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2021/4/19 (月)============================== 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: 2021/4/12 (月)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: 2021/3/8 (月)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)On Lorentz-invariant (bi-)spin-2 theories †Daisuke Yamauchi †In this talk, I will discuss the Lorentz-invariant massless and massive spin-2 theories 2021/2/22 (月)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). 2021/2/15 (月)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)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)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 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. Refs: 2021/1/25 (月)Low Scale Leptogenesis and the Perspectives to Test it in the Laboratory †Marco Drewes †Right-handed Majorana neutrinos can simultaneously explain the light neutrinos via the seesaw mechanism and generate the baryon asymmetry of the universe through leptogenesis. If their Majorana masses are at or below the electroweak scale they can be searched for in collider and fixed target experiments. We study the viable leptogenesis parameter space in this region to make predictions for the heavy neutrino properties, such as their mass spectrum, flavour mixing pattern and CP-properties. If any heavy neutral leptons are found in the laboratory, combining different observables allows to test whether these particles can indeed be responsible for baryogenesis and the neutrino masses. 2021/1/18 (月)Pion/axion domain wall in the presence of the magnetic fields and chiral asymmetry †Kohei Kamada †I will introduce the chiral soliton lattice which is the layer of the pion/axion domain walls References: 2020/12/21 (月)Toward detection of gravitational waves by pulsar timing arrays †Jun'ichi Yokoyama †An introduce will be given about the detection of gravitational waves by pulsar timing data including recent NANOGrav data. 2020/12/14 (月)Soft de-Sitter effective field theory (continue) †Jason Kristiano †de-Sitter space plays important role in cosmology, such as inflation of the universe is described by a scalar field in de-Sitter space. de-Sitter space is characterized by a dimensionful quantity, which is Hubble constant and corresponds to the typical energy scale of de-Sitter space. To predict some observable quantities, cosmologist usually interested in the late time limit of de-Sitter space. The energy scale at the late time limit is far below the typical energy of de-Sitter space. Such limit is called soft limit. An example in cosmology is power spectrum of inflation is evaluated at the superhorizon scale or soft limit. To calculate it, we usually use the full theory of de-Sitter space, then evaluate the soft limit at the end of the calculation. However, can we construct the effective field theory of soft de-Sitter space? In this talk, I will discuss the construction of soft de-Sitter effective field theory. Reference: 2020/12/7 (月)Soft de-Sitter effective field theory †Jason Kristiano †de-Sitter space plays important role in cosmology, such as inflation of the universe is described by a scalar field in de-Sitter space. de-Sitter space is characterized by a dimensionful quantity, which is Hubble constant and corresponds to the typical energy scale of de-Sitter space. To predict some observable quantities, cosmologist usually interested in the late time limit of de-Sitter space. The energy scale at the late time limit is far below the typical energy of de-Sitter space. Such limit is called soft limit. An example in cosmology is power spectrum of inflation is evaluated at the superhorizon scale or soft limit. To calculate it, we usually use the full theory of de-Sitter space, then evaluate the soft limit at the end of the calculation. However, can we construct the effective field theory of soft de-Sitter space? In this talk, I will discuss the construction of soft de-Sitter effective field theory. Reference: 2020/11/30 (月)Classical information geometry and its application to physics †Koki Tokeshi †Information geometry is a theory for statistical manifolds, on which many kinds of probability distributions are described geometrically. In particular, the so-called exponential family type probability distributions can be treated uniformly in the language of information geometry. The dually flat connection plays a crucial role in the theory as well as the Fisher information metric and Hessian structure of the manifolds. In this talk, I first review the general formulation of the theory. Although the theory has been succeeded to (re-)interpret the already known results in the statistical theory, its application to physics seems to be under construction, that is, many quantities have been calculated with their physical meanings remain uninterpreted, but some of them are referred to in this talk especially on the relation to general relativity or black hole physics. References: 2020/11/2 (月)Saddle-point solutions from gradient flow †Fumio Uchida †In most of the scenarios of baryogenesis, sphalerons play an important role because they violates baryon number. However, when one wants to find a sphaleron solution and estimate its energy, a difficulty sometimes arises because sphalerons are not local minima but saddle-points. Also, when one studies vacuum decay, to find bounce configurations is an important problem. In this case, one has to overcome the same difficulty because bounce solutions are saddle-points. However, a method that is intuitively understandable to find these solutions is proposed recently. In the seminar, I introduce the methodology and prove its validity, mainly following [1]. References: 2020/10/26 (月)Baryon isocurvature perturbations from non-helical magnetic fields †Fumio Uchida †The origin of the baryon asymmetry of the universe is a fundamental problem in cosmology. Among many proposed mechanisms that could solve it, baryogenesis from helical magnetic fields is characteristic in that it relates baryon asymmetry directly to the cosmic magnetic fields. In the scenario, the chiral anomaly in the Standard Model converts hypermagnetic helicity into baryon number. The observed baryon-to-entropy ratio gives conditions on the strength and the coherence length of the magnetic fields, and the generation of baryon isocurvature perturbations gives another constraint because it affects the big-bang nucleosynthesis. Counterintuitively, while the net baryon asymmetry is produced only from the helical part, the baryon isocurvature perturbations are generated from both helical and non-helical parts of the magnetic power spectrum. In the seminar, I focus on and explain how even non-helical magnetic fields can generate baryon isocurvature perturbations. References: 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]. References: 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. 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] The reference is listed below. |