Cosmophysics Group Seminar


Nagisa Hiroshima「Scaling relation of the astrophysical J-factor of the dwarf spheroidal galaxies」

Abstract: Recently, arXiv:1802.06811 provided an updated scaling relations for the astrophysical J-factor , which is a key quantity in the indirect search of the dark matter, of dwarf spheroidal galaxies. Their results indicate that J(<0.5deg) can be scaled with the line-of-sight velocity dispersion, distance, and half-light radius of the target dwarf spheroidal galaxy. In this talk, I briefly review the paper and explain the physical origin of the derived scaling law.
reference: A.B. Pace et al. (2018) [arXiv: 1802.06811 (astro-ph)] N.W. Evans et al. (2016) [arXiv: 1604.05599 (astro-ph)]


Takuya Hasegawa「Production of Dark Matter Sterile Neutrino in the Early Universe」

Abstract: In this talk, I will present the production process of sterile neutrinos in the early universe. Especially, I focus on the massive sterile neutrino with masses in the keV-MeV range which is a plausible candidate of Dark Matter. Such massive sterile neutrinos are produced from primordial plasma via neutrino oscillations mainly at around a temperature of O(100)MeV. Therefore, the final abundances of sterile neutrinos strongly depends on the QCD dynamics. This time, I will review the paper on this topic.
reference: i ) T.Venumadhav, F.Cyr-Racine, K.N.Abazajian, C.M.Hirata 2015 (arXiv:1507.06655) ii ) K.N.Abazajian 2017 (arXiv:1705.01837)


Aya Ishihara「Recent progresses on the neutrino astronomy from IceCube」

Abstract: IceCube is a cubic kilometer scale, deep-ice Cherenkov neutrino detector at the South Pole. IceCube's cosmic neutrino searches cover an energy region all the way from TeV and less to EeV and higher. Following the first observation of PeV neutrino events and successful measurements of high energy extraterrestrial neutrino flux in the energy region between a few tens of TeV and PeV significantly above the atmospheric neutrino background flux by IceCube, the field of neutrino astrophysics is becoming more and more active. Also in the EeV energy region, a flux of 'cosmogenic' neutrinos generated by interactions of ultra-high energy cosmic rays on intervening radiation backgrounds is expected. The cosmogenic neutrinos will give us insight into the sources of the ultra-high energy cosmic- rays (UHECRs). In this talk, I review the recent results from IceCube on cosmic neutrino observations. In particular I highlight the multi-messenger observation using the neutrino as the trigger for the gamma-ray/x-ray/optical telescopes, and constraints on the UHECR sources from the observation of cosmogenic neutrinos. Finally I discuss the future prospects for such activities.


Taro Mori「Slow-roll v.s. Constant-roll」

Abstract: In this talk, I will briefly review so-called constant-roll inflation. Especially, I focus on the violation of the consistency relation of scalar/tensor perturbation. I also discuss the possibility to enhance the amplitude of perturbation on small scale(smaller than CMB scale).


Koutarou Kyutoku「GRB170817A」

Abstract: I review the current status of multiband observations for GRB 170817A, the first short gamma-ray burst associated with gravitational waves from a binary-neutron-star merger. A top-hat ultra-relativistic jet is disfavored from increasing brightness of radio and X-ray observations, and rather cocoon(-like) models are preferred.


Yoshiyuki Inoue「Toward the understanding of magnetic fields in the vicinity of active supermassive black holes」

Abstract: Magnetic fields of active supermassive black holes have not been measured yet, although it is one of the most fundamental parameters in black hole physics such as for launching mechanism of relativistic jets and generation of hot coronae. In this talk, I will report our new measurements of magnetic fields in several tens Schwarzschild radii of central black holes. Utilizing ALMA and NuSTAR, we successfully detected coronal synchrotron emission. The estimated magnetic field strength is not strong enough to keep corona hot.


Toyokazu Sekiguchi「Cosmological applications of axion electrodynamics」

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’m going to focus on their rolls at later epoch when the Universe is dominated by the radiation. In the first part of my talk, such an axion is identified with the QCD axion. It is 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 production of the axion fluctuations plays a crucial roll and the suppression is moderated significantly. In the latter part, I will discuss an application to the cosmological relaxation, which we show can be compatible with reheating temperature higher than the electroweak scale.


Takashi Hiramatsu「Field-theoretic simulations of cosmic strings」

Abstract: Cosmic strings had been studied originally as potential seeds of the large-scale structure of the Universe, which had eventually been replaced by inflation, but the recent concern on them would be an aspect as a possible source of gravitational waves. The cosmic strings have a network structure spread over superhorizon scales, and their reconnection process is crucial for their networks to evade the strong observational constraints and thus to survive in the present time. In this talk, we show our recent numerical works on the reconnection process and the time-evolution of cosmic strings networks based on the 3D field-theoretic simulations. Furthermore, we introduce some preliminary results on the gravitational waves radiated from the cosmic strings networks.


