black holes whose merger LIGO has detected were primordial in origin. I will briefly summarise the evidence and challenges behind this claim, and then focus on how current and future data can be used to discriminate between astrophysical and primordial black holes. I will discuss the black hole mass function, the mass ratio of the merging pairs, and how the merger rate is modified by (small) amounts of primordial non-Gaussianity. Finally, I will briefly show that the QCD transition when the horizon mass was about one solar mass may lead to a large enhancement in the formation rate of primordial black holes with mass below the Chandrasekhar mass limit.

• 13:50 14:35
Cristiano Germani :　Universal threshold and non-linear PBHs statistics 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

Abs: In the first part of this talk I will show that the threshold for PBHs formation, in a radiation dominated universe, can be analytically given by the use of a universal formula for the averaged compaction function. In the second part, I will present the full non-linear statistics of PBH abundance related to Gaussian curvature perturbations.

• 14:40 15:00
Tomohiro Harada: Effect of Inhomogeneity on Primordial Black Hole Formation in the Matter Dominated Era 20m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

We investigate the effect of inhomogeneity on primordial black hole formation in the matter dominated era. In the gravitational collapse of an inhomogeneous density distribution, a black hole forms if the apparent horizon prevents information of the central region of the configuration from leaking. Since information cannot propagate faster than the speed of light, we identify the threshold of the black hole formation by considering the finite speed for propagation of information. We show that the production probability β_inhom(σ) of primordial black holes, where σ is the density fluctuation at horizon entry, is significantly enhanced from that derived in previous work in which the speed of propagation was effectively regarded as infinite. For σ≪1, we obtain βinhom≃3.70σ^(3/2), which is larger by about an order of magnitude than the probability derived in earlier work by assuming instantaneous propagation of information.

• 15:00 16:00
IPMU Tea Time 1h 3rd floor, Fujiwara Hall

#### 3rd floor, Fujiwara Hall

• Thursday, 5 December
• 10:00 10:45
Masahiro Kawasaki : Particle physics models for primordial black hole formation 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

We present two scenarios for primordial black hole (PBH) formation. One is PBH formation
by Affleck-Dine mechanism which produces high baryon bubbles after inflation.
Those high baryon bubbles produce high density contrasts and collapse, which leads to
formation of PBHs. This scenario can account for LIGO PBHs or seeds for supermassive BHs.
The other scenario is PBH formation from non-topological solitons. We perform lattice simulation for
oscillon formation and show that oscillons may produce density fluctuations large enough for PBH formation.
This scenario could account for a significant fraction of dark matter.

• 10:50 11:10
Coffee break 20m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan
• 11:10 11:55
Alexander Dolgov: Astronomical data in favor of abundant population of PBH in the universe and the mechanism of their formation. 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

A review is presented on the recent astronomical data, which indicate that the bulk of
black holes in the universe in all mass ranges are mostly primordial. A mechanism of
PBH formation with the necessary properties is discussed.

• 12:00 13:00
Lunch 1h Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan
• 13:00 13:45
Antonio Riotto: Primordial Black Holes and Gravitational Waves 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

We will discuss some aspects of primordial black holes and how LISA will be able to detect the associated gravitational waves.

• 13:50 14:35
Alexandros Kehagias: Unstable modes in Kerr Black Holes 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

I will discuss stability issues of classical fields in the Kerr black hole backgrounds.
In particular, I will describe mass bounds for instability of massive scalar and vector modes in the subcritical regime. In addition, some results on the construction of massive vector growing modes will be presented.

Blackboard was used. No material files attached.

• 14:40 15:00
Minxi He: On the formation threshold of rotating primordial black holes 20m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

The spin of black holes is one of the important observables in gravitational wave detection. As LIGO/Virgo have found a number of events of black hole mergers and some of them are of small effective spins, one possibility is that these black holes are primordial because it is believed that primordial black holes (PBH) have small spin. One important factor that affects the spin distribution of PBHs is the modified formation threshold by rotation. We discuss the effect of the angular momentum on the formation threshold of primordial black holes formed in the radiation-dominated epoch by direct gravitational collapse of large primordial density perturbations. We find that the threshold is increased in proportional to the square of the angular momentum.

• 15:00 15:20
IPMU Tea time 20m 3rd floor, Fujiwara Hall

#### 3rd floor, Fujiwara Hall

• Friday, 6 December
• 10:00 10:45
Savvas Koushiappas: Detecting primordial black holes 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

I will discuss how we can infer the existence of primordial black holes. I will focus on stellar kinematics, merger rates and the possibility of a WIMP particle dark matter / primordial black hole co-existence using 1-point function statistics.

• 10:50 11:10
Coffee break 20m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan
• 11:10 11:55
Misao Sasaki: TBA 45m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

TBA

• 12:00 13:00
Lunch 1h Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan
• 13:00 13:20
Yuichiro Tada: Primordial black hole tower: Dark matter, earth-mass, and LIGO black holes 20m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

We investigate a possibility of primordial black hole (PBH) formation with a hierarchical mass spectrum in multiple phases of inflation. As an example, we find that one can simultaneously realize a mass spectrum that has recently attracted a lot of attention: stellar-mass PBHs (~O(10)M_\odot) as a possible source of binary black holes detected by LIGO/Virgo collaboration, asteroid-mass (~O(10^-12)M_\odot) as a main component of dark matter, and earth-mass (~O(10^-5)M_\odot) as a source of ultrashort-timescale events in Optical Gravitational Lensing Experiment microlensing data. The recent refined de Sitter swampland conjecture may support such a multiphase inflationary scenario with hierarchical mass PBHs as a transition signal of each inflationary phase.

• 13:25 13:45
Anand Hedge: Gravitational Thomas Precession: New Window to Study PBHs 20m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

Thomas Precession e?ect is purely a relativistic e?ect which has been studied extensively in the premises of Special Relativity. From a relativist's perspective, it is due to the non-commutative nature of Lorentz groups. Thomas Precession in the context of general relativity can be realized as a Fermi-Walker transport equa- tion. In this paper, we study Thomas Precession of spin vector upon a passage of gravitational waves. We observe that, the gravitational waves produce Gravi- tational Thomas Precession when they pass through the system characterized by spin. Conventionally the consequence of Thomas Precession in special relativistic regime can be observed in the ?ne structure of Hydrogen. We ?nd out that, in the case if simpli?ed toy model of binary mergers, the ?ne structure energy levels of hydrogen to be shifted. This is unique signature of the source and encoded like a memory in the atomic spectra. We further propose this e?ect can be observed in events involving Primordial Black Holes(PBHs) of mass as light as O(10􀀀13)M?. In this paper we examine the signatures of distribution of primordial blackholes us- ing aforementioned gravitational Thomas Precession. Further we propose the idea of mapping the background in terms of this new Gravitational Thomas Precession e?ect and thereby, use it to probe and put constraint on mass spectrum.

• 13:50 14:10
Yi-Peng Wu: Statistical bias for black hole mass functions from the inflationary power spectrum 20m Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan

The spatial configuration near local extrema of the primordial density perturbation restricts the statistics of high-density peaks valid for the formation of
primordial black holes (PBHs). We argue that the condition for density extrema to be maxima in the peak statistics leads to an universal enhancement to the extended mass function of PBH estimated by the Press-Schechter method. For the inflationary spectrum
in the narrow-spike class, the mass function from the peak theory is up-lifted in all mass range, indicating a severer constraint to models in the favor of considering PBHs as all dark matter.

• 14:15 14:35
Round table discussion Lecture Hall

### Lecture Hall

#### Kavli IPMU, Kashiwa, Japan

Kashiwa, Japan
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