Cosmology Seminars

Seminars in 2018

Speaker Keigo Shimada
Title Metric-affine Geometry and Scalar-tensor Theories
Date 9/20 (Thu.) 16:00-
Place H239
Abstract
Scalar-tensor theories in metric-affine geometry are formulated. General Relativity is currently the most successful gravitational theory which has surpassed countless of observations. However, in recent years, it has been noticed that GR cannot explain some cosmological phenomenon such as inflation, dark energy and dark matter. To solve this, countless alternative gravitational theories beyond General Relativity has been proposed. However, most require the geometry to be Riemannian, just as GR. In this talk, it will be shown how one could extend theories of gravity by 'deforming' Riemann Geometry into what is called metric-affine geometry, in which not only the metric but also that connection is an independent variable that is decided from the gravitational action. By applying metric-affine formalism to scalar-tensor theories, one notices that there are different and fruitful characteristics that appear when compared to the Riemann counterpart. Furthermore, application to inflation and its influence on observational variables would be shown. Finally, some possible applications would be discussed. References:Phys.Rev. D98 (2018) no.4, 044038 [arXiv:1806.02589]

Speaker Evangelos Sfakianakis (NIKHEF )
Title Higgsed Gauge-flation and beyond: Inflation with Non-abelian Gauge Fields
Date 9/3 (Mon.) 16:00-
Place H155B
Abstract
I will introduce a class of inflationary models, where inflation is driven exclusively or in part by non-abelian gauge fields. I will describe how the spectrum of density fluctuations and gravitational waves is computed, and show that these models can produce observationally viable spectra, if the gauge symmetry is spontaneously broken by a Higgs sector. The background gauge field texture violates parity, resulting in a chiral gravitational wave spectrum. This arises due to an exponential enhancement of one polarization of the spin-2 fluctuation of the gauge field. Higgsed Gauge-flation and related models can produce observable gravitational waves at inflationary energy scales well below the GUT scale.

Speaker Xian Gao (Sun Yat-sen University)
Title Spatially covariant gravity with velocity of the lapse function
Date 7/25 (Wed.) 16:30-
Place H156
Abstract
Covariant scalar-tensor theories of gravity can be reformulated as spatially covariant gravity (SCG) theories in the unitary gauge. In the previous work [1406.0822, 1409.6708], an important ingredient --- time derivative of the lapse function --- was overlooked. Based on our recent paper [1806.02811], I will discuss how to build SCG theories with kinetic terms for both the spatial metric and the lapse function. Generally such kind of theories propagate 4 physical degrees of freedom, one of which is a ghost mode. Through a detailed Hamiltonian analysis, we derive two conditions that the Lagrangian must satisfy in order to get rid of this ghost mode. I will also discuss some simple examples as well as cosmological implications.

Speaker Tomo Takahashi (Saga University)
Title Tensions in H_0 and sigma_8: Current status and implications
Date 6/28 (Thu.) 17:00-
Place H239
Abstract
It has been argued that there are discrepancies on the values of H_0 and sigma_8 between those derived from Planck satellite (CMB) and other (low redshift) observations such as direct measurement of H_0, weak lensing surveys and so on. We review the current status of the tensions and some possible solutions suggested so far. Its implications to cosmological models beyond the standard Lambda CDM are also discussed.

Speaker Tomoya Kinugawa (ICRR)
Title Remnants of first stars for the gravitational wave source
Date 6/13 (Wed.) 16:00-
Place H156
Abstract
Using our population synthesis code, we found that the typical chirp mass of binary black holes (BH-BHs) whose origin is the first star (Pop III) is ~30Msun with the total mass of ~60Msun so that the inspiral chirp signal as well as quasi normal mode (QNM) of the merging black hole are interesting targets of LIGO,VIRGO and KAGRA (Kinugawa et al.2014 and 2016). The detection rate of the coalescing Pop III BH-BHs is ~ 180 events/yr (SFR_p/(10^{-2.5} Msun /yr/Mpc^3))*([f_b/(1+f_b)]/0.33)*Err_sys in our standard model where SFR_p, f_b and Err_sys are the peak value of the Pop III star formation rate, the binary fraction and the systematic error with Err_sys=1 for our standard model, respectively. Furthermore, we found that the chirp mass has a peak at ~30Msun in most of parameters and distribution functions (Kinugawa et al.2016). This result predicted the gravitational wave events like GW150914 and LIGO paper said ‘recently predicted BBH total masses agree astonishingly well with GW150914 and can have sufficiently long merger times to occur in the nearby universe (Kinugawa et al. 2014)’ (Abbot et al. ApJL 818,22 (2016)).Thus, there is a good chance to check indirectly the existence of Pop III massive stars by gravitational waves.

Speaker Junsei Tokuda (Kyoto University)
Title Classical stochastic interpretation of infrared loops in de Sitter space
Date 2/27 (Tue.) 16:00-
Place H284A
Abstract
It is known that in the theory of a light scalar field during inflation, correlation functions suffer from infrared (IR) divergences or large IR loop corrections, leading to the breakdown of perturbation theory. Since such large IR loop corrections come from the deep-IR modes beyond the observable scale, these corrections are usually neglected in cosmology. However, in quantum mechanics, we cannot eliminate deep-IR modes from the theory by hand. Therefore, in order to clarify whether or not IR loops affect observables for local observers (us), we need to understand the physical meaning of IR loop corrections and what are observables for us. Motivated by these observations, we derive the IR dynamics which can correctly describe the IR loop corrections in the theory of a light scalar field with a general potential on de Sitter background. We show that the IR dynamics which can correctly recover all the IR loop corrections is equivalent to a classical stochastic process. This means that at least in our model, it is consistent to interpret all the IR loop corrections as an increase of the classical statistical variance. Ref: JCAP02(2018)014 (arXiv:1708.01734)

Speaker Emre Kahya (Istanbul Technical University)
Title GW170817 Falsifies Dark Matter Emulators
Date 2/1 (Tur.) 16:00-
Place H239
Abstract
The gravitational wave (GW) signal (GW170817) from the coalescence of binary neutron stars was simultaneously seen throughout the electromagnetic (EM) spectrum from radio waves to gamma-rays. We point out that this simultaneous detection rules out a class of modified gravity theories, and provides another indirect evidence for the existence dark matter.

