## 2021/7/19 (月)## Title: SU(N) gauge natural inflation †## Speaker: Tomohiro Fujita †Abstract: I'll discuss general attractor solutions for SU(N) gauge fields coupled to a rolling axion via the Chern-Simons interaction during inflation. It has been known that the SU(2) gauge field has an isotropic and homogeneous attractor solution, which leads to rich phenomenology including the amplification of primordial gravitational waves. We find a generic way to construct similar background solutions for SU(N), and enumerate all the isotropic stationary solutions for SU(3) and SU(4). We also confirm that numerical solutions with random initial conditions converge into these attractors. Interestingly, the gauge field amplitudes (i.e. VEVs) tend to increase as N of SU(N) grows, which may provide interesting observational signatures. ## 2021/7/12 (月)## Title: Black hole information paradox and fine grained entropy in gravitational system †## Speaker: Yusuke Yamada †Black hole information paradox has been one of the unsolved problems in quantum gravity. One can quantify the problem by looking at the behavior of the von Neumann entropy, which is also known as fine-grained entropy. Recently, the solution to the information paradox was proposed, which is the new rule to obtain improved fine-grained entropy formula in gravitational system. The improved formula gives the fine-grained entropy consistent with the unitarity. Refs: ## 2021/7/5 (月)## Title: Ceterum autem censeo, Gravitas esse quantizo †## Speaker: Kohei Kamada †Niels Bohr has once argued in his private communications Refs. ## 2021/6/28 (月)## Title: Environmental noise estimation and detector characterization of gravitational wave detectors †## Speaker: Jun’ichi Yokoyama †In the first part, I will introduce our recent paper References: ## 2021/6/21 (月)## An Introduction to skyrmions †## Fumio Uchida †Abstract: References: ## 2021/6/7 (月)## Title: Quantum Simulation (for cosmology?) †## Speaker: Soichiro Hashiba †Abstract: References: ## 2021/5/31 (月)## Inflationary bispectrum from symmetry and locality. †## Jason Kristiano †Abstract: Inflationary bispectrum was first derived by Maldacena in his famous paper [1]. Recently, inspired by the development of scattering amplitude in the particle physics community, some cosmologists are trying to derive inflationary bispectrum, or generally higher-point function, by only considering symmetries of cosmological perturbations. First, Nima Arkani-Hamed et. al. [2] successfully derived inflationary bispectrum by only using slightly broken de-Sitter space isometries. However, inflation generally breaks de-Sitter isometries, either small or large. For that motivation, Enrico Pajer et.al. [3-5] introduced a new bootstrap method without assuming de-Sitter isometries. In this seminar, I will review his method and compare the advantages and disadvantages to the previous one. References: ## 2021/5/24 (月)## Title: Primordial black holes from fifth forces †## Speaker: Koki Tokeshi †Primordial black holes could have formed in the early universe and various formation mechanisms have been proposed. Among them, I will review the relativly new one, first applied to PBHs in [1], in which the long range attractive forces between the light scalar and heavy fermion fields are exploited. I will also review the complementary paper [2], in which more general and quantitative analyses have been done, supporting the idea and estimations given in [1]. Related discussion about PBHs vs PMDHs in [3] and observational implications in [4] will also be reviewed. References: ## 2021/5/17 (月)## Title: Entanglement harvesting from vacuum in cosmology †## Speaker: Takumi Hayashi †Abstract: References: ## 2021/5/10 (月)## Title: Inflation from the point of view of quantum estimation theory †## Speaker: Jun’ya Kume †Abstract: Recently, application of the quantum estimation theory to inflation has been discussed in several works by C. Gomez and R. Jimenez. Focusing on two of their papers, I introduce their perspectives that the properties of primordial curvature perturbation generated during inflation can be quantum mechanically explained by the quantum Fisher information with respect to the appropriate parameter. After I briefly give a review on the quantum Fisher information and Cramer-Rao bound, I present how the amplitude and the tilt of the curvature perturbation can be related to the Fisher information with respect to the time and the energy scale, respectively. References: ## 2021/4/26 (月)## Title: Electroweak-like Baryogenesis with New Chiral Matter †## Speaker: Kohei Fujikura †Abstract: References: ## 2021/4/19 (月)============================== ## Title: Dark Matter and Inflation in Einstein-Cartan Gravity †## Speaker: Inar Timiryasov †Abstract: It is known that the gravitational force can be obtained by gauging the Poincaré group. Once the resulting theory – Einstein-Cartan gravity – is coupled to matter, its action contains more terms of mass dimension at most 4 than metric gravity. In this talk, I will discuss them and explore two phenomenological consequences. First, Einstein-Cartan theory generalizes inflationary scenarios driven by the Higgs field. As in the known metric and Palatini models, the resulting predictions agree with observations. Secondly, we obtain a new mechanism for producing singlet fermions in the Early Universe. In a large range of masses, these fermions can constitute all of the dark matter. The seminar will be preceded by three short (one