2020/10/26 (月)


Topic 1: Baryon isocurvature perturbations from non-helical magnetic fields

Topic 2: Saddle-point solutions from gradient flow

Fumio Uchida

The first topic: 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.

The second topic: 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 [3].

The seminar will be organized as follows. The first 40 min is devoted to the first topic, based on [1,2], then 10 min break, and the next 40 min to the second, based on [3,4].

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

2020/10/19 (月)


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].

[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)


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)


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)


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''

2020/7/20 (月)


Attracted by Particle Production

Soichiro Hashiba

Whenever a value of field changes, particles coupled with this background field are produced. This particle production is quite universal and it can induce considerable backreaction by extracting the energy of the background field. In other words, this particle production behaves as a kind of attractive force. By using this effect, we can trap the background field at a certain point where coupled particles are produced most efficiently. In this talk, I will explain the mechanism of field trapping by particle production and then introduce several applications, moduli trapping and trapped inflation.
This talk is mainly based on the following references.

[1] L. Kofman et al., JHEP 05 (2004) 030.
[2] N. Itzhaki and E. Kovetz, JHEP 10 (2007) 054.
[3] D. Green et al., Phys. Rev. D 80 (2009) 063533.

2020/7/13 (月)


Thermodynamical interpretation for the second law of Cosmology

Ido Ben-Dayan

The area of a future holographic screen increases monotonically. Associating this area with entropy results in a generalized second law for Cosmology (GSLC). Unlike black hole horizons, screens relevant to Cosmology have no thermodynamical interpretation.
Past efforts have tried to express thermodynamical quantities related to gravity in terms of phase space variables and microstates. Such phase space constructions of black holes and accelerated observers have been carried out. We will utilize this notion for holographic screens.
Relating the entropy of the screens to spacetime degrees of freedom surface density derived from a phase space enables us to identify the entropy of any holographic screen as the entropy detected by accelerating observers due to their acceleration. Using Unruh's temperature and the equivalence principle, this gives the holographic screens' temperature and yields a thermodynamical interpretation of the GSLC.

2020/7/6 (月)


Introduction of peak statistics of Gaussian random fields

Minxi He

In this talk, the statistical theory of peaks of Gaussian random fields are introduced, based on Bardeen, Bond, Kaiser, and Szalay (1986) which is usually called BBKS or peak theory. This theory is originally used to study the statistical properties of the structure formation in the Universe. Recently, this theory is applied to the study of primordial black holes.
Bardeen, Bond, Kaiser, Szalay, Astrophys. J. 304, 15 (1986)
Young, Byrnes, Sasaki (2014)
Yoo, Harada, Garriga, Kohri, 1805.03946
Germani, Musco, Phys. Rev. Lett. 122, no. 14, 141302 (2019)
Suyama, Yokoyama, 1912.04687
Wu, 2005.00441
Tokeshi, Inomata, Yokoyama, 2005.07153

2020/6/29 (月)


From scattering amplitudes to binary system

Hiroaki Tahara

I will briefly introduce a few recent letters which calculate classical Hamiltonian for compact spinless binaries up to third post-Minkowskian order with modern tools for scattering amplitudes.

[1] C. Cheung, et al., “From scattering amplitudes to classical potentials in the post-Minkowskian expansion,” PRL 121, no.25, 251101 (2018) [arXiv:1808.02489 [hep-th]].
[2] Z. Bern, et al., “Scattering amplitudes and the conservative Hamiltonian for binary systems at third post-Minkowskian order,” PRL 122, no.20, 201603 (2019) [arXiv:1901.04424 [hep-th]].
[3] Z. Bern, et al., “Black Hole Binary Dynamics from the Double Copy and Effective Theory,” JHEP 10, 206 (2019) [arXiv:1908.01493 [hep-th]]

2020/6/22 (月)


Experiments towards the Quantum Nature of Gravity and the Related Theoretical Challenges

Tomohiro Fujita

Quantum gravity is one of the most outstanding problems in physics. In fact, we do not know if
gravity should be quantized in a similar way to the other degree of freedoms. Recently, a couple
of experiments are proposed to test whether gravitational fields become quantum superposition or not.
After the proposal, a number of questions are raised and the possible implications of the experiments
are under discussion. I will introduce the proposed experiments and the related questions
and hope to specify the remaining theoretical challenges to clarify the implications.

[1] Phys.Rev.Lett. 119 (2017) 24, 240401 [1707.06050]
[2] Phys.Rev.Lett. 119 (2017) 24, 240402 [1707.06036]
[3] Phys.Lett.B 792 (2019) 64-68 [1808.05842]
[4] arXiv: 2005.14596

2020/6/15 (月)


de Sitter wave function and Euclidean AdS

Yusuke Yamada

In cosmology, correlation functions are important observables. One can evaluate them e.g. within the in-in perturbation formalism or using wave functional(=path integral). The wave functional/path integral approach manifests the relation to the path integral in Euclidean AdS.
In this talk, I will review the relation between dS and EAdS path integral and discuss the subtlety of their relation. As I will show, the Bunch-Davies wave functional can be given by the analytic continuation of EAdS path integral, which can be evaluated by the technique used in AdS/CFT context.
Independently of the main topic, I will also review vacuum states in de Sitter QFT, if time allows. (It is unlikely, though. )

[1] D. Harlow, D. Stanford, arXiv:1104.2621
[2] D. Anninos, T. Anous, D. Z. Freedman, G. Konstantinidis, JCAP11(2015)048
[3] J. Maldacena, JHEP0305(2003)013

2020/6/8 (月)


Cosmological superfluids and phonons

Kohei Kamada

Spontaneous space-time symmetry breaking is now known to be a key to explore cosmology
such as inflation, dark energy, and dark matter.
In this talk, I will introduce the way to understand the cosmological perturbation
as the Nambu-Goldstone mode of the space-time symmetry breaking
and to construct their effective field theory, following the references [1,2].
I start from the brief introduction of the coset construction and inverse Higgs constraints,
Then I discuss the way to understand the perturbation to be the phonons in superfluids,
which is related to e.g., DBI and cuscuton
and construct their effective Lagrangian.
If time allows, I will also discuss the appearance of other (unconventional) types of phonons.

[1] Enrico Pajer, David Stefanyszyn, "Symmetric Superfluids", arXiv:1812.05133
[2] Tanguy Grall, Sadra Jazayeri, David Stefanyszyn, "The Cosmological Phonon: Symmetries and Amplitudes on Sub-Horizon Scales”, arXiv:2005.12937
[3] Sidney R. Coleman, J. Wess, Bruno Zumino, “Structure of phenomenological Lagrangians. 1“, Phys.Rev. 177 (1969) 2239-2247
[4] Curtis G. Jr. Callan, Sidney R. Coleman, J. Wess, Bruno Zumino, “Structure of phenomenological Lagrangians. 1“, Phys.Rev. 177 (1969) 2247-2250
[5] E.A. Ivanov, V.I. Ogievetsky, "The Inverse Higgs Phenomenon in Nonlinear Realizations”, Teor.Mat.Fiz. 25 (1975) 164-177,
[6] D.T. Son, “Low-energy quantum effective action for relativistic superfluids”, hep-ph/0204199

2020/6/1 (月)


Stochastic dynamics from Quantum mechanics

Jun'ichi Yokoyama

I will introduce several very old papers to extract
stochastic dynamics from quantum mechanics starting
from the Schroedinger equation, with a wish to apply
for quantum tunneling in real time. I will not go into
field theory, though.

References (chronological order)
E. Nelson "Derivation of Schroedinger equation from Newtonian mechanics"
Physical Review 150(1966)1079
F Guerra and P Ruggiero "New interpretation of the Euclidean-Markov
field in the
frame work of physical Minkowski spacetime" Physical Review Letters
K. Yasue "Detailed time dependent description of tunneling phenomena
arising from stochastic quantization" Physical Review Letters 40(1978)665
D.L. Weaver "Tunneling, stochastic quantization, and escape over a barrier"
Physical Review Letters 23(1978)1473
F. Guerra "Structural aspects of stochastic mechanics and stochastic
field theory"
Physics Reports 77(1981)263

2020/5/25 (月)


Quantum entanglement in the early Universe

Kouki Tokeshi

The quantum state becomes squeezed after inflation due to cosmological perturbations, and hence there exists quantum entanglement. In this talk, we start with the simplest example which shows quantum entanglement, and see the relation between Hawking temperature of a BH and the entangled particle pair near the horizon, in the viewpoint of themo field dynamics (TFD). After these examples, we consider the quantum squeezed state after inflation, and from the result we finally draw some implications on PBHs.

[1] A. Albrecht et al., PRD 50 (1994) 4807 [astro-ph/9303001]
[2] T. Prokopec, Class. Quantum Grav. 10 (1993) 2295-2306
[3] M. Hashizume and M. Suzuki, Physica A 392 (2013) 3518-3530 [cond-mat/1305.4679]
[4] L. Espinosa-Portalés and J. García-Bellido, PRD 101 (2020) 043514 [gr-qc/1907.07601]

2020/5/18 (月)


Inhomogeneous nucleosynthesis

Fumio Uchida

A nonstandard model for big-bang nucleosynthesis (BBN) is introduced in this talk. The standard theory of BBN developed after the famous paper written in 1948 by R. Alpher, H. Bethe, and G. Gamow, to have achieved a great success in describing the abundance of light nuclei in our universe. However, there remain some open questions such as the Lithium 7 problem. Attempts to model inhomogeneous big-bang nucleosynthesis (IBBN) is introduced, and so are the motivations for it and latest results.
In the standard BBN (SBBN) theory, homogeneity and isotropy of the distribution of all the constituents is assumed, as is supported by the Planck observation at large scales. In the IBBN models, on the contrary, baryon isocurvature perturbations at small scales is taken into account. Some theories lead to this situation, and the theory of baryogengesis from a (hyper)magnetic field is introduced as an example. Most IBBN studies simply use a 2-phase model, and the resultant nuclei abundance can be different from that SBBN predicts.
[1] J. F. Lara, Phys. Rev. D 72.2, 023509 (2005).
[2] B. D. Fields, et al. JCAP 2020.03, 010 (2020).
[3] R. A. Malaney, and G. J. Mathews, Phys. Rept. 229.4 145-219 (1993).