Speaker Taotao Qiu (Institute of Astrophysics, Central China Normal University)
Title Effective Field Theory of Nonsingular Cosmology
Date 12/19 (Tue.) 16:00-
Place H284A
Abstract
Due to the recently proved no-go theorem, any nonsingular cosmological models based on the cubic Galileon suffer from pathologies. In this talk, I'll show how the EFT could help us clarify the origin of the no-go theorem, and offer us solutions to break the no-go. Particularly, we point out that the gradient instability can be removed by using some spatial derivative operators in EFT. Based on the EFT description, we obtain a realistic healthy nonsingular cosmological model, and show the perturbation spectrum can be consistent with the observations.

Speaker Toyokazu Sekiguchi (RESCEU)
Title Cosmological abundance of axions coupled to hidden photons
Date 12/5 (Tue.) 16:00-
Place H284A
Abstract
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 will consider the possibility that the QCD axion couples to hidden photons. It was recently argued that coupling of the axion to a hidden U(1) field suppresses the axion CDM 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 that production of the axion fluctuations plays a crucial roll and the suppression is moderated significantly. We also show that axion abundance can be enhanced rather than suppressed if the coupling is large enough.

Speaker Kenji Kadota (IBS)
Title light dark matter
Date 8/31 (Turs.) 16:00-
Place H239
Abstract
A few examples for the light dark matter will be
 presented along with their cosmological (e.g. galaxies, supernova) and 
the particle physics (collider, dark matter search) constraints to illustrate the complementarity 
between the particle physics and cosmology probes.

Speaker Ayuki Kamada (IBS)
Title Light Axinos from Freeze-in: production processes, phase space distributions, and Ly-alpha constraints
Date 8/18 (Fri.) 16:00-
Place H155B
Abstract
While decaying 7 keV dark matter (DM) is one of the most promising explanations of the 3.5 keV line excess, it is usually in tension with Ly-alpha forest observations when DM particles are produced from the thermal plasma. In this talk, we consider freeze-in production of axino DM in the supersymmetric (SUSY) Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) model in order to resolve this tension. Interestingly, freeze-in production of axinos results in a colder phase space distribution than the thermal one. By directly comparing the linear matter power spectra with Ly-alpha forest constraints, we demonstrate to what extent the tension with the Ly-alpha forest data can be mitigated. We introduce benchmark points with Higgsino next-to-light supersymmetric particle (NLSP) and wino NLSP. In the case of Higgsino NLSP, the most stringent Ly-alpha forest constraint can be evaded with mild entropy production from saxion decay inherent in the supersymmetric DFSZ axion model.

Speaker Karim Noui (Laboratoire de Mathématiques et Physique Théorique)
Title Towards a classification of higher order scalar tensor theories
Date 7/25 (Tue.) 16:00-
Place H284A
Abstract
There have been numerous attempts to modify or extend general relativity, with either the motivation to account for dark energy (and sometimes dark matter) or, more modestly, to construct benchmark models that are useful to test general relativity quantitatively. Scalar- tensor theories have often played a prominent role in these attempts and, lately, special attention has been devoted to scalar-tensor theories whose Lagrangians contain second-order derivatives of a scalar field. Lagrangians of this type, which contain “accelerations”, are generically plagued by an instability due to the presence, in addition to the usual scalar mode and tensor modes, of an extra scalar degree of freedom (unless the higher order terms can be treated as perturbative terms in the sense of low energy effective theories). Until recently, it was believed that only theories that yield second-order Euler-Lagrange equations were free of this dangerous extra degree of freedom. In the last couple of years, it has been realized that there in fact exists a much larger class of theories that satisfy this property. After introducing these theories, I present a few phenomenological aspects. In cosmology, these theories can be included in the unified effective description of dark energy and modified gravity.

Speaker Keisuke Inomata (ICRR)
Title Inflationary primordial black holes for the LIGO gravitational wave events and pulsar timing array experiments
Date 6/21 (Wed.) 16:00-
Place H156
Abstract
Primordial black holes (PBHs) are one of the candidates to explain the gravitational wave (GW) signals observed by the LIGO detectors. Among several phenomena in the early universe, cosmic inflation is a major example to generate PBHs from large primordial density perturbations. In this talk, we discuss the possibility to interpret the observed GW events as mergers of PBHs that are produced by cosmic inflation. The primordial curvature perturbation should be large enough to produce a sizable amount of PBHs, and thus we have several other probes to test this scenario. We point out that the current pulsar timing array (PTA) experiments already put severe constraints on GWs generated via the second-order effects, and that the observation of the cosmic microwave background puts severe restriction on its μ distortion. In particular, it is found that the scalar power spectrum should have a very sharp peak at k ∼ 10^6 Mpc−1 to fulfill the required abundance of PBHs while evading constraints from the PTA experiments together with the μ distortion. We propose a mechanism that can realize such a sharp peak. In the future, simple inflation models that generate PBHs via almost Gaussian fluctuations could be probed/excluded.

