## Cosmophysics Group Seminar

**1-Jun-2017**

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

**18-May-2017**

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]].

**13-Apr-2017**

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.