Kazunori Kohri「Revisiting Big-Bang Nucleosynthesis Constraints on Decaying Particles」

Abstract: We study effects of massive particles decaying during the epoch of big-bang nucleosynthesis (BBN) on the primordial values of light element abundances. We improved our computations related with effects by electronically and hadronically decaying modes. We update reaction rates of the standard and non-standard processes, deserving particular attention on effects by anti-nucleons emissions, interconversion reactions of neutron and proton at inelastic scatterings of energetic nucleons. Compared our theoretical predictions of light-element abundances with latest observational values, we obtain upper bounds on the abundance of the decaying particle as a function of its lifetime. Even if we adopt the observational 4He abundance reported by Izotov, Thuan and Guseva (2014), we find an allowed region in the parameter space. We also apply the results to unstable gravitino which appears in supergravity and obtain an upper bound on reheating temperature after inflation. Implications to leptogenesis scenarios based on particle-physics models beyond the standard model are also discussed.
reference: Kawasaki, Kohri, Moroi and Takaesu (2017) in progress


Takahiro Terada「Curvaton as Dark Matter - with the aid of a second inflation -」

Abstract: We consider a novel cosmological scenario in which a curvaton is long-lived and plays the role of cold dark matter (CDM) in the presence of a short, secondary inflation. Non-trivial evolution of the large scale cosmological perturbation in the curvaton scenario can affect the duration of the short term inflation, resulting in the inhomogeneous end of inflation. Non-linear parameters of the curvature perturbation are predicted to be f_NL ~ 5/4 and g_NL ~ 0. The curvaton abundance can be well diluted by the short-term inflation and accordingly, it does not have to decay into the Standard Model particles. Then the curvaton can account for the present CDM with the isocurvature perturbation being sufficiently suppressed because both the adiabatic and CDM isocurvature perturbations have the same origin. As an explicit example, we consider the thermal inflation scenario and a string axion as a candidate for this curvaton-dark matter.
reference: J.O.Gong, N.Kitajima and T.Terada,``Curvaton as dark matter with secondary inflation,''JCAP (2017) no.03, 053 [arXiv:1611.08975 [hep-ph]].


Takafumi Kokubu「Stability of Wormholes with Singular Hypersurfaces in Einstein and Gauss-Bonnet theories of gravity」

Abstract:We introduce a way to a spacetime short-cut that might be realized in theoretical physics. Such a short-cut provides us a very fast travel connecting the distant two points, namely, a faster-than-light travel. It is surprising that such science-fiction-like topics are put on the subject to theoretical physics. In classical theory of gravitational physics, one of these topics is a wormhole. Wormhole is a spacetime structure which connects two different universes or even two points of our universe. General relativity, the most successful and the simplest theory of classical gravitational theories, predicts a wormhole spacetime. Besides, quantum physics may support the possibility for existence of wormholes. In this presentation, we pursue the possibility for eternal existence of such objects. First, we introduce properties of wormholes with its history of discoveries. Next, we review thin-shell wormholes that are categorized into a class of wormhole solutions. After that, we investigate negative tension branes as stable thin-shell wormholes in Reissner-Nordstr anti-de Sitter spacetimes in d dimensional Einstein gravity. Imposing Z2 symmetry, we construct and classify traversable static thin shell wormholes in spherical, planar (or cylindrical) and hyperbolic symmetries. In spherical geometry, we find the higher dimensional counterpart of Barcel'o and Visser's wormholes, which are stable against spherically symmetric perturbations. We also find the classes of thin shell wormholes in planar and hyperbolic symmetries with a negative cosmological constant, which are stable against perturbations preserving symmetries. In most cases, stable wormholes are found with the appropriate combination of an electric charge and a negative cosmological constant. However, as special cases, we find stable wormholes even with vanishing cosmological constant in spherical symmetry and with vanishing electric charge in hyperbolic symmetry. Finally, the effect of the Gauss-Bonnet term on the existence and dynamical stability of thin-shell wormholes as negative tension branes is studied in the arbitrary dimensional spherically, planar, and hyperbolically symmetric spacetimes with a cosmological constant. We consider radial perturbations against the shell for the solutions, which have the Z2 symmetry. The effect of the Gauss-Bonnet term on the stability depends on the spacetime symmetry. For planar symmetric wormholes, the Gauss-Bonnet term does not affect their stability and they are at most marginally stable. If the coupling constant is positive and small, our setup proves that spherically symmetric wormholes are unstable against perturbations and the Gauss-Bonnet term tends to destabilize the wormholes. For hyperbolically symmetric wormholes, the Gauss-Bonnet term tends to stabilize them and there are stable wormholes.

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