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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. 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. References: 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. 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. 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]]. 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 References: 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. References 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 References 2020/6/1 (月)Stochastic dynamics from Quantum mechanics †Jun'ichi Yokoyama †I will introduce several very old papers to extract References (chronological order) 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. Refs. 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. |