hour) lectures introducing Einstein-Cartan theory and other required tools. References: ## 2021/4/12 (月)## Introduction of Wash-In Leptogenesis †## Minxi He †The generation mechanism of baryon asymmetry in our Universe is still a mystery. A simple and elegant scenario is leptogenesis which converts the lepton asymmetry to baryon asymmetry. However, the standard leptogenesis requires very heavy right-handed neutrinos and the generated B-L asymmetry may be strongly washed out by inverse decay of right-handed neutrinos and some scattering process. Recently, a new leptogenesis mechanism called wash-in leptogenesis is proposed in 2011.09347 which makes use of the non-trivial chemical potential structure to generate non-vanishing B-L at late time. In wash-in leptogenesis, the right-handed neutrino mass can be as small as ~100TeV and the strong wash-out process turns out to enhance the asymmetry. Also, it is independent of the CP violation in the right-handed neutrino section. I will introduce this mechanism in this seminar. References: ## 2021/3/8 (月)## Higgs Inflation, Unitarity, and Emergence of Scalaron †## Yohei Ema †Higgs inflation introduces a large non-minimal coupling between the Ricci scalar and Higgs that causes tree-level unitarity violation well below the Planck scale. After reviewing the unitarity issue during and after inflation, we show that the unitarity can be restored by summing over vacuum polarization-type diagrams that are leading-order in the large-N limit. The scattering amplitude develops a pole after the resummation, which we identify as the scalar component of the metric, or the scalaron. We show that this phenomenon can be understood in the language of the non-linear sigma model (NLSM), with the scalaron identified as the sigma-meson that linearizes the NLSM. ## 2021/3/1 (月)## On Lorentz-invariant (bi-)spin-2 theories †## Daisuke Yamauchi †In this talk, I will discuss the Lorentz-invariant massless and massive spin-2 theories ## 2021/2/22 (月)## On polarizations of non-luminal gravitational waves †## Hiroaki Tahara †If GWs are subluminal or superluminal, there must be a special frame (e.g. unitary frame of a scalar tensor theory). ## 2021/2/15 (月)## Schwinger Effect during Axion inflation †## Tomohiro Fujita †Axion inflation model is well motivated and has been extensively studied. In particular, if the axionic inflaton is coupled to the U(1) gauge field, the particle production takes place and its backreaction slows down the inflaton which assists slow-roll inflation. In the previous works, however, charged particles and its pair productions due to Schwinger effect have not been taken into account. To analyze this complicated system, we develop a stochastic formalism of the gauge field and obtain a self-consistent value of the electric conductivity induced by the charged particles. Using these novel tools, we find several interesting features, for instance, the generated magnetic field can be stronger than the electric field, which could not be realized without charged particles. ## 2021/2/8 (月)## Stochastic background of induced gravitational waves: recent developments †## Shi Pi †Our universe is fulfilled by stochastic gravitational wave background (SGWB) with a large range of frequencies, which may have various astrophysical/cosmological origins in the early universe. In this talk I will briefly review the properties of SGWB induced by the scalar perturbation, as well as its connection to the primordial black holes (PBHs). Recently, NANOGrav reported the discovery of a common-spectrum process of the time residuals in their 12.5-yr data set of pulsar timing array, which might be the first detection of nano-hertz SGWB. Based on the recent developments on the spectral shape of SGWB, I will show that this signal could be connected to the planet-mass PBH candidates found by OGLE recently, if there is a dust-like stage prior to the radiation dominated era. ## 2021/2/1 (月)## Dynamically assisted Schwinger effect †## Yusuke Yamada †Schwinger pair production in electric field background is one of the most well-known non-perturbative effects in quantum field theory. Its non-perturbative nature typically leads to an exponentially small production rate, like tunneling in quantum mechanics. In order to describe the mechanism quantitatively, we have two options, WKB formalism and "world line instanton" method. Although the latter method seems not common in our (cosmology?) group, it gives an interesting description of how perturbative corrections affect non-perturbative pair production. So I will give a brief review of the world line method and discuss the dynamically assisted Schwinger mechanism with such a formalism. Refs: ## 2021/1/25 (月)## Low Scale Leptogenesis and the Perspectives to Test it in the Laboratory †## Marco Drewes †Right-handed Majorana neutrinos can simultaneously explain the light neutrinos via the seesaw mechanism and generate the baryon asymmetry of the universe through leptogenesis. If their Majorana masses are at or below the electroweak scale they can be searched for in collider and fixed target experiments. We study the viable leptogenesis parameter space in this region to make predictions for the heavy neutrino properties, such as their mass spectrum, flavour mixing pattern and CP-properties. If any heavy neutral leptons are found in the laboratory, combining different observables allows to test whether these particles can indeed be responsible for baryogenesis and the neutrino masses. |