Speaker Norihiro Tanahashi (Osaka University)
Title Wave propagation and shock formation in the most general scalar-tensor theory
Date 5/24 (Wed.) 17:00-
Place H156
Abstract
We study the wave propagation in the most general scalar-tensor theory focusing on the shock formation caused by nonlinear effects. For this study we use the Horndeski theory, which is the most general scalar-tensor theory that gives second order equations of motion.The propagation speeds of the scalar field wave and gravitational wave depend on the environment and also their own amplitudes in this theory, and it causes various phenomena which cannot be seen in GR. To study the shock formation, we focus on transport of weak discontinuity in the metric and scalar field. We find that amplitude of the discontinuity generically diverges within finite time, which corresponds to shock formation. It turns out that the canonical scalar field and the scalar DBI model, among other theories described by the Horndeski theory, are free from such shock formation even on nontrivial background. We also observe that gravitational wave is protected against shock formation when the background has some symmetries at least. We will discuss implications of these findings.

Speaker Graziano Rossi(Sejong University)
Title Cutting-Edge Cosmology with Large-Volume Surveys
Date 4/27 (Thu.) 17:00-
Place H156
Abstract
Despite considerable progress in cosmology over the last decade, our knowledge of the Universe is still profoundly incomplete, forcing to invoke a mysterious dark energy component that propels the apparent acceleration in the present-day expansion rate of the Universe and affects the growth rate of large-scale structures and the motion of galaxies. Large-volume surveys are then the key to achieve substantial progress in the field. Currently, there is also an intense activity in neutrino cosmology, after the breakthrough discovery in particle physics that neutrinos are massive (2015 Nobel Prize in Physics). In the first part of my talk, I will provide a broad introductory cosmological tutorial, focused on the cosmic web and on cosmological surveys. In the second, I will introduce some state-of-the-art research topics that are currently top-priorities for eBOSS science, and also relevant for future large-volume surveys.

Speaker Sadra Jazayeri (IPM)
Title Unpromotable Residual Diffeomorphisms and Violation of Consistency Conditions in Cosmology
Date 4/11 (Tue.) 13:00-
Place H155B
Abstract
Having plenty of inflationary models consistent with the data, the importance of model-independent predictions of single field models is undeniable. Consequently, in the last decade there have been many efforts to understand consistency conditions in cosmology. These discussions mainly originated from the well known Maldacena consistency condition. This CR relates the squeezed limit of three point function of curvature perturbations into its two point function, in a single field model. The main idea behind the rigorous proof of CR s in cosmology is to use the residual diffeomorphisms of a certain gauge in cosmological perturbation theory and construct Ward identities associated with these new symmetries. In fact, the same strategy might be exploited to prove the conservation of curvature perturbations on super horizon scales. There are few known single field models in the literature violating 3 point function consistency condition. These include solid inflation and non attractor inflation. In the first part of this talk, I would address the technical reason on how these certain models evade 3pt CRs. In the second part, I would discuss about violation of CRs in a curved universe and show that this is closely related to the fact that residual diffeomorphisms in the curved background can not be extended into physical solutions.

Seminars in 2016

Speaker Takashi Hiramatsu(Rikkyo University)
Title CMB bispectrum
Date 3/15 (Wed.) 16:00-
Place H156
Abstract
We have developed a new Einstein-Boltzmann solver for CMB anisotropy to quantify the significance of the non-Gaussianity induced during the non-linear evolution of the primordial fluctuations after the last-scattering surface. In our numerical code, we implemented a 2nd-order line-of-sight formula, so-called "curve"-of-sight formula, which has been developed in our recent work in JCAP 1410 (2014) 051. In this talk, we show the numerical results on the non-Gaussianity of scalar perturbations at the late time. Furthermore, we are now extending this code to include the tensor modes, so we will also show the preliminary results on the bispectra induced by the tensor modes.

Speaker Cosimo Bambi (Fudan University)
Title Testing the Kerr paradigm using X-ray reflection spectroscopy
Date 1/26 (Thu.) 16:00-18:00
Place H156
Abstract
The spacetime metric around astrophysical black holes is supposed to be well described by the Kerr solution. However, deviations from the Kerr geometry are expected in a number of different scenarios beyond Einstein's gravity coupled to ordinary matter. In this talk, I will show that the study of the reflection spectrum of thin accretion disks is a promising approach to probe the metric around black holes and test the Kerr paradigm.

Speaker Toshifumi Noumi (Kobe Univ.)
Title Unitarity Constraints on the EFT of Inflation
Date 12/14 (Wed.) 17:30-
Place H156,Main building
Abstract
We provide a class of consistency conditions constraining the EFT of single-field inflation based on the unitarity of inflationary perturbations and the de Sitter symmetry. By analogy with the optical theorem, we introduce a set of posi tivity conditions on inflationary 4pt functions in the collapsed limit. In particular our unitarity constraints require the subluminality of inflationary perturbations in addition to positivity conditions on the EFT parameters, one of w hich was derived by Baumann et al. from analyticity of non-relativistic scatterings. We demonstrate that angular dependence of inflationary correlation functions plays a crucial role to reproduce the subluminality conditions. Interestin gly, we find that our positivity conditions may be used to constrain the inflationary 3pt functions, which are experimentally more accessible than 4pt functions. Observational prospects of our unitarity constrains are also discussed.

Speaker Joint Seminar Speaker 1: Rampei Kimura (Titech)
Title Extended vector-tensor theories
Date 9/13 (Tue.) 14:00-15:00
Place H111, Main Building
Abstract
Recently, several extensions of massive vector theory in curved space-time have been proposed in many literatures. In this talk, we consider the most general vector-tensor theories that contain up to two derivatives with respect to metric and vector field. By imposing a degeneracy condition of the Lagrangian in the context of ADM decomposition of space-time to eliminate an unwanted mode, we construct a new class of massive vector theories where five degrees of freedom can propagate, corresponding to three for massive vector modes and two for massless tensor modes. We find that the generalized Proca and the beyond generalized Proca theories up to the quartic Lagrangian, which should be included in this formulation, are degenerate theories even in curved space-time. Finally, introducing new metric and vector field transformations, we investigate the properties of thus obtained theories under such transformations.

