Cosmophysics Group Seminar
20-February 2020
Yoshihisa Kitazawa, "Holographic β function in de Sitter Space"
Abstract:
We investigate infra-red logarithms in de Sitter space from holographic perspective. We employ Fokker-Planck equation to investigate the evolution of de Sitter entropy 1/g = π/GN H2: here H2 is Hubble parameter and GN is Newton’s constant. Under the Gaussian approximation, we obtain the dynamical β function of g. It is asymptotically free toward the future. It also possesses the UV fixed point indicating the Universe started de Sitter expansion at the Planck scale with minimal entropy. The pre-inflation period exists to prepare the special initial conditions for inflationary universe. reference: H. Kitamoto and Y. Kitazawa, T. Matsubara, Phys.Rev. D101, no. 2, 023504 (2020) [arXiv:1908.02534][hep-th].
30-January 2020
Tomomi Sunayama, "Cosmology with Subaru Prime Focus Spectrograph (PFS)"
Abstract:
FS (Prime Focus Spectrograph), a next generation facility instruments on the Subaru telescope, is a wide-field, massively multiplexed, and optical & near-infrared spectrograph. In the PFS cosmology survey, emission line galaxies (ELGs) in the wide redshift range from z= 0.6 to 2.4 over 1400 square-degree will be observed. The unique redshift range for the PFS cosmology survey is at z>2. We envision that we will start our survey from 2022, and I will describe strategies to achieve the scientific goals as well as the possible systematic problems for future fiber-fed spectroscopic surveys. reference:
24-October-2019
Toyokazu Sekiguchi, "Smallest haloes in wino dark matter"
Abstract:
Neutral wino is a natural candidate of dark matter in split- supersymmetry. Indirect detection is a promising probe of wino dark matter, with its annihilation enhanced non-perturvatively (i.e. Sommerfeld enhancement). In theoretical prediction, halo formation at the low-mass end is a key ingredient. For this purpose, we investigate kinetic decoupling of wino dark matter and consequent dark matter density perturbations. We show that inelastic processes involving charged wino, which are relevant for kinetic equilibrium at late times, shuts off abruptly. This results in boosted acoustic peaks in density power spectrum at horizon scales around the kinetic decoupling. Based on an analytic modeling of subhalo evolution, we estimate the subhalo mass function of (dwarf) galaxy-sized haloes and effects on the annihilation boost factor. We also discuss an application of our analysis to SU(2)_L multiplet minimal dark matter. reference:
29-August-2019
Kenji Toma, "Theoretical Interpretation of the M87 black hole shadow imaged by EHT"
Abstract:
The Event Horizon Telescope (EHT) observed the central compact radio source of the elliptical galaxy M87 at 1.3mm with unprecedented angular resolution, and on April 10, 2019, the EHT Collaboration reported the asymmetric ring-like image reconstructed from the data. We generated a theoretical model image library of magnetohydrodynamic simulations and photon ray tracing calculations in the steady rotating black hole spacetime with broad ranges of parameter values to compare the observed data with the models. As a result, we confirmed that the asymmetric ring is consistent with strong gravitational lensing and Doppler beaming of synchrotron emission from a hot plasma orbiting near the supermassive black hole event horizon. In this seminar, we mainly review our theoretical interpretation, shown in the EHT Collaboration paper V. This time EHT did not have sufficient number of stations for detecting extended emission component such as the jet and accretion flow. We try to discuss how to identify the driving mechanism of the jet in M87 with next EHT observations. reference:
08-Aug-2019
Sai Wang「EDGES constraints on SMBH seeds」
Abstract: We investigate the influence of accreting SMBH seeds on the global 21cm spectrum in cosmic dawn. The electromagnetic radiations emitted are expected to enhance the IGM kinetic temperature, which can diminish the 21cm brightness temperature. We use the EDGES low-band measurement to constrain the initial redshfit and mass of SMBH seeds. reference:
01-August-2019
Takuya Hasegawa, "MeV-scale reheating temperature and thermalization of active neutrinos by radiative and hadronic decays of massive particles"
Abstract:
