## Cosmophysics Group Seminar

**17-May-2018**

Terada Takahiro「Semi-Analytic Calculation of Gravitational Wave Spectrum Induced from Primordial Curvature Perturbations」

Abstract:

Whether or not the primordial gravitational wave (GW) produced during inflation is sufficiently strong to be observable, GWs are necessarily produced from the primordial curvature perturbations in the second order of perturbation. The induced GWs can be enhanced by curvature perturbations enhanced at small scales or by the presence of matter-dominated stages of the cosmological history, both of which are motivated in primordial black hole scenarios to explain dark matter or the LIGO/Virgo event rate. We analytically calculate the integral in the expression of the power spectrum of the induced GWs which is a universal part independent of the primordial spectrum. This makes the subsequent numerical integrals significantly easy. In simple cases, we derive fully analytic formulae for the induced GW spectrum.

reference:

K. Kohri and T. Terada, 1804.08577 [gr-qc] (and 1802.06785 [astro-ph.CO])

**10-May-2018**

Takafumi Kokubu「Inhomogeneity Effect of Density Perturbations on Condition of Primordial Black Hole Formation」

Abstract:

Condition for Primordial Black Hole (PBH) formation in matter dominated era including inhomogeneity effect is derived with treating general relativity. We investigate how the inhomogeneity of density perturbation affects the formation of PBHs.

Black hole formation is determined by comparing two characteristic times, the formation time of the horizon and of the singularity. We investigate the threshold of the PBH formation by taking account of the speed of light propagating between the central singularity and the surface of a perturbation to be trapped.

reference:

[1] M. Y. Khlopov and A. G. Polnarev, Phys. Lett. 97B, 383 (1980).

[2] A. G. Polnarev and M. Y. Khlopov, Sov. Astronomy, 25, 406 (1981).

[3] T. Harada, C. M. Yoo, K. Kohri, K. i. Nakao and S. Jhingan, Astrophys. J. 833, no. 1, 61 (2016).

**26-April-2018**

Yohei Ema「Electroweak vacuum metastability in inflationary universe」

Abstract:

The recent measurements of the Higgs/top quark masses indicate that the electroweak (EW) vacuum may be metastable in the high energy region. In this talk, I will give an overview of implications of the EW vacuum metastability on the inflationary cosmology. A particular attention will be paid to the dynamics of the Higgs field during/after inflation.

reference:

[1] Y. Ema, K. Mukaida, K. Nakayama, “Fate of Electroweak Vacuum duringPreheating,” arXiv:1602.

[2] Y. Ema, M. Karciauskas, O. Lebedev, M. Zatta, “Early Universe Higgsdynamics in the presence of the Higgs-inflaton and non-minimal Higgsgravity couplings,” arXiv:1703.04

[3] Y. Ema, K. Mukaida, K. Nakayama, “Electroweak Vacuum Metastability andLow-scale Inflation,” arXiv:1706.089

[4] J. R. Espinosa, G. F. Giudice, E. Morgante, A. Riotto, L. Senatore, A.Strumia, N. Tetradis, “The cosmological Higgstory of the vacuuminstability,” arXiv:1505.04

[5] W. E. East, J. Kearney, B. Shakya, H. Yoo, K. M. Zurek, “SpacetimeDynamics of a Higgs Vacuum Instability During Inflation,” arXiv:160700

**19-April-2018**

Misao Sasaki「Scalaron as a Heavy Field and Formation of Primordial Black Holes」

Abstract:

We propose a model of two-stage inflation in which a scalar field, chi, is non-minimally coupled to Starobinsky's R2 gravity. The scalaron, phi, in Starobinsky’ model drives the first stage of inflation, and chi drives the second. At the end of the first stage, the scalaron becomes heavy and undergoes damped oscillations. This causes enhancement and oscillatory features in the curvature perturbation power spectrum. The peak in these features may give rise to copious production of primordial black holes (PBHs). For a suitably chosen set of model parameters, these PBHs may even dominate the CDM of the Universe.

reference:

**12-April-2018**

Takahiko Matsubara「The Large-scale Structure and the Lagrangian Perturbation Theory」

Abstract:

The Lagrangian perturbation theory (LPT) is an alternative method of perturbation theory for the cosmologically nonlinear structure formation. In this talk, I will introduce a formulation dubbed integrated Perturbation Theory (iPT), which is based on the LPT, and present its consequences in correlation analyses of galaxy surveys.

reference: