2021/7/19 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

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)

UTAPwiki/セミナー/初期宇宙・相対論速報

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.
Interestingly, it can be derived by the semiclassical method, which is unexpectedly simple.
In this talk, I will briefly review the information paradox and review the derivation of the fine-grained entropy formula in gravitational system, which is crucial to understand the recent proposals. If I have time, I will also explain how the expected behavior of the entropy, known as Page curve, is realized. (It is quite unlikely to have enough time, though.)

Refs:
A. Almheiri, T. Hartman, J. Maldacena, E.Shaghoulian, A. Tajdini, “The entropy of Hawking radiation” [2006.06872]
A. Lewkowycz, J. Maldacena, “generalized gravitational entropy” [1304.4926]
T. Faulkner, A. Lewkowycz, J. Maldacena, “Quantum corrections to holographic entanglement entropy” [1307.2892]
X. Dong, A. Lewkowycz, “Entropy, extremality, euclidean variations, and the equations of motion” [1705.08453]
A. Almheiri, T. Hartman, J. Maldacena, E.Shaghoulian, A. Tajdini, “Replica Wormholes and the Entropy of Hawking Radiation” [1911.12333]
G. Penington, S. Shenker, D. Stanford, Z. Yang, “Replica wormholes and the black hole interior” [1911.11977]

2021/7/5 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Title: Ceterum autem censeo, Gravitas esse quantizo

Speaker: Kohei Kamada

Niels Bohr has once argued in his private communications
that the consistency of the double-slit experiment of charged particle
with the detectors that can measure the Coulomb force
requires the quantization of the electromagnetic fields.
Motivated by the idea, recently people have studied the necessity
for the quantization of gravitons.
In this talk, I will introduce two studies on that,
where the later one concluded that graviton must be quantized.

Refs.
[1] Baym & Ozawa, PNAS 106, 3035, 2009, arXiv: 0902.2615 [quant-ph]
[2] Belenchia et al., PRD 98, 126009, 2018, arXiv:1807.07015 [quant-ph]

2021/6/28 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

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
on environmental noise estimation via injection
test toward removing them to improve sensitivity.
Then I will introduce works on detector characterization
which remove transient noises using auxiliary channels
to obtain cleaner data of strain channel.

References:
T. Washimi, T. Yokozawa, T. Tanaka, Y. Itoh, J. Kume, and J. Yokoyama Class. Quantum Grav 38(2021)125005 [arXiv:2012.09294 [gr-qc]]

2021/6/21 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

An Introduction to skyrmions

Fumio Uchida

Abstract:
In the last decade there seems to be a trend of studying skyrmions [1], mostly in the field of condensed matter physics. Although we are in the field of not condensed matter physics but cosmology, it could be worth understanding the concept of it, since it is a family of the important concepts in cosmology, e.g., monopoles, cosmic strings, and domain walls, in that they all are topological solitons. In the talk, I will introduce magnetic skyrmions [2], which are the very concept attracting much attention from condensed matter physicists, and also talk about skyrmion dark matter [3], in the context of cosmology.

References:
[1] T. H. R. Skyrme, “A Nonlinear field theory,” Proc. Roy. Soc. Lond. 260 (1961) 127,
[2] A. N. Bogdanov and D. A. Yablonskii, “Thermodynamically stable ‘vortices’ in magnetically ordered crystals. The mixed state of magnets,” Zh. Eksp. Teor. Fiz 95 (1989) 178,
[3] H. Murayama and J. Shu, “Topological Dark Matter,” Phys. Lett. B 686 (2010) 162. [arXiv:0905.1720 [hep-ph]]

2021/6/7 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Title: Quantum Simulation (for cosmology?)

Speaker: Soichiro Hashiba

Abstract:
Quantum computing is one of the most rapidly progressing fields in science. Nowadays, quite a few people try to use quantum computers in cosmology. The most straightforward use of quantum computer is the quantum simulation by which we can simulate (in other words, make analogy experiments of) quantum phenomena in the early Universe. In this talk, I will explain the concept of quantum simulation [1] and show some primitive examples [2,3]. Current quantum computers, however, are too noisy for practical simulations. Therefore, I will also give a brief explanation of a quantum-classical hybrid method called the variational quantum eigensolver (VQE) [4] and recent examples of its application for cosmology [5,6].

References:
[1] M. Nielsen and I. Chuang, “Quantum Computation and Quantum Information”, Cambridge University Press (2000).
[2] C. Argüelles and B. Jones, Phys. Rev. Res. 1, 033176 (2019).
[3] B. Hall, A. Roggero, A. Baroni and J. Carlson, 2102.12556 [quant-ph].
[4] A. Peruzzo et al., Nat. Commun. 5, 4213 (2014).
[5] J. Stecenko, Y. Feng and M. McGuigan, 2105.02032 [quant-ph].
[6] A. Joseph et al., 2105.13849 [quant-ph].

