Colloquiums archive 2022

Expanding edges in quantum Hall systems can become a simulator of quantum 1+1 dimensional expanding universes. In these systems, edge exciations are represented as a chiral scalar field in curved spacetimes. We investigate Hawking radiation and entanglement behavior predicted by this model assuming that the expansion law of the edge region corresponds to a de Sitter universe.  As observable quantities for the quantum field, local spatial modes associated with detection regions are introduced using window functions for the field, and their correlations are evaluated. We found impact of Hawking radiation caused by the edge expansion on auto-correlation functions of the local modes, and confirmed that entanglement death due to Hawking radiation occurs. This behavior of entanglement is related to ``quantum to classical transition" in cosmic inflations.

AdS Instablity conjecture is that AdS spacetime may be unstable under large class of perturbation. This conjecture has begun since work of Bizon and Rostworowski.

In this instability, AdS confinement and non-linearity of Einstein equation can be considered as essential. So, we can expect that other objects which obey non-linear equation and reflection boundary condition shows phenomenon corresponding to AdS instability. We will consider test Nambu-Goto string on Global AdS spacetime as one of such things.

In this talk, we first will review the AdS instability and previous works which deal with test Nambu-Goto string on AdS spacetime. Then we consider Nambu-Goto string on AdS spacetime whose endpoints are fixed on AdS boundary.

In quantum field theory, unitary representation is a powerful tool to classify the elementary particles in given spacetime. It can apply to not only flat space but also curved spacetime whose isometry is maximally symmetric case. In this seminar, we consider the unitary representation of SO(2,2) group in AdS3 spacetime. Then, the following results are obtained: 1.mass bounds for each spin, 2. relationship between mass in AdS3 and conformal dimension in CFT2, 3. mode functions in AdS3 for any spin. [Slides]

 I will (try to) derive the F theorem in (1+2)-dimensional spacetime, using AdS/CFT correspondence.

The F theorem implies the decrease of free energy in QFT in RG flow, which can be expressed by the decrease of “renormalized entanglement entropy”. I show our holographic analysis to show the F-theorem.

I will review the basics of a certain decomposition for spacetimes including famous ADM decomposition and present several key equations with some applications such as area bound, uniqueness theorem etc. [Note]

In this QG colloquium, I will discuss the simulation of PBH formation (type I) when a time-dependent equation of state is considered,  showing some specific features of this scenario compared with the standard case of a pure radiation-dominated Universe. Two cases of relevant interest will be explored: the QCD crossover and a possible smooth phase transition beyond SM theories. [Slides]

In the context of the quantum field theory in curved spacetime, detectors are used to pick up the quantum nature of the fields. By using the detectors we can observe the thermal behavior of the Hawking radiation or the nonlocality of the quantum fields(i.e. quantum entanglement). However, most of the analysis of the detector is evaluated perturbatively, and we don’t know what happens when the fields and the detectors couple strongly.

The instantaneous switching detector is the detector model which interacts with fields for a very short time, and it is possible to evaluate nonperturbatively.

In this talk, I will shortly review the concept of quantum measurement and the detector, after that, I will talk about the behavior of the instantaneous detectors in spacetime with the moving boundary which is the toy model of the black hole radiance. We will discuss how the thermality, nonlocality, and quantum fluctuation of the fields can be seen via the detectors. [Slides]

We investigate the formation of type II primordial black holes (PBHs), clarified by non-monotonic areal radius functions with respect to radial coordinates and induced by the gravitational collapse of several large amplitude of density fluctuations in the radiation-dominant universe. Type II PBHs have unique features of the spacetime structure compared to a typical model of PBHs (type I). We will show some results of the calculations about the relation between mass and amplitude of the primordial fluctuation. [Slides]

We consider the periapsis shifts of bound orbits of stars around a spherically symmetric black hole dressed by accreting matter. Assuming that the accreting matter is very sparse, we construct a perturbed metric corrected for the existence of the accreting matter. Then we consider massive particle orbits around the black hole taking the perturbed geometry into account.

First, we numerically solve the geodesics equations. Second, we analytically calculate the periapsis shifts using the osculating method. Finally, we numerically calculate the redshifts, and discuss the observability of the effects of the accreting matter. [Slides]

 I will review the singularity theorem based on the classical entropy bound established by Bousso and Shahbazi-Moghaddam (arXiv:2201.11132), which is a topic I am interested in recently. I will start with a brief review on the Bekenstein's entropy bound, which provides an upper bound of the entropy by its surface area and is expected to hold when gravity is weak enough. Then I will explain the Bousso's entropy bound, that is a generalization of the Bekenstein bound to curved space, with examples of cosmological spacetime. The singularity theorem by Bousso and  Shahbazi-Moghaddam relates a spatial region breaking the Bekenstein bound to the presence of singularity, assuming the Bousso bound is satisfied.

