2017/10/30 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報
Speaker: Masaki Yamada (Tufts Univ.)


2017/10/23 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報
Joint seminar at IPMU


2017/10/16 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報
No seminar due to Kavli IPMU 10th anniversary symposium


2017/10/9 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報
Holiday


2017/10/2 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報
Speaker: Jun'ichi Yokoyama (RESCEU)


2017/9/4 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報
Speaker: Yuichiro Tada (IAP)
Title: Stochastic Formalism in Curved Field Space
Abstract:
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)

UTAPwiki/セミナー/初期宇宙・相対論速報
Speaker: Katsuki Aoki (Waseda Univ.)
Title: condensate of massive graviton as dark matter
Abstract:
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)

UTAPwiki/セミナー/初期宇宙・相対論速報
Speaker: Hiroaki Tahara (RESCEU) [from 1PM]
Title: Anisotropic inflation with the generalized G-inflation
Abstract:
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)

UTAPwiki/セミナー/初期宇宙・相対論速報
Speaker: Atsuhisa Ota (Tokyo Tech)
Title: Secondary isocurvature perturbations from acoustic reheating
Abstract:
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
order.

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


2017/6/19 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報
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)

UTAPwiki/セミナー/初期宇宙・相対論速報
Speaker: Rampei Kimura (Tokyo Tech)
Title : Field theoretical extension of higher derivative fermionic theory
Abstract:
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)

UTAPwiki/セミナー/初期宇宙・相対論速報
Speaker: Takashi Hiramatsu (Rikkyo Univ.)
Title: Reconstruction of primordial tensor power spectrum from CMB B-mode observations
Abstract:
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.


2017/5/29 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報
Speaker: Naritaka Oshita (RESCEU)
Title: Thermal excitation of the universe and the second law of thermodynamics
Abstract:
The stochastic inflation describes the thermal fluctuation of a homogeneous
scalar field in a de Sitter patch. The scalar field could transient from the bottom to the
top of an effective potential with a finite probability due to its thermal dissipation.
However, its horizon entropy that is proportional to the inverse of the square of Hubble
parameter would decrease during the transition, which apparently looks like the second
law of thermodynamics was broken. I will show that taking into account the entropy production
due to the cosmological decoherence leads to the consistent picture of the thermal excitation
of the universe where the second law of thermodynamics can be satisfied.


2017/5/22 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報
Speaker: Yi-Peng Wu (RESCEU)
Title: On loops in inflation: Can these corrections become large?
Abstract:
In next Monday I would like to talk about loop corrections to the curvature perturbation
in the single-field inflation paradigm, where we intend to draw a new perspective to
Weinberg's question: "Can loop corrections become large?" [hep-th/0605244].
I will introduce a special in-in formalism to show that the loop-contribution may be more
important than the previous conclusion due to the transition of vacuum expectation
values of spectator fields during inflation.


2017/5/15 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報
Speaker: Daisuke Yamauchi (Kanagawa Univ.) [from 1PM]
Title: Collisions of superconducting strings with Y-junctions
Abstract:
In this talk, I will discuss the dynamics of Nambu-Goto and superconducting strings
with Y-junction at which three strings meet. When the two strings collide, they may
be linked by a third string with different tension (and current), instead of intercommutation.
In particular, I show the kinematical constraints on this process and the requirement to
form the Y-junction when two strings collide.


2017/5/8 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Joint seminar at Rikkyo Univ.