2021/5/31 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

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:
[1] J. Maldacena, JHEP 05, 013 (2003).
[2] N. Arkani-Hamed, et. al., JHEP 04, 105 (2020).
[3] E. Pajer, JCAP 01, 023 (2021).
[4] S. Jazayeri, et.al., arXiv: 2103.08649.
[5] H. Goodhew et. al., JCAP 04, 021 (2021).

2021/5/24 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

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:
[1] Phys. Rev. D 97 (2018), 081302
[2] arXiv:2104.05271 (hep-th)
[3] Phys. Rev. D 100 (2019), 083518
[4] Phys. Rev. Lett. 126 (2021), 041101

2021/5/17 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Title: Entanglement harvesting from vacuum in cosmology

Speaker: Takumi Hayashi

Abstract:
Entanglement is a characteristic notion of quantum physics and important physical resources which can be used for quantum computation, quantum teleportation, and so on. Recently, the entanglement acquisition process in which an unentangled pair of cubits become entangled by the Unruh-Dewitt detector interaction with the vacuum of quantum field, called “entanglement harvesting”, has been well studied. In this talk, I review the entanglement harvesting process in various situations, including Minkowski spacetime and cosmological backgrounds. And I also introduce the case with cubits uniformly accelerated, which can induce the nontrivial resonant production of entanglement. I briefly mention recent works as well regarding this topic.

References:
[1] Martin-Martinez, Eduardo and Menicucci, Nicolas C., Class. Quantum Grav. 31, 214001 (2014),[arXiv:quant-ph/1408.3420], “Entanglement in curved spacetimes and cosmology”
[2] Reznik, B.; Retzker, A.; Silman, J., Phys. Rev. A 2005, 71, 042104, [arXiv:quant-ph/0310058], “Violating Bell’s inequalities in vacuum”.
[3] Steeg, G.V.; Menicucci, N.C., Phys. Rev. D 2009, 79, 044027, [arXiv:quant-ph0711.3066], “Entangling power of an expanding universe”.
[4] G. Salton, R. B. Mann, and N. C. Menicucci, New J. Phys. 17, 035001 (2015), [arxiv:quant-ph/1408.1395v2], “Acceleration-assisted entanglement harvesting and rangefinding”.

2021/5/10 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

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:
[1]C. Gomez and R. Jimenez, Phys. Rev. D 102 (2020) no.6, 063511, [arXiv:2002.04294 [hep-th]].
[2]C. Gomez and R. Jimenez, arXiv:2012.04003 [hep-th].
[3]M. G. A. Paris, Int. J. Quantum. Inform. 07, 125 (2009).
[4]A. Albrecht, P. Ferreira, M. Joyce and T. Prokopec, “In-flation and squeezed quantum states,” Phys. Rev. D50,4807-4820 (1994) [arXiv:astro-ph/9303001 [astro-ph]]

2021/4/26 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

Title: Electroweak-like Baryogenesis with New Chiral Matter

Speaker: Kohei Fujikura

Abstract:
We propose a framework where a phase transition associated with a gauge symmetry breaking that occurs (not far) above the electroweak scale sets a stage for baryogenesis similar to the electroweak baryogenesis in the Standard Model. New chiral fermions charged under the extended gauge symmetry have nonzero lepton numbers, which makes the B-L symmetry anomalous. The new lepton sector contains a large flavor-dependent CP violation, similar to the Cabibbo-Kobayashi-Maskawa phase, without inducing sizable electric dipole moments of the Standard Model particles. A bubble wall dynamics associated with the first-order phase transition and sphaleron processes generate a lepton asymmetry, which is transferred into a baryon asymmetry via the ordinary electroweak sphaleron process. Unlike the Standard Model electroweak baryogenesis, the new phase transition can be of the strong first order and the new CP violation is not significantly suppressed by Yukawa couplings, so that the observed asymmetry can be produced. The model can be probed by collider searches for new particles and the observation of gravitational waves. This seminar is based on ref. [4].

References:
[1] G.R. Farrar and M.E. Shaposhnikov, “Baryon asymmetry of the universe in the minimal standard model" [hep-ph/9305275]
[2] M.B. Gavela, P.Hernandez, J. Orloff and O. Pene, “Standard Model CP violation and baryon asymmetry” [hep-ph/9312215]
[3] P. Huet and E. Sather, “Electroweak baryogenesis and standard model CP violation" [hep-ph/9404302]
[4] K. Fujikura, K. Harigaya, Y. Nakai and R. Wang “Electroweak-like Baryogenesis with New Chiral Matter” [arXiv:2103.05005]

2021/4/19 (月)

UTAPwiki/セミナー/初期宇宙・相対論速報

==============================

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.


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