2021/4/19 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

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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:
https://arxiv.org/abs/2007.14978
https://arxiv.org/abs/2007.16158
https://arxiv.org/abs/2008.11686

2021/4/12 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

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:
Domcke, Kamada, Mukaida, Schmitz, Yamada, 2011.09347
Domcke, Ema, Mukaida, Yamada, 2006.03148
Kolb, Turner, The Early Universe

2021/3/8 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

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)

UTAPwiki/セミナー/初期宇宙・相対論速報

On Lorentz-invariant (bi-)spin-2 theories

Daisuke Yamauchi

In this talk, I will discuss the Lorentz-invariant massless and massive spin-2 theories
in flat spacetime. Starting from the most general action of a rank-2 symmetric tensor field
whose Lagrangian contains up to quadratic in first derivatives of a field, I classify
ghost-free theories based on the number of degree-of-freedom and constraint structures.
I also discuss the possible extension to the ghost-free bi-spin-2 theories.

2021/2/22 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

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).
Although the simultaneous observations of binary merger with light and GWs have given a constraint on the speed of GWs around 100 Hz,
its propagation speed at lower frequency (<10 Hz) is still free from that constraint.
I will talk about polarizations of sub/superluminal GWs in Horndeski theory, considering the relative speed of the solar system to the unitary frame.

2021/2/15 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

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)

UTAPwiki/セミナー/初期宇宙・相対論速報

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)

UTAPwiki/セミナー/初期宇宙・相対論速報

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 this seminar, I will review “dynamically assisted Schwinger effect” where a “perturbative” weak electric field significantly enhances the “non-perturbative” production rate caused by a strong field. This is a very interesting example of interplay between perturbative and non-perturbative effects, and similar mechanism may work in other contexts such as cosmology.

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.
If (and only if) I have time, I will also discuss the dynamical assist mechanism with WKB formalism.

Refs:
R. Schutzhold, H. Gies, G. V. Dunne, Phys.Rev.Lett. 101 (2008) 130404,
G.V. Dunne, C. Schbert, Phys.Rev.D 72 (2005) 105004,
G. Torgrimsson, et al. JHEP 06 (2017) 043
G. Torgrimsson, Phys.Rev.D.99.096002 …etc.

2021/1/25 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

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.

2021/1/18 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Pion/axion domain wall in the presence of the magnetic fields and chiral asymmetry

Kohei Kamada

I will introduce the chiral soliton lattice which is the layer of the pion/axion domain walls
in the presence of the chiral chemical potential and magnetic fields.
If my preparation goes well, I will try to explain how it could appear in the compact stars
and what happens if they have boundaries.

References:
[1] Brauner and Yamamoto, “Chiral Soliton Lattice and Charged Pion Condensation in Strong Magnetic Fields”,
JHEP 04 (2017) 132 [1609.05213 [hep-ph]]
[2] Eto, Hashimoto, and Hatsuda, “Ferromagnetic neutron stars: axial anomaly, dense neutron matter, and pionic wall”,
Phys.Rev.D 88 (2013) 081701 [1209.4814 [hep-ph]]
[3] Komargodski “Baryons as Qunatum Hall Droplets”, 1812.09253 [hep-th]
[4] Fukushima and Imaki, "Anomaly inflow on QCD axial domain-walls and vortices”,
Phys.Rev.D 97 (2018) 11, 114003 [1802.08096 [hep-ph]]

2020/12/21 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Toward detection of gravitational waves by pulsar timing arrays

Jun'ichi Yokoyama

An introduce will be given about the detection of gravitational waves by pulsar timing data including recent NANOGrav data.

2020/12/14 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Soft de-Sitter effective field theory (continue)

Jason Kristiano

de-Sitter space plays important role in cosmology, such as inflation of the universe is described by a scalar field in de-Sitter space. de-Sitter space is characterized by a dimensionful quantity, which is Hubble constant and corresponds to the typical energy scale of de-Sitter space. To predict some observable quantities, cosmologist usually interested in the late time limit of de-Sitter space. The energy scale at the late time limit is far below the typical energy of de-Sitter space. Such limit is called soft limit. An example in cosmology is power spectrum of inflation is evaluated at the superhorizon scale or soft limit. To calculate it, we usually use the full theory of de-Sitter space, then evaluate the soft limit at the end of the calculation. However, can we construct the effective field theory of soft de-Sitter space? In this talk, I will discuss the construction of soft de-Sitter effective field theory.