Speaker Joint Seminar Speaker 2: Daisuke Yoshida (Titech)
Title Gravitational Scalar-Tensor Theory
Date 9/13 (Tue.) 15:15-16:15
Place H111, Main Building
Abstract
We consider a new form of theories of gravity in which the action is written in terms of the Ricci scalar and its first and second derivatives. Despite the higher derivative nature of the action, the theory is free from ghost under an appropriate choice of the functional form of the Lagrangian. This model possesses 2+2 physical degrees of freedom, namely 2 scalar degrees and 2 tensor degrees. We exhaust all such theories with the Lagrangian of the form f(R, (\nabla R)^2, \Box R), where R is the Ricci scalar, and then show some examples beyond this ansatz. In course of analysis, we prove the equivalence between these examples and a subclass of generalized bi-Galileon theories.

Speaker Joint Seminar Speaker 3: Toyokazu Sekiguchi (IBS)
Title A new constraint on millicharged dark matter from galaxy clusters
Date 9/13 (Tue.) 16:30-17:30
Place H111, Main Building
Abstract
Despite numerous attempts, little is known about the nature of dark matter (DM). Provided that the quantization of electromagnetic charge (e.g., monopole or GUT) is yet to be probed, DM may have a tiny but still nonzero electromagnetic charge. The charge of DM has been a subject of constraints from a variety of experiments and observations, including collider, direct detection, stellar evolution and cosmic microwave background. After reviewing these constraints, we will present our new constraint on millicharged DM from galaxy clusters. With nonzero charge, DM should undergo cyclotron motion in magnetic fields of O(1) Gauss in typical clusters. This leads to a density profile different from the CDM prediction and observations based on gravitational lensing. Once DM is assumed to consist only of millicharged particle, our constraint surpasses any of previous constraints by orders of magnitude for the relevant mass scale of DM. We will also discuss the backreaction onto magnetic fields, which is found negligible in most cases.

Speaker Robert Brandenberger (McGill U.)
Title Unified Description of Dark Energy and Dark Matter from Axion-Gauge Field Coupling
Date 8/29 (Mon) 10:30-
Place H156, Main Building
Abstract
We study cosmological solutions in a model in which a complex scalar field couples to the Pontryagin term of a higher scale non-Abelian gauge symmetry, in analogy to how the axion field couples to the chiral symmetry group of QCD. We find that if the potential energy function of the scalar field has the typical {\it Mexican hat} form, then radial fluctuations of the field can act as {\it Dark Matter}, and the phase becomes a candidate for tracking {\it Dark Energy}. The phase of dark energy domination, however, does not continue for ever. Eventually a new component of dark matter, namely that coming from the energy density of the new gauge fields, takes over.

Speaker Moslem Zarei(IUT& IPM)
Title Anisotropic inflation and CMB polarization
Date 8/24 (Wed) 16:30-
Place H284A, Main building
Abstract
In this talk, first I will review CMB physics and anisotropic inflation models. Then, I will talk about our recent work on anisotropic inflation with charge scalar field. In this model, a U(1) gauge field and charged scalar fields are coupled. I will show that there exist attractor solutions where the anisotropies produced during inflation becomes comparable to the slow-roll parameters. In the models where the inflaton field is a charged scalar field the gauge field becomes highly oscillatory at the end of inflation ending inflation quickly. Then, I will discuss about models of anisotropic inflation with the generalized non-vacuum initial states for the inflaton field and the gauge field. At the end, I will consider a dipole asymmetry in tensor modes and talk about the effects of this asymmetry on the angular power spectra of CMB.

Speaker Erfani, Encieh(IASBS, Zanjan)
Title Dark Matter Primordial Black Holes & their Formation
Date Aug. 24 (Wed.) 15:00-
Place H284A, Main building
Abstract
Primordial Black Holes (PBHs) are gravitationally collapsed objects that may have been created by density fluctuations caused by Inflation in the early universe. A broad range of single field models of inflation are analysed in light of all relevant recent cos- mological data, checking whether they can lead to the formation of long-lived PBHs as candidate for dark matter (DM). We also study the possibility that particle production during inflation can source the re- quired power spectrum for DM PBH formation. We consider the scalar and the gauge quanta production in inflation models and we do not assume any specific potential for the inflaton field. We show that in the gauge production case, the non-production of DM PBHs puts stronger upper bound on the particle production parameter. Our analysis show that this bound is more stringent than the bounds from the bispectrum and the tensor-to-scalar ratio derived by gauge production in these models. In the scenario where the inflaton field coupled to a scalar field, we put an upper bound on the amplitude of the generated scalar power spectrum by non-production of PBHs.

Speaker Jerome Quintin(McGill U.)
Title No-Go Theorem in Nonsingular Bouncing Cosmology
Date 6/7 (Tue.) 16:00-
Place 155B, Main building
Abstract
I will present the idea of matter bounce cosmology as an alternative to inflation. I will introduce a specific model with a non-canonical scalar field, a Galilean field, that avoids the Big Bang singularity by violating the Null Energy Condition. I will show how one can study the evolution of the cosmological perturbations through the nonsingular bounce and discuss the implications for the model in light of the observational constraints from the CMB (e.g., scalar tilt, tensor-to-scalar ratio, non-Gaussianity). In the end, I will present the no-go conjecture that nonsingular bouncing cosmology is believed to suffer from and briefly comment on some of its possible extensions and how one could get around it.