2020/5/11 (月)


Thermodynamical aspects of gravity and its application.

Takumi Hayashi

The entropy of the blackhole was proposed by Hawking and others several decades ago and developed into the notion of thermodynamics in gravity. It has still been intensively studied, because it could possibly offer the window to the nature of the quantum gravity such as the microstates of the theory.
In this seminor, I start from the emergence of thermodynamics at the classical BH horizon in the usual manner, and introduce the Euclidean analysis via quantum (or thermal) partition function, making use of Hamiltonian formulation of gravity.
Then I extend the thermodynamics to the modified gravity with higher curvatures with the help of the Wald’s entropy. I show examples in some theories how the entropy formula and thermodynamics works, and briefly introduce a recent application aimed at supporting the conjecture in quantum gravity.
The reference is listed below.
[1]T. Padmanabhan, Class. Quant. Grav.19 (2002) 5387-5408
[2]S. W. Hawking, Gary. T. Horowitz, Class. Quant. Grav. 13 (1996) 1487
[3]T. Jacobson, G. Kang, R. C. Myers, Phys. Rev. D 49 (1994) 6587
[4] C. Cheung, J. Liu, and G. N. Remmen, JHEP 10 (2018) 004

2020/4/27 (月)


Gravitational Wave Production right after Primordial Black Hole Evaporation

Keisuke Inomata

We discuss the footprint of evaporation of primordial black holes (PBHs) on stochastic gravitational waves (GWs) induced by scalar perturbations. We consider the case where PBHs once dominate the Universe but eventually evaporate before the big bang nucleosynthesis. The reheating through the PBH evaporation could end with a sudden change in the equation of state of the Universe compared to the conventional reheating caused by particle decay. We show that this "sudden reheating" by the PBH evaporation enhances the induced GWs, whose amount depends on the length of the PBH-dominated era and the width of the PBH mass function.
We explore the possibility to constrain the primordial abundance of the evaporating PBHs by observing the induced GWs. We find that the abundance parameter \beta >~ 10^{-5} - 10^{-8} for O(10^3 - 10^5) g PBHs can be constrained by future GW observations if the width of the mass function is smaller than about a hundredth of the mass.
This seminar will be based on our recent work, arXiv:2003.10455.

2020/4/20 (月)


Gravitational lensing of gravitational wave

Junya Kume

The effect of gravitational lensing on the propagation of gravitational wave (GW) have been extensively studied recently. Since the sensitivity of the GW detector improved and many GW events are now observed, we can have the opportunity to observe "lensed" GW.
In this seminar, I review the basic formulae of the gravitational lensing and explain what is the main difference between the lensing of light and that of GW. Then I briefly explain how this phenomena can affect observational result of GW from compact binary coalescence.

2020/4/13 (月)


Deformation of the gravitational wave spectrum by density perturbations

Ryusuke Jinno

We study the effect of primordial scalar perturbations on the propagation of cosmological gravitational waves (GWs). We point out that such scalar perturbations deform the power spectrum of any stochastic GWs of the early-Universe origin. We first show that, adopting linear order results in scalar perturbations, this deformation is described by the convolution of the original GW spectrum and a linearly biased Gaussian kernel, and then discuss how large this effect can be, taking the latest bounds on primordial black holes (PBHs). We also point out that full understanding of this effect requires calculations of second order in scalar perturbations.

2020/4/6 (月)


Time complexification for particle production

Soichiro Hashiba

Particle production, such as Hawking radiation, Schwinger effect and gravitational particle production, occurs in real time, however, we have to consider complex time in these phenomena for a proper estimation. That is because zeros of an effective frequency of a particle, which usually are located in complex time plane, plays a crucial role. Classically, a transition from a vacuum state to an excited state is prohibited anywhere but at these zeros, and in quantum field theory, a path integral picks up an effect of the zeros in complex time and particle production is realized. I will introduce several calculation methods of using time complexification for particle production. This talk will be mainly based on these papers:
Picard-Lefschetz theory - 1403.1277, 1510.03435, 1907.12224
Stokes phenomenon - 0911.4692, 1001.2933, 1004.2509, 1405.0302

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Last-modified: 2013-11-20 (水) 11:11:56 (2529d)