Reference:
[1] T. Cohen and D. Green, Soft de Sitter Effective theory (2020), arxiv:2007.03693

2020/12/7 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Soft de-Sitter effective field theory

Jason Kristiano

de-Sitter space plays important role in cosmology, such as inflation of the universe is described by a scalar field in de-Sitter space. de-Sitter space is characterized by a dimensionful quantity, which is Hubble constant and corresponds to the typical energy scale of de-Sitter space. To predict some observable quantities, cosmologist usually interested in the late time limit of de-Sitter space. The energy scale at the late time limit is far below the typical energy of de-Sitter space. Such limit is called soft limit. An example in cosmology is power spectrum of inflation is evaluated at the superhorizon scale or soft limit. To calculate it, we usually use the full theory of de-Sitter space, then evaluate the soft limit at the end of the calculation. However, can we construct the effective field theory of soft de-Sitter space? In this talk, I will discuss the construction of soft de-Sitter effective field theory.

Reference:
[1] T. Cohen and D. Green, Soft de Sitter Effective theory (2020), arxiv:2007.03693

2020/11/30 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Classical information geometry and its application to physics

Koki Tokeshi

Information geometry is a theory for statistical manifolds, on which many kinds of probability distributions are described geometrically. In particular, the so-called exponential family type probability distributions can be treated uniformly in the language of information geometry. The dually flat connection plays a crucial role in the theory as well as the Fisher information metric and Hessian structure of the manifolds. In this talk, I first review the general formulation of the theory. Although the theory has been succeeded to (re-)interpret the already known results in the statistical theory, its application to physics seems to be under construction, that is, many quantities have been calculated with their physical meanings remain uninterpreted, but some of them are referred to in this talk especially on the relation to general relativity or black hole physics.

References:
[1] S. Amari, Information geometry and its applications, 2016.
[2] A. Fujiwara, Foundations of information geometry, 2015.
[3] H. Matsueda, Emergent GR from Fisher information metric, arXiv:1310.1831 [gr-qc].
[4] J. E. Aman and N. Pidokrajt, Critical phenomena and information geometry in black hole physics, arXiv:1001.2220 [gr-qc].
The first two are textbooks in English and Japanese respectively. The first person has extensively contributed to construct the theory while the second person studies quantum information geometry which is not referred to in this talk.

2020/11/2 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Saddle-point solutions from gradient flow

Fumio Uchida

In most of the scenarios of baryogenesis, sphalerons play an important role because they violates baryon number. However, when one wants to find a sphaleron solution and estimate its energy, a difficulty sometimes arises because sphalerons are not local minima but saddle-points. Also, when one studies vacuum decay, to find bounce configurations is an important problem. In this case, one has to overcome the same difficulty because bounce solutions are saddle-points. However, a method that is intuitively understandable to find these solutions is proposed recently. In the seminar, I introduce the methodology and prove its validity, mainly following [1].

References:
[1] S. Chigusa, T. Moroi, and Y.Shoji, PLB800, 135115 (2020)
[2] R. Sato, PRD101, 016012 (2020); Y. Hamada, and K. Kikuchi, PRD101, 096014 (2020).

2020/10/26 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Baryon isocurvature perturbations from non-helical magnetic fields

Fumio Uchida

The origin of the baryon asymmetry of the universe is a fundamental problem in cosmology. Among many proposed mechanisms that could solve it, baryogenesis from helical magnetic fields is characteristic in that it relates baryon asymmetry directly to the cosmic magnetic fields. In the scenario, the chiral anomaly in the Standard Model converts hypermagnetic helicity into baryon number. The observed baryon-to-entropy ratio gives conditions on the strength and the coherence length of the magnetic fields, and the generation of baryon isocurvature perturbations gives another constraint because it affects the big-bang nucleosynthesis. Counterintuitively, while the net baryon asymmetry is produced only from the helical part, the baryon isocurvature perturbations are generated from both helical and non-helical parts of the magnetic power spectrum. In the seminar, I focus on and explain how even non-helical magnetic fields can generate baryon isocurvature perturbations.

