2018/1/29 (月)

Joint seminar at TiTech

2017/12/18 (月)

Speaker: Shun-Pei Miao (National Cheng Kung Univ.) [from 1:30 PM]
Title: Deducing Cosmological Observables from the S-matrix
We study one loop quantum gravitational corrections to the long range force induced by the exchange of a massless scalar between two massive scalars. The various diagrams contributing to the flat space S-matrix are evaluated in a general covariant gauge and we show that dependence on the gauge parameters cancels at a point considerably before forming the full S-matrix, which is unobservable in cosmology. It is possible to interpret our computation as a solution to the effective field equations --- which could be done even in cosmology --- but taking account of quantum gravitational corrections from the source and from the observer.

2017/12/11 (月)

No seminar due to CosPA2017

2017/12/4 (月)

Speaker: Keisuke Inomata (ICRR)

2017/11/27 (月)

No seminar due to JGRG27

2017/11/20 (月)

Speaker: Toyokazu Sekiguchi (RESCEU)
Title: Abundance of dark matter axion in the coupling with dark photon
The axion and axion-like particles originate from a variety of UV theories based on QFT as well as string theory. Couplings of those axions with U(1) gauge fields are fairly ubiquitous. They are known to offer rich phenomenology in the very early Universe, for instance, inflation and preheating. In my talk I’m going to focus on their effects on the abundance of the QCD axion produced from the misalignment mechanism. It is recently argued that the coupling of the axion to a hidden U(1) field suppresses the axion abundance, which can open up the axion window up to the GUT scale. The claim postulates that production of the axion fluctuations is negligible. We revisit the argument by performing 3d lattice simulations of the axion electrodynamics, where the nonlinear dynamics in the coevolution of the axion fluctuations and gauge field is incorporated to the full extent. We show production of the axion fluctuations plays a crucial role and the suppression is moderated significantly.

2017/11/13 (月)

Speaker: Louis Yang (IPMU)
Title: Postinflationary Higgs Relaxation and Leptogenesis
The discovery of the Higgs boson with a mass of 125 GeV implies a relatively slow rise of the Standard Model Higgs potential at large energy scales. A scalar field with such a shallow potential can develop a large vacuum expectation value (VEV) during inflation through quantum fluctuation. The relaxation of the large VEV of the Higgs field after inflation may play an important role in the early stage of the universe. In this talk, I will talk about the generation of large Higgs VEV and how to do leptogenesis using the postinflationary relaxation of the Higgs field. I will also discuss one observable consequence of this relaxation leptogenesis model, which could explain the excess found in the cosmic infrared background (CIB) radiation anisotropy.

2017/11/6 (月)

Joint seminar at IPMU

2017/10/30 (月)

Speaker1: Masaki Yamada (Tufts Univ.)
Title: On false vacuum decay catalyzed by black holes
False vacuum states are metastable in quantum field theories, and true vacuum bubbles can be nucleated due to the quantum tunneling effect. It was recently suggested that an evaporating Black Hole (BH) can be a catalyst of bubble nucleations and dramatically shortens the lifetime of the false vacuum. In particular, in the context of the Standard Model valid up to a certain energy scale, even a single evaporating BH may spoil the successful cosmology by inducing the decay of our electroweak vacuum. In this talk, we reinterpret catalyzed vacuum decay by BHs, using an effective action for a thin-wall bubble around a BH to clarify the meaning of bounce solutions. We calculate bounce solutions in the limit of a flat spacetime and in the limit of negligible backreaction to the metric, where it is much easier to understand the physical meaning, and compare these results with the full calculations done in the literature. As a result, we give a physical interpretation of the enhancement factor: it is nothing but the probability of producing states with a finite energy. This makes it clear that all the other states such as plasma should also be generated through the same mechanism, and calls for finite density corrections to the tunneling rate which tend to stabilize the false vacuum.

Speaker2: Ayuki Kamada (IBS)
Title: Galactic Rotation Curves with Dark Matter Self-Interactions
It has been shown that the LambdaCDM model can successfully explain large-scale structure of the universe. But it is not clear that it can accommodate observations on (sub-)galactic scales. In fact, the LambdaCDM model predicts dark matter halos, which are systematically denser than inferred from astrophysical observations. One interesting alternative assumes that dark matter particles are strongly interacting with each other. Dark matter self-interactions will lead to heat transfer from the hotter outer region to the cold inner region, reducing the central dark matter density. In this talk, I will show that the observed rotation curves of spiral galaxies are consistent with predicted in the SIDM model. I will also emphasize the importance of the baryonic disk in determining the SIDM halo profile.