We determine the minimum value of the reheating temperature set by Big-Bang Nucleosynthesis by considering both radiative and hadronic decays of long-lived massive particles. In the current study, effects of neutrino oscillation and neutrino self-interactionare taken into account in the calculation of the neutrino thermalization. Also, we adopt updated observational bounds on the primordial abundance of light elements. An obtained lower bound on the reheating temperature is T_RH > 1.8MeV in the case of the 100% radiative decay, and T_RH > 2 MeV – 5 MeV in the case with hadronic decays depending on the mass of the massive particles and the hadronic branching ratio of the decay. reference:
25-July-2019
Kazunori Kohri, "SMBHs"
Abstract:
TBA reference:
11-July-2019
Shohei Saga, "The vector mode in the observational cosmology"
Abstract:
n the context of the cosmological perturbation theory, perturbations can be decomposed into the scalar, vector, and tensor modes. Current observations strongly support only the scalar mode. It is known that the vector mode does not arise from the linear perturbation theory in the standard cosmology. In this talk, I will introduce the second-order vector mode, which is inevitably generated in the standard cosmology. I also show the way to observe the vector-mode signature. Finally, I will mention a role of the second-order vector mode in observational cosmology. reference:
26-June-2019
Tomo Takahashi, "Aspects of non-minimal inflation"
Abstract:
We discuss models of inflation where the inflaton is non-minimally coupled to gravity in some general setting, focusing on their predictions on inflationary observables such as the spectral index and tensor-to-scalar ratio. We consider both metric and Palatini formalisms of gravity and argue that how some inflation models can be alleviated by introducing a non-minimal coupling to gravity. The issues of distinguishing variants of such models are also discussed. reference:
20-June-2019
Yu Sang "Stochastic gravitational wave background from reheating"
Abstract:
In preheating stage after inflation, fluctuations of inflaton field may be enhanced exponentially by parametric resonance. The large and time dependent density inhomogeneity will produce stochastic gravitational wave background. We use lattice simulation to study the non-perturbative evolution of the inflaton field and the production of gravitational waves. We focus on axion monodromy inflation model and find that significant single-peak stochastic gravitational wave backgrounds are generated during preheating. reference:
13-June-2019
Takahiko Matsubara, "On the large-separation expansion of peak clustering"
Abstract:
The statistics of density peaks have many applications to theories of structure formation. A pioneering work by Bardeen et al. (1989) derived analytic expression of peak abundance. However, analytic treatment of peak clustering is not easy because of its multi-dimensional integrals. In this seminar, I will report some progress of the analytic results in large-separation expansion of peak clustering. I will discuss possible applications to the clustering of primordial black holes. reference:
30-May-2019
Takahiro Terada, "Bouncing Universe from Nothing"
Abstract:
We find a class of solutions for a homogeneous and isotropic universe in which the initially expanding universe stops expanding, experiences contraction, and then expands again (the "bounce"), in the framework of Einstein gravity with a real scalar field without violating the null energy condition nor encountering any singularities. Two essential ingredients for the bouncing universe are the positive spatial curvature and the scalar potential which becomes flatter at large field values. Depending on the initial condition, either the positive curvature or the negative potential stops the cosmic expansion and begins the contraction phase. The flat potential plays a crucial role in triggering the bounce. After the bounce, the flat potential naturally allows the universe to enter the slow-roll inflation regime, thereby making the bouncing universe compatible with observations. If the e-folding of the subsequent inflation is just enough, a positive spatial curvature may be found in the future observations. Our scenario nicely fits with the creation of the universe from nothing, which leads to the homogeneous and isotropic universe with positive curvature. As a variant of the mechanism, we also find solutions representing a cyclic universe. reference: H. Matsui, F. Takahashi and T. Terada, "Bouncing Universe from Nothing," arXiv:1904.12312 [gr-qc].