Speaker Rio Saitou (HUST)
Title Canonical invariance of spatially covariant scalar tensor theory
Date 6/3 (Fri.) 16:00-
Place H155B, Main building
Abstract
We investigate invariant canonical transformations of a spatially covariant scalar-tensor theory of gravity, called the XG theory. This theory includes gauge fixed forms of Horndeski theory which has been considered as the most general healthy scalar-tensor theory and the GLPV theory which is one of the beyond Horndski theory as its subclasses. We derive the Hamiltonian in a non perturbative manner and complete the Hamiltonian analysis for all regions of the XG theory. We confirm that the theory has at most 3 degrees of freedom in all regions of the theory as long as the theory has the symmetry under the spatial diffeormorphism. Then, we derive the invariant canonical transformation by using the infinitesimal transformation. The invariant metric transformation of the XG theory contains a vector product as well as the disformal transformation. The vector product and the disformal factor can depend on the higher order derivative terms of the scalar field and the metric in their general covariant forms. In addition, we discover the invariant canonical transformation which transforms the momentum of the metric. Using the invariant transformation, we study the relation between the Horndeski theory and the GLPV theory, and find that we can not obtain the arbitrary GLPV theory from the Horndeski theory through the invariant canonical transformation we have found.

Speaker Razieh Emami(HKUST)
Title TBA.
Date 4/27 (Wed.) 14:00-
Place 284A, Main building
Abstract
TBA.

Speaker Ryo Namba (IPMU)
Title Toward testing vacuum fluctuations of gravitational waves
Date 4/27 (Wed.) 16:00-
Place 284A, Main building
Abstract
The forthcoming missions for the cosmic microwave background anisotropies are expected to probe the tensor-to-scalar ratio with the sensitivity of order 10^(-3), highly surpassing the current upper bound. Detection of the tensor mode is often identified as originated from vacuum fluctuations of the gravitational waves (GWs) during inflation, a direct measure of the inflationary energy scale. Given the high expectation by the future experiments, it would be of great interest to test the validity of this identification. In this talk, I will discuss one of few existing models as a counterexample to this standard lore. In this model, GWs are significantly sourced by a vector field, leading to a visible signal for the upcoming observations. This contribution to GWs is uncorrelated with the vacuum fluctuations, and the three-point statistics can serve as a strong discriminator for such signals. I will present the theoretical setup of the model and the testability of the sourced tensor-mode signal with a special interest in LiteBIRD-like sensitivities.

Speaker Yasuho Yamashita(YITP)
Title Bigravity from gradient expansion
Date 3/29 (Tue.) 16:00-
Place 284A, Main building
Abstract
In general, bigravity, i.e., the gravitational model that contains two gravitons interacting with each other, was known to be suffered from an unavoidable ghost mode, which is called Boulware-Deser (BD) ghost. Recently, however, the restriction of the interaction to the specific form of non-derivative coupling proposed by de Rham, Gabadadze and Tolley is found to evade the BD ghost problem. We discuss how this ghost free bigravity coupled with a single scalar field can be derived from a braneworld setup. We consider DGP two-brane model without radion stabilization and obtain the effective four-dimensional action by solving the bulk configuration for given boundary metrics and substituting back the solution into the action under the gradient expansion. In the obtained effective theory, two gravitons interact through the Fierz-Pauli mass term and the radion remains as a scalar field, but its coupling to the metrics is non-trivial.

Speaker Alexander Vikman(YITP)
Title Mimetic Gravity
Date 2/18 (Thu.) 16:00-
Place H156, Main building
Abstract
In this talk I will discuss recently introduced mimetic scalar-tensor theories which are Weyl-invariant. Surprisingly these theories can naturally describe Dark Matter on linear scales. Moreover, these theories are interesting to model Inflation and Dark Energy. I will review the results from our recent papers arXiv:1512.09118, arXiv:1403.3961 and arXiv:1412.7136.

Speaker Satoshi Iso (KEK)
Title Dynamical fine-tuning of initial conditions in Small field inflations
Date 1/19 (Tue.) 16:00-
Place 284A, Main building
Abstract
LHCでのHiggs発見で、素粒子物理学は新たな時代に入った。 一方で、電弱スケールの安定性や起源には大きな謎が残っている。 この講演では、コールマンワインバーグ機構に基づく素粒子模型を説明し、 初期宇宙のインフレーションとCMB観測にどのような示唆があるかを説明する。 特に、最近、CMB観測でテンソルスカラー比に上限が与えられ、 small field inflationが再度、注目されている。 ところが、SFIでは場の初期条件に極めて不自然な微調整が必要なことが知られ ている。 上記の素粒子物理的な動機から出発して、この問題が、 インフレーション後の非摂動的な熱化(プレヒーティング)により ダイナミカルに解決できることを話す。

Seminars in 2015

Speaker Maresuke Shiraishi(IPMU)
Title Cosmological asymmetric correlators
Date 12/17 (Thu.) 16:00-
Place Room H239, Main building, Oookayama campus, Tokyo Institute of Technology
Abstract
There are many possibilities to violate symmetries (e.g., Gaussianity, parity and isotropy) in primordial cosmological fluctuations. Such asymmetries can produce a lot of distinctive signatures in late-time harmonic-space observables, since they are very sensitive to the statistical, spin and angular dependences. In this talk, I would discuss general responses of cosmological correlators (e.g., CMB) to such asymmetries, some theoretical models creating interesting signatures, and aspects of the data analysis.

Speaker Hassan Firouzjahi
Title Primordial anisotropies and asymmetries and cosmic inflation
Date 11/24 (Tue.) 16:00-
Place Room H155B, Main building, Oookayama campus
Abstract
In this talk I present our works on anisotropic inflation and primordial statistical anisotropies. I will review the power anisotropy in scalar, tensor and their cross-correlations in the model of anisotropic inflation. In addition, I present the EFT approach to study anisotropic inflation model independently. Finally I will review our work on generating hemispherical asymmetry from primordial domain walls during inflation.