2017/10/23 (月)

Joint seminar at IPMU -> postponed to 11/6 due to a typhoon

2017/10/16 (月)

No seminar due to Kavli IPMU 10th anniversary symposium

2017/10/9 (月)


2017/10/2 (月)

Speaker: Jun'ichi Yokoyama (RESCEU)
Title: Spontaneous genesis after G-inflation
In kinetically driven G-inflation, the Universe after inflation
is dominated by the kinetic energy density of a free scalar field
whose energy density is dissipated in proportion to $a^{-6}$
so that the cosmic expansion law is like $a(t) \propto t^{1/3}$.
The reheating is achieved by gravitational particle production
or through direct coupling preserving the shift symmetry of the
model. We discuss spontaneous baryogenesis and asymmetric
dark matter genesis may be possible if these charge densities
have a derivative coupling with the inflaton.

2017/9/4 (月)

Speaker: Yuichiro Tada (IAP)
Title: Stochastic Formalism in Curved Field Space
In multi-field inflation scenarios, the scalar fields can span curved field spaces in general.
Recently it was pointed out that the sufficiently large field curvature can make the entropic
degrees of freedom tachyonic during inflation and alter the inflation dynamics [PRL117, 141301].
This geometrical destabilization can be applied to a large class of inflationary models.
To analyze the destabilization, we formalize the stochastic approach in the general curved
field space from first principles.
Also I’ll briefly mention the slow-roll phase well after the geometrical destabilization.

2017/7/10 (月)

Speaker: Katsuki Aoki (Waseda Univ.)
Title: condensate of massive graviton as dark matter
We study coherently oscillating massive gravitons in the ghost-free bigravity theory. This coherent field could be interpreted as a "condensate" of the massive gravitons, or it is regarded as a spacetime anisotropy from the geometrical point of view. We present that the condensate of massive graviton (or the spacetime anisotropy) is a viable candidate for dark matter. We also discuss a production of the spacetime anisotropy and find that the extragalactic magnetic field of a primordial origin can yield a sufficient amount of dark matter.

2017/7/3 (月)

Speaker: Hiroaki Tahara (RESCEU) [from 1PM]
Title: Anisotropic inflation with the generalized G-inflation
I give a talk about our ongoing work on anisotropic inflation.
First, I review [1] which proves that homogeneous universes with
a positive cosmological constant evolve toward the de Sitter solution.
Next, I show that some classes of the generalized Galileon theories
cause anisotropic inflation. The anisotropic inflation can change the
background action dramatically, and I argue its application.
[1] Robert M. Wald, Phys. Rev. D 28(1983), 2118(R)

2017/6/26 (月)

Speaker: Atsuhisa Ota (Tokyo Tech)
Title: Secondary isocurvature perturbations from acoustic reheating
We discuss the secondary isocurvature perturbations from mode coupling
effects called acoustic reheating in the Thomson scattering dominant
universe. The perfect fluid approximation for the photon energy
momentum tensor is invalid deep inside the horizon so that shear
viscosity and thermal conduction occur. They produce the deviations
from the ideal Planck distribution, which can be recast into the entropy
perturbation on superhorizon scales. We also show that the latter effect breaks the
conservation law for the photon energy momentum tensor because
there exists heat conduction from the baryon sector. Therefore, the
superhorizon isocurvature perturbations are generated at second

#This seminar is based on our recent work; arXiv:1705.05196.

2017/6/19 (月)

Joint seminar at RESCEU
Speaker1: Soichiro Morisaki [14:00- @ science building 4, #1116]
Speaker2: Leo Tsukada [15:00- @ science building 4, #1116]
Speaker3: Misao Sasaki [16:15- @ science building 1, #233]
(The third talk by Misao Sasaki doubles as RESCEU colloquium)

2017/6/12 (月)

Speaker: Rampei Kimura (Tokyo Tech)
Title : Field theoretical extension of higher derivative fermionic theory
Next Monday, I will talk about our ongoing work : higher derivative fermionic field theory.
I will first give a brief introduction of the Grassmann algebra
and a fermionic version of Ostrogradsky’s ghost, based on arXiv:1704.02717.
Then I will introduce multiple scalar fields and Weyl fermion
to construct a field theoretical extension of the previous work
and derive sufficient (degeneracy) conditions to have an appropriate number of degrees of freedom
for avoiding Ostrogradsky’s ghost. I will give a successful model,
and the consistency between Hamiltonian and Lagrange formulation will be discussed as well.

2017/6/5 (月)

Speaker: Takashi Hiramatsu (Rikkyo Univ.)
Title: Reconstruction of primordial tensor power spectrum from CMB B-mode observations
We present our on-going work on the reconstruction of
primordial tensor power spectrum from the CMB B-mode
observations. Directly computing the Fisher information
matrix, we estimate the 1-\sigma error around the fiducial
power spectrum of tensor perturbations. Using the results,
we demonstrate that our approach is useful to constrain the
models which predict characteristic features on the
primordial tensor power spectrum.

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Last-modified: 2013-11-20 (水) 11:11:56 (1464d)