References:
[1] M. Giovannini, and M. E. Shaposhnikov, PRD57, 2186 (1998)
[2] K. Bamba, PRD74, 123504 (2006); T. Fujita, and K. Kamada, PRD93, 083520 (2016); K. Kamada, and A. J. Long, PRD94, 063501 (2016); K. Kamada, and A. J. Long, PRD94, 123509 (2016)

2020/10/19 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

An introduction to astrophysical gravitational wave background

Jun'ya Kume

The recent result of NANOGrav attracts our interests since it might be interpreted as the first ever detection of stochastic gravitational wave background (GWB). Although we cannot reject the possibility that the GWB has its origin in a primordial universe, the dominant source in its frequency range is usually expected to be supermassive blackhole binaries. In this situation, understanding the nature of astrophysical GWB might be useful to the cosmologists. What characterizes the spectral shape of it? How does it prevent the detection of primordial gravitational wave backgrounds in the forthcoming observation?

In this seminar, I will make a review on the stochastic gravitational wave background from compact binary coalescences based on [1]. Then I discuss its resolvability which is studied in [1, 2] and briefly introduce recent ideas to remove it as foreground noise [3, 4].

References:
[1] P. A. Rosado, Phys. Rev. D 84, 084004 (2011).
[2] S. Sachdev, T. Regimbau and B. Sathyaprakash, (2020), arXiv:2002.05365 [gr-qc].
[3] A. Sharma and J. Harms, (2020), arXiv:2006.16116 [gr-qc].
[4] S. Biscoveanu, C. Talbot, E. Thrane and R. Smith, (2020), arXiv:2009.04418 [astro-ph.HE]

2020/10/12 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Stable, ghost-free solutions in UV non-local gravity

Shubham Maheshwari

Abstract: I consider higher derivative, UV modifications to GR. In particular, I will focus on a specific kind of string theory-inspired higher derivative gravity where one includes derivatives to all orders in the action. First, I will discuss how such a non-local theory of gravity admits stable, non-singular bouncing solutions in the absence of matter. Moreover, around this bouncing background, there exists only one propagating (and ghost-free) scalar mode, and no vector or tensor modes. Next, I will discuss the general analysis of scalar-vector-tensor perturbations in non-local gravity - in particular, I will show how non-local gravity is ghost-free around (A)dS and certain non-maximally symmetric backgrounds, and how certain (A)dS backgrounds have special physical spectra in that the propagating degrees of freedom are different from usual expectations.

2020/10/5 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Non-minimal coupling in quantum field theory

Ayuki Kamada

A non-minimal coupling of an inflaton to the Ricci scalar changes the inflationary prediction such as the spectral index and running.
Its importance has been growing, since the recent CMB data favor the chaotic inflation models with a non-minimal coupling such as the Higgs inflation model.
On the other hand, a non-minimal coupling is not well understood, especially in terms of quantum field theory (QFT).
In this talk we will discuss its properties such as perturbative Unitarity and renormalization.
This talk is aimed at locating our QFT-based works (1909.04228, 1909.04229) in the context of inflationary models or more broadly modified gravity models.
We assume only a basic knowledge of QFT (e.g., meaning of Feynman diagrams).

2020/9/28 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Entanglement entropy and its relation to blackhole entropy

Takumi Hayashi

Abstract: The well known “area law” of blackhole entropy is precisely formulated in the context of thermodynamics of classical gravity. However the dynamical origin of blackhole entropy is still mysterious given No-hair theorem which uniquely determined classical configuration. With such kinds of interest, there are many attempts to connect blackhole entropy with quantum statistical entropy, called ”entanglement entropy”. [#ue4dc349]
In this talk, I show the “area law” of entanglement entropy in quantum field theory, and introduce more concrete computational method to obtain it, which is so called replica trick. Finally I mention its relation to blackhole entropy and relevant argument in the context of AdS/CFT correspondence.

The reference is listed below.
[1]S. N. Solodukhin, Living Rev. Rel. 14, 8 (2011), [arXiv:1104.3712], ”Entanglement entropy of black holes”.
[2]C. G.~Callan, Jr. and F. Wilczek, Phys. Lett. B 333, 55-61 (1994), [arXiv:hep-th/9401072], “On geometric entropy''.
[3]A. Lewkowycz and J. Maldacena, JHEP 08, 090 (2013),[arXiv:1304.4926], ”Generalized gravitational entropy''


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Last-modified: 2021-04-12 (月) 11:04:35 (6d)