Speaker Takahisa Igata (Rikkyo Univ.)
Title Gravitational two-soliton solutions in Levi-Civita spacetime
Date 11/4 (Wed.) 16:00-
Place Room H155B, Main building, Oookayama campus
Abstract
Recently, we have constructed a cylindrically symmetric exact solution to the Einstein equation by using Pomeransky's inverse scattering method. The solution shows gravitational soliton waves in the Levi-Civita spacetime and describes nonlinear interaction of gravitational waves such as gravitational Faraday rotation. We will discuss the physical property of this solution and will compare it with the other gravitational wave solutions with the same symmetry.

Speaker Jinn-Ouk Gong (APCTP)
Title WIMP isocurvature perturbation and small scale structure
Date 8/7 (Fri.) 16:00-
Place Room H155B, Main building, Oookayama campus
Abstract
The adiabatic component of perturbations is damped during the kinetic decoupling due to the collision with relativistic component on sub-horizon scales. However the isocurvature part is free from the damping and could be large enough to make a substantial contribution to the formation of small scale structure. We explicitly study the weakly interacting massive particles as dark matter with an early matter dominated period before radiation domination and show that the isocurvature perturbation is generated during the phase transition and leaves imprint in the observable signatures for the small scale structure.

Speaker Ivan Arraut (ITP)
Title The Higgs mechanism at the graviton level: The Vainshtein mechanism in time-domain
Date 7/21 (Tue.) 16:00-
Place Room H239, Main building, Oookayama campus
Abstract
I analyze the Higgs mechanism at the graviton level inside the non-linear theory of massive gravity. In the standard formulation, the graviton mass appears as a parameter multiplying the whole massive action and it cannot appear dynamically. Then the theory contains three free-parameters, namely, two inside the potential and the graviton mass. The spherically symmetric solutions of the theory revealed the existence of vacuum degeneracy. It appears due to the preferred time direction when the Stuckelberg function is non-trivial. Then any generator related to the time coordinate is potentially broken at the vacuum level, remaining then the spherical symmetry. For the gauge symmetries involved, I formulate the Higgs mechanism at the graviton level as a consequence of the Vainshtein mechanism but formulated in time domains by working in a 'free falling frame' of reference.

Speaker Katsuki Aoki (Waseda University)
Title Stability of the Early Universe in Bigravity Theory
Date 7/7 (Tue.) 16:00-
Place Room H284A, Main building, Oookayama campus
Abstract
It has been known that, when the graviton mass is smaller than the Hubble parameter, homogeneous and isotropic spacetimes suffer from the Higuchi-type ghost or the gradient instability against the linear perturbation in the bigravity. Hence, the bigravity theory has no healthy massless limit for cosmological solutions at linear level. In this talk, however, I will show that the instabilities can be resolved by taking into account nonlinear effects of the scalar graviton mode for an appropriate parameter space of coupling constants. The early history of the Universe in the bigravity is restored to the result in GR, in which the Stuckelberg fields are nonlinear and there is neither ghost nor gradient instability. Therefore, the bigravity theory has the healthy massless limit, and cosmology based on it is viable even when the graviton mass is smaller than the Hubble parameter.

Speaker Jinn-Ouk Gong (APCTP)
Title Searching for relativistic signatures on large scales
Date 5/13 (Wed.) 16:00-
Place Room H156, Main building, Oookayama campus
Abstract
TBA.

Speaker Yuki Sakakihara (Kyoto University)
Title Inflation in Bimetric Gravity
Date Apr. 30 (Wed.) 16:00-
Place Room H156, Main building, Oookayama campus
Abstract
Recently, a consistent theory including massive gravitons is constructed by de Rham, Gabadadze and Tolley although it has been thought to be difficult to obtain such a theory for a long time. The theory includes two metrics and it is called bimetric gravity when we treat both of them as dynamical variables. We investigated a minimal bimetric model. There are several branches of de Sitter solutions and we examined the stability of de Sitter solutions and finally we obtained the only stable solution. Then, we discussed inflationary solutions under slow-roll approximation. Concretely, we calculated tensor perturbations on the background solution and we showed the features of the primordial tensor spectrum.

Speaker Keiju Murata (Keio University)
Title Turbulent strings in AdS/CFT
Date Apr. 22 (Wed.) 16:30-
Place Room H156, Main building, Oookayama campus
Abstract
We study nonlinear dynamics of the flux tube between an external quark-antiquark pair in N=4 super Yang-Mills theory using the AdS/CFT duality. In the gravity side, the flux tube is realized by a fundamental string whose endpoints are attached to the AdS boundary. We perturb the endpoints in various ways and numerically compute the time evolution of the nonlinearly oscillating string. As a result, cusps can form on the string, accompanied by weak turbulence and power law behavior in the energy spectrum. When cusps traveling on the string reach the boundary, we observe the divergence of the force between the quark and antiquark. Minimal amplitude of the perturbation below which cusps do not form is also investigated. No cusp formation is found when the string moves in all four AdS space directions, and in this case an inverse energy cascade follows a direct cascade.

Speaker Shunichiro Kinoshita (Chuo University)
Title Non-equilibrium phenomena in holographic QCD
Date Apr. 22 (Wed.) 15:00-
Place Room H156, Main building, Oookayama campus
Abstract
The D3/D7 system is one of models constructing a QCD-like strongly coupled gauge theory, by using the AdS/CFT correspondence. In this model, fluctuations on the D7-brane describe excitations of mesons in the dual boundary theory, and configurations of the brane describe stable/unstable phases for the mesons. To explore non-equilibrium phenomena, we numerically solved dynamics of the D7-brane under time-dependent boundary conditions. In this talk, I will review a series of our works and show some results such as 'turbulent meson condensation' causing a non-equilibrium quark deconfinement transition.