The event page is HERE.

We investigate the probability distribution of the effective spin, the mass ratio, and the chirp mass of primordial black hole (PBH) binaries, incorporating the effect of the critical phenomena of gravitational collapse.

As a simplest model, the binaries are assumed to be formed from two PBHs that are randomly chosen according to their probability distribution.

We find that, although the critical phenomena can lead to formation of rapidly spinning PBHs, the RMS of the effective spin of PBH binaries is very small, $\sqrt{\langle\chi_{\mathrm{eff}}^2\rangle}=8.41\times10^{-4}$.

We also see that there is no anti-correlation between the RMS of the effective spin and the mass ratio, which is inferred from observations, in this model. [Slides]

The Einstein-Vlasov system is a collisionless self-gravitating many-particle system in general relativity. For the static and spherically symmetric case, the existence and stability of the solutions are well-known. However, for the stationary rotating case, these properties have not been fully revealed due to the less symmetry.

In this talk, we investigate a method to construct the rotating Einstein-Vlasov system by considering the five-dimensional spacetime with a R×SU(2)×U(1) symmetry. The five-dimensional rotating black hole solution, which called the Myers-Perry black hole, has a R×U(1)×U(1) symmetry corresponding to two independent planes of rotation. When the angular momenta are equal, the spatial symmetry gets enhanced to SU(2)×U(1) symmetry.

Therefore the spacetime has five conserved charges for a particle. We try to construct rotating solutions to the Einstein-Vlasov system by considering in the spacetime with the same symmetry and using the five charges. [slides]

It is worth evaluating the entanglement entropy(EE) and the logarithmic negativity(LN) of two disjoint intervals as basic examples so that we will understand the behavior of the quantum entanglement of a general system in QFT. In general, it is difficult to evaluate an entanglement entropy of QFT as it has a vast amount of degree of freedom. Fortunately, in 2-dimensional CFT, the replica trick is a helpful evaluation method. In this talk, we will review the replica method in 2-dim CFT and see why the Liouville field theory is essential for holographic CFTs. Then, we will present the EE and the LN of two disjoint intervals of the Liouville CFT by constructing the numerical solution of the Liouville equation. [slides]

Due to the infrared(IR) divergence problem, the S-matrix, which describes scattering processes, is not well-defined in the standard formulation in the quantum field theory. Dressed state formalisms are formulations that respect the existence of the S-matrix. Kulish-Faddeev(KF) formalism may be the most famous dressed state formalism. KF formalism is constructed based on the Dollard formalism for scattering in quantum mechanics. However, several issues make the formulation ill-defined.

In this talk, I propose a dressed state formalism. It can construct divergence-free S-matrix at least in the quantum electromagnetism. This formulation includes asymptotic symmetry and memory effect, as expected from Strominge’s IR triangle relations. By comparing with the Dollard formulation in the non-relativistic case, I will examine the validity of the formulation from a mathematical perspective. If possible, I also discuss soft graviton in our formalism and its implication.

Neutron stars (NSs) are one of the final forms of star evolution. Due to the high density and the pressure, strong gravity and quantum interaction among matters (i.e. eq. of state) are dominant. And they determine even macroscopic physical quantities such as mass and radius. Thus it is hoped that we can test the gravity theory through observation of NSs. However, when we consider NSs under modified gravity theory, these two factors (gravity theory and EOS) degenerate, and their effects cannot be distinguished by a single physical quantity.

 To overcome this problem, we are considering the way to observationally determine them by utilizing a few observables, mass-radius relation, and tidal deformability. In this talk I will discuss the spherical hydrostatic star configuration (as background) and tidal deformation phenomena under F(R) gravity with some preliminary results. [slides]

A tabletop experiment of the quantumness of gravity has been explored for the last few years such as BMV proposals. Its main purpose is to find out whether the gravitational field is also superposed for superposed mass source. Although many proposals similar to the BMV proposals are investigated, they do not work when the other quantum interaction is mixed in the setup. Therefore, we must get rid of the other quantum interactions during the experiment, or consider a new proposal to pick out the quantum nature unique to gravity.

In my talk, I will present a new quantum gravity witness using a quantum clock collaborated with Maeda-kun, Nambu-san, and Osawa-kun. The main idea of this talk is to find a quantum nature unique to gravity in the interference experiment by using the specific feature that only the gravity couples to the energy level system. We will see that the decoherence time scale of the interference pattern will reflect the quantum property peculiar to the gravity, unlike BMV proposals. [slides]

False vacuum decay is a transition of a field from a metastable state (false vacuum) to a more stable state (true vacuum). The phenomenon is triggered by e.g. a quantum tunneling, a thermal excitation. In this talk, we discuss false vacuum decay induced by a rotating black hole. Especially, we study the decay of Kerr de-Sitter false vacuum. 