2017/5/1 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報
Speaker: Takahiro Hayashinaka (RESCEU)
Title: Negativity of electromagnetic response in de Sitter space
Abstract:
This talk will be dedicated to show recent activities on the topic. In the first half of the talk, I explain how negative, i.e. unstable, electromagnetic responses have been revealed. Though this phenomenon was repeatedly reported by several groups recently [1], neither physical origin or intellectual interpretation of the phenomenon have been explicitly given up to the present.
The rest of my talk involve this point. Inspired by a relevant negative response phenomenon in a specific ferroelectric matter [2], I recognized importance of spontaneous symmetry breaking and the effective action or potential of the system. A variation of renormalization group technique, which is called non-perturbative renormalization group (NPRG) [3], can be used for evaluation of the effective action. I re-developed a gauge-invariant formulation of flow equation for scalar QED effective action in curved spacetime. In this formalism, one introduces an IR cutoff scale. As a result, undesirable behavior of the IR modes in de Sitter spacetime can be virtually suppressed [4]. The solution for the flow equation has not been found yet though, I will explain about construction of the gauge-invariant flow equation.
References:
[1] T. M. B. Frob, J. Garriga, S. Kanno, M. Sasaki, J. Soda, T. Tanaka, and A. Vilenkin, JCAP 2014 (2014) 04 009, R. G. Cai and S. P. Kim, JHEP 09 (2014) 072, T. Kobayashi and N. Afshordi, JHEP 2014 (2014) 10 1-36, C. Stahl, E. Strobel, and S. S. Xue, Phys. Rev. D 93 (Jan, 2016) 025004, E. Bavarsad, C. Stahl, and S. S. Xue, arXiv:1602.0655, C. Stahl and S. Xue Phys.Lett. B760 (2016) 288-292, T. Hayashinaka and J. Yokoyama, JCAP 1607 (2016) 07 012, T. Hayashinaka, T. Fujita and J. Yokoyama, JCAP 1607 (2016) 07 010
[2] G. Catalan et al., Nature matt. (2015) 14, 137, S. Kasamatsu et al., Adv. Mater. 2016, 28, 335-340
[3] Christof Wetterich. Phys. Lett., B301:90-94 (1993), M. Reuter and C. Wetterich. Nucl. Phys., B417:181-214 (1994)
[4] For recent application of NPRG in cosmology, see: Maxime Guilleux, Theses, Universite paris Diderot, September, 2016


2017/4/24 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報
Speaker: Kazufumi Takahashi (RESCEU)
Title: Field transformation and mimetic gravity
Abstract:
In our recent paper arXiv:1702.01849, we proved that the number of physical degrees of freedom (DOFs) is not changed by an invertible field transformation.
On the other hand, this is not the case if the field transformation is not invertible.
In the context of scalar-tensor theories, the so-called mimetic gravity model is obtained by a noninvertible conformal transformation of the Einstein-Hilbert action, which introduces an extra scalar DOF.
In this talk, I will summarize the results of arXiv:1702.01849 on invertible transformations, and then mention mimetic gravity as an example of a theory resulting from a noninvertible transformation.


2017/4/17 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報
Speaker 1: Ken'ichi Saikawa (DESY)
Title: Inflationary gravitational waves and equation of state in the Standard Model
Abstract:
The existence of relic gravitational waves (GWs) is one of the most important predictions of inflationary models, and the knowledge about their detailed spectrum will become indispensable in future direct detection experiments. In this work, we investigate thermodynamic properties of high temperature plasma composed of elementary particles of the Standard Model, and identify how such properties affect the spectrum of primordial GWs. The evolution of the equation of state of radiations with interacting particles is estimated based on the latest results of perturbative and non-perturbative calculations, and the effective degrees of freedom for the energy density and the entropy density as a function of temperature are obtained. After obtaining the effective degrees of freedom, we numerically compute the spectrum of GWs by taking account of the evolution of the Standard Model equation of state. It is shown that the spectrum of GWs at high frequencies is corrected due to the effect of particle interactions, and that such corrections are relevant to future high sensitivity experiments.

Speaker 2: Shi Pi (Chinese Academy of Sciences)
Title: Power-law oscillating features in the primordial spectrum
Abstract:
The effects of heavy fields modulate the scalar power spectrum during inflation, and quadratic heavy field will generate logarithmic oscillations. If we extend the potential to power-law, i.e. V(φ)~φ^n, a power-law oscillating signal ~sin(Cμk^p) will appear in the power spectrum. The power-law oscillations can also be generated in the ``standard clock scenario'' which has the power-law scale factor a~t^s but a quadratic heavy field. The difference between these two models relies only on the coefficients.


2017/4/10 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Speaker: Takeshi Chiba (Nihon Univ.)

On a paper by
T. W. Baumgarte and C. Gundlach, "Critical collapse of rotating radiation fluids"
Phys. Rev. Lett. 116, no. 22, 221103 (2016) [arXiv:1603.04373 [gr-qc]]


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Last-modified: 2013-11-20 (水) 11:11:56 (1400d)