Speaker Kohei Kamada (EPFL)
Title Higgs G-inflation and its self-consistency
Date Mar. 13 (Fri.) 16:00-
Place Room H155B, Main building, Oookayama campus
Abstract
After the discovery of Higgs particle at LHC, we can now say that the Standard Model of particle physics (SM) is completed. However, we cannot still explain the Universe with the SM as it is. In this talk, I will focus on inflation, which is now an (almost) indispensable ingredient in the modern cosmology, and explain the possible ways to drive inflation by the SM Higgs field itself. In particular, I will explain its nontrivial realization, Higgs G-inflation, and its self consistency. I will also discuss its remaining issues and the comparison with other Higgs inflation scenarios.

Speaker Hirotada Okawa (Waseda University)
Title On the nonlinear instability of confined geometries
Date Feb. 18 (Wed.) 16:00-
Place Room H156, Main building, Oookayama campus
Abstract
It was recently pointed out that anti-de Sitter(AdS) spacetime is unstable against gravitational collapse. The pertubation in AdS does not simply decay away and can be reflected by AdS boundary to nonlinearly interact with one another. Confinement would play an important role in the nonlinear instability. For instance, how does the gravitational collapse occur by an effective confinement? In this presentation, I would like to show our results and discuss open problems in this field after making a brief review.



Seminars in 2014

Speaker Masaki Yamada (ICRR)
Title 重力波で超対称性のスケールを探る
Date November 25 (Tue.) 16:00-
Place Room H284A, Main building, Oookayama Campus
Abstract
超対称性理論においては平坦方向とよばれるポテンシャルが平坦なスカラー場が大量に存在し、それらは宇宙初期に多彩なダイナミクスを引き起こす可能性がある。 このセミナーでは、インフレーション終了直後からしばらくの間に平坦方向がcosmic stringを形成する可能性を示す。 このcosmic stringから放射される重力波をとらえることができればそこから超対称性のスケールを知ることができ、加速器 実験と相補的な情報が得られることが期待される。 さらに 拡張として、フレーバー対称性があった場合にはどのようなことが起きるかも説明する。

Speaker Kentaro Tanabe (KEK)
Title Large D gravity
Date October 24 (Fri.) 15:00-
Place Room H115, Main building, Ookayama Campus
Abstract
General Relativity has one dimensionless parameter D, which is a spacetime dimension. We consider the infinite limit of D. Then we find that the gravity is drastically simplified but its structure remain non-trivial in that limit. Using this feature we can solve various problem of gravitational physics in analytic way. The analytic solution obtained by this large D method gives more deeper understanding to the gravity. In this talk we will review the fundamental property of large D gravity, and next apply the large D expansion method to some simple and advanced problem of black holes.

Speaker Hiroyuki Tashiro (Nagoya univ.)
Title Search for CP violation in the gamma ray sky
Date July 23 (Wed.) 16:00-
Place Room H156, Main building, Ookayama Campus
Abstract
Cosmological magnetic fields are one of cosmic relics which can be used as probes of the early universe and high energy particle physics. In this talk, motivated by the possible existence of a cosmological magnetic field with non-trivial helicity, we propose a CP odd statics to measure the magnetic helicity, using a gamma ray observation. We evaluate this statistic with gamma ray data obtained from Fermi satellite observations at high galactic latitudes. Observed values of Q are found to be non-zero at the 2¥sigma level and also deviate at the same level from values obtained from simulated data. Assuming that the excess is indeed due to a helical cosmological magnetic field, our results indicate left-handed magnetic helicity and field strength, 10^-14 G on about 10Mpc scales.

Speaker Kai Schmitz (IPMU)
Title Hybrid Inflation in the Complex Plane (http://arxiv.org/abs/1404.1832)
Date June 12 (Thu.) 16:45-
Place Room H156, Main building, Ookayama Campus
Abstract
Supersymmetric hybrid inflation is an exquisite framework to connect inflationary cosmology to particle physics at the scale of grand unification. Ending in a phase transition associated with spontaneous symmetry breaking, it can naturally explain the generation of entropy, matter and dark matter. Coupling F-term hybrid inflation to soft supersymmetry breaking distorts the rotational invariance in the complex inflaton plane---an important fact, which has been neglected in all previous studies. Based on the delta-N formalism, we analyze the cosmological perturbations for the first time in the full two-field model, also taking into account the fast-roll dynamics at and after the end of inflation. As a consequence of the two-field nature of hybrid inflation, the predictions for the primordial fluctuations depend not only on the parameters of the Lagrangian, but are eventually fixed by the choice of the inflationary trajectory. Recognizing hybrid inflation as a two-field model resolves two shortcomings often times attributed to it: The fine-tuning problem of the initial conditions is greatly relaxed and a spectral index in accordance with the PLANCK data can be achieved in a large part of the parameter space without the aid of supergravity corrections. Our analysis can be easily generalized to other (including large-field) scenarios of inflation in which soft supersymmetry breaking transforms an initially single-field model into a multi-field model.

Speaker Ivan Arraut (Osaka univ.)
Title Schwarzschild de-Sitter space and non-linear massive gravity
Date April 21 (Mon.) 17:00
Place Room H147B, Main building, Ookayama Campus
Abstract
Massive gravity theories have emerged as possible alternatives for solving the Dark Energy problem. Interestingly, in some scenarios like dRGT, the Schwarzschild de-Sitter (S-dS) solution share some common features with respect to the standard result obtained from the theory of General Relativity (GR) when we analyze some specific family of solutions. However some differences might appear due to the extra degrees of freedom in dRGT, which become highly relevant after the Vainshtein radius. I explore the possible differences between both approaches (dRGT and GR) inside the S-dS solutions by analyzing the results for black holes and some general aspects related to the local physics.