 In the colloquium, we first present the conventional method to analyze false vacuum decay in spherically symmetric spacetimes and the difficulty stems from the non-spherically. Next, we show some assumptions e.g. the transition with the time-independent shell, to circumvent the difficulties. Finally, we discuss how the spin in the BH affects false vacuum decay. [slides]

The propagation of magnetohydrodynamic (MHD) waves is studied in an inner region of black hole magnetosphere. By using a canonical type formulation for the propagation of MHD disturbances in magnetized plasma around a black hole, the basic properties and the numerical calculations of motion of the locus of simultaneous fronts of wave packets are presented. We define the “magneto-acostic metric”. Hence, we discuss the “magneto-acostic horizon”, which corresponds to the magnetosonic surface for ingoing flows, and “magneto-acostic ergoregion” where is the super-magnetosonic region for flows in the toroidal direction. In order to determine the magneto-acostic metric, it is necessary to solve the background fluid distribution and magnetic field distribution. Therefore, we discuss the constraints to  the solution that satisfies the boundary conditions at the magneto sonic surface and the event horizon. [slides]

A massive black hole candidate of 4 million solar mass exists at the center of our galaxy, which is called Sagittarius A*; Sgr A*. The observational identification of Sgr A* as a black hole candidate was awarded 50% of the Nobel Prize in Physics 2020. Within a few 1000AU  (1AU = 1.5x10^8 km) of Sgr A*, a star (named S0-2 or S2) is orbiting around Sgr A* with an orbital period of 16 years. (I and collaborators have been observing S0-2 since 2014 using the Subaru telescope.) The motion of S0-2 around Sgr A* provides us with a laboratory of black hole spacetime. The 1st post-Newtonian (1PN) effect of Schwarzschild spacetime has already been detected from the observational data of S0-2 motion, and then the Newtonian gravity is rejected. (Kerr and Schwarzschild metrics cannot be distinguished at 1PN order, because the BH-spin effect appears from 1.5PN order in the stellar motion.) 

However note that, in the analysis detecting the 1PN Schwarzschild effect, the Newtonian gravity and 1PN Schwarzschild spacetime were compared, and the possibility of spacetime other than 1PN Schwarzschild has not been investigated. Therefore, in order to compare the 1PN Schwarzschild and the other spacetime, we have been performing a Parametrized Post-Newtonian (PPN) test of black hole spacetime. ("PPN formalism" is a technique to investigate various spacetime metrics without specifying the gravity theory.) Currently (when I am writing this abstract), I think I may have found a deviation from 1PN Schwarzschild metric from S0-2 observational data. If my numerical calculation of the chi-squared fitting (of observational data and PPN model) finishes before this QG colloquium, the result will be fixed. If the deviation from 1PN Schwarzschild is true, then it may be regarded as evidence of a modified gravity theory, or evidence that a vacuum spacetime such as Schwarzschild does not match with observational data and we need to introduce some matter effects. 

In this colloquium, I report the current status of my PPN analysis. [slides]

After reviewing the scalar-Gauss-Bonnet-Einstein gravity theory, we show how we can construct models which realize the FRW Universe or static and spherically symmetric space-time. [slides]

We study the effects of velocity dispersion on the formation of primordial black holes (PBHs) in a matter-dominated era.  The velocity dispersion originates from smaller-scale inhomogeneities. We consider two mechanisms for the production of the velocity dispersion depending on whether the virialization of the smaller-scale perturbations with the wave number $\tilde k$ takes place before the collapse of the larger would-be PBH scale with the wave number $k_{\rm PBH}$. We assume that, if the virialization is completed before the collapse of the larger scale, the velocity dispersion is released at the time when the mean density of the larger scale is equal to the virialized halo density. On the other hand, if the collapsing time of the larger scale is shorter than the virialization time of the smaller scale perturbations, the velocity dispersion is assumed to be directly released when the smaller-scale density perturbations reach the non-linear regime. Once the velocity dispersion is shared in the larger scale, we can estimate the effect of the velocity dispersion comparing the two time scales of the free-fall and the particle crossing. As a result, we obtain the threshold $\delta_{\rm th}$ of the amplitude of the density perturbation for the would-be PBH scale at the horizon entry.  Letting the primordial density power spectrum be $P(k)= \sigma^2 (k/k_{\rm PBH})^n$, we obtain $\delta_{\rm th}\sim \sigma^{2/5}$ for $n\leq 2$ and $\delta_{\rm th}\sim \sigma^{4/(8+n)}$ for $n> 2$.

The handwriting note (to be updated in real-time) will be shared in here.