Seminars in 2013 (Including in Japanese)

日時 1月18日(火)16:00@H284
講師 住友洋介氏(KEK)
タイトル Probabilistic Approach for Vacuum Stability in String Theory
アブストラクト
We explore the possibility of positive vacuum energy in the string theory landscape, from the standpoint of probability that all eigenvalues of moduli mass matrix at extremal points turn to be positive. The positivity of mass matrix is motivated by stable de-Sitter hunting. Starting from the analysis of a large class of landscape, we discuss the property of the mini-landscapes characterized by models.

日時 12月18日(水)16:00@H156
講師 大橋勢樹氏(KEK)
タイトル Generalization of Horndeski's theory
アブストラクト
Recently Horndeki's theory has attracted much attention as the most general single-field inflationary model with second order field equations. In order to consider the wide variety of inflationary models in a unified manner, the more general theoretical framework is needed. In this talk, I will discuss about the generalization of Horndeski's theory. Mainly I will explain its generalization into two scalar fields case. I will also comment on some other generalizations.

日時 10月23日(水)16:00@H156
講師 藤田智弘氏(IPMU)
タイトル Curvature Perturbation in Stochastic Inflation
アブストラクト
In our recent paper arXiv:1308:4754, we propose a new approach for calculating the curvature perturbations produced during inflation in the stochastic formalism. In our formalism, the fluctuations of the e-foldings are directly calculated without perturbatively expanding the inflaton field and they are connected to the curvature perturbations by the $\delta N$ formalism. In this talk, we would like to introduce our formalism and discuss several unclear points: differences between our result and standard result obtained by the perturbative approach, differences between our formalism and the others proposed so far and to extend our formalism to general potential and/or multi-field cases.

日時 7/3(水) 16:00- @H156
講師 遠藤基氏
タイトル LHC実験の現状とMuon g-2が示唆するTeVの物理
アブストラクト
LHC実験におけるHiggs粒子の発見や新粒子探索の最新の結果が、TeV scaleの物理 とくに超対称標準模型にどのようなインパクトを与えるかについて最近の研究結果を紹介する。 TeV scaleの新しい物理のシグナルとして、ミューオン異常磁気モーメント(Muon g-2)の 精密測定が有望な候補の1つとして知られているが、Higgsの探索結果を取り入れることで TeV scaleにおけるSUSY模型はかなり特定されてくる。LHC実験におけるヒッグス粒子の 発見や新粒子探索の最新の結果に基づいて、Muon g-2から示唆されるSUSY模型について議論する。
関連論文:arXiv:1303.4256, 1212.3935

日時 6/11(火) 16:00- @H284A
講師 棚橋典大氏(IPMU)
タイトル 極限ブラックホールの地平面における新しい不安定性
アブストラクト
4次元時空におけるブラックホールについては長い研究の歴史があり、例えばそ の摂動に対する安定性がこれまでにほぼ確立されている。この常識に反して、表 面重力がゼロとなる極限ブラックホールについては、その地平面上の摂動が不安 定性を示すことが近年発見された。この不安定性が成長することで、摂動のエネ ルギー密度は最終的に地平面上で不連続となる。本発表では、この新しい不安定 性が発見された経緯を紹介するとともに、より一般的な状況でも同種の不安定性 が発生することを示してその物理的意義を考察する。また、重力反作用の効果を 考慮した場合に実現する時空の最終状態についても検証を行う。


日時 5/14(火) 16:00- @H284A
講師 小林直也氏(CITA)
タイトル Rolling in the Modulated Reheating Scenario
アブストラクト In the modulated reheated scenario, the field that drives inflation has a spatially-varying decay rate, and the resulting inhomogeneous reheating process generates adiabatic perturbations. In this talk, I describe the power spectrum and the bispectrum of the density perturbations generated in this scenario. Our derivation carefully follows the dynamics of the field that perturbs the inflaton decay rate. I will show that the statistics of the perturbations can be greatly modified by the dynamics of this modulus field, even if the field has a simple potential and a small effective mass compared to the Hubble scale. I will present examples in which non-Gaussianity is amplified by the modulus dynamics to values that are forbidden by Planck. Therefore, a proper treatment of the modulus dynamics is important in order to accurately calculate the resulting perturbations from modulated reheating.




日時 3/18(月) 13:30- @H284AB
講師 中村康二氏(国立天文台)
タイトル Progress and problems in general-relativistic higher-order gauge-invariant perturbation theory
アブストラクト Higher-order general-relativistic perturbation theory is one of topical subjects in recent researches on general relativity, e.g., cosmological perturbations, black hole perturbation and so on. To develop the higher-order general-relativistic perturbation theory in a gauge-invariant manner, the definition of gauge-invariant variables for 2nd- and 3rd-order perturbations was proposed in the paper [K. Nakamura, Prog. Theor. Phys. vol.110 (2003), 723.]. From this work in 2003, we have been developing the higher-order general-relativistic gauge-invariant perturbation theory. In this talk, I will explain the outline of our higher-order gauge-invariant perturbation theory and the recent progress of this theory. I also point out the problem to be clarified for the completion of our general-relativistic higher-order gauge-invariant perturbation theory.


日時 3/18(月) 16:00- @H284AB
講師 佐々木伸氏 (北里大学)
タイトル 超対称高階微分理論とインフレーション
アブストラクト 近年議論されている、高々2階の微分運動項とともに高次の微分相互作用を含む 系の超対称化を紹介する。特にwarped geometry上のD-brane有効作用(Dirac- Born-Infeld作用)のoff-shell超対称化とインフレーション模型について議論す る。時間があれば1/2 BPS解とその性質も紹介する。


2011年度以前に行われたコロキウム