## Cosmophysics Group Seminar

**4-Oct-2018**

Tomoaki Ishiyama「Supercomputer simulations of dark matter structure formation in the Universe」

Abstract:

We report the results of ultralarge cosmological N-body simulations of small and large scale structure formation in the Universe. In particular, I focus on the formation and evolution of dark matter halos and subhalos near the free streaming scale, and their impact on the indirect dark matter detection experiments. We also present the implementation of our massively parallel simulation code for modern huge supercomputers.

reference:

**26-Jul-2018**

Taro Mori「Multi-Moduli inflation」

Abstract:

I will introduce multi-field inflation model which contains two dynamical moduli fields. These two moduli fields play a role of inflatons. In the first part of this talk, I'II briefly review what are moduli fields and why we need to stabilize them. After that, I'll talk about Kähler Moduli Inflation. This model is based on stabilized moduli potential. In the end of this talk, I'll briefly report the analysis on Multi-Moduli Inflation by my dN code.

reference:

・ J.Conlon, F.Quevedo (2005) [arXiv:0509012]

・ M.Cicoli et al. (2008) [arXiv:0808.0691]

**19-Jul-2018**

Takuya Hasegawa「Neutrino Thermalization in the Low Reheating Temperature Universe」

Abstract:

Some kinds of massive particles in the Beyond standard model (e.g. gravitino, flaton) can have a very long lifetime and decay at around the epoch of Big Bang Nucleosynthesis (BBN). The entropy production induced by the decay induces the reheating of the universe, which reset the background and gives the initial condition of the Universe. In the case where the reheating temperature is of O(1)MeV (which corresponds to the lifetime of O(1)sec), the current success of BBN theory might be spoiled mainly due to the incomplete thermalization of neutrinos which feebly interact with other particles. Therefore, in order to obtain the accurate bound on the reheating temperature, it is important to understand the production process of neutrinos considering the effect of flavor oscillations. In addition, if the long-lived massive particles have a branching ratio into hadrons, there are some extra effects on BBN, which affects the constraint on the late-time entropy production. In this talk, I will explain the thermalization process of neutrinos in the Universe with O(1)MeV and show some results on BBN.

reference:

・ P.F. Salas et al. (2015) [arXiv: 1511.00672]

・ K. Ichikawa et al. (2005) [arXiv: astro-ph/0505395]

・ M. Kawasaki et al. (2000) [arXiv: astro-ph/0002127]

**12-Jul-2018**

Nobuya Nishimura「Synthesis of the Trans-Iron Elements in Stars」

Abstract:

Stellar nucleosynthesis beyond iron-group nuclei provides additional problems that we do not encounter when studying nucleosynthesis processes for light nuclei. Besides ambiguity in the determination of astrophysical sites/conditions, uncertainties in nuclear physics inputs have signifiant impacts on the theoretical nucleosynthesis predictions. In this talk, I present recent results of comprehensive nucleosynthesis studies, focusing on the uncertainties of relevant nuclear reactions and decays. The evaluated uncertainty range for the several nucleosynthesis processes, e.g., s- and r-processes, gamma-process and nup-process, are discussed. I also show the lists of “key reactions”, which have strong impacts on the nucleosynthesis yields, based on our Monte-Carlo based analysis.

reference:

**28-Jun-2018**

Chinami Kato「The evolutions of massive stars and the importance of neutrino observations」

Abstract:

Massive stars with more than 8 solar mass initially are supposed to be progenitors of core-collapse supernovae, however, there still remain many problems about the understanding of their evolutions and deaths, especially the interior profile of cores and the explosion mechanism. In order to solve these problems, some observations are necessary. Then we focus on the observations of “neutrinos”, which are emitted inside the stellar core or proto-neutron stars and decide their evolutionary paths. In this talk, I want to talk about what we can learn about the stellar evolutions from the observations of pre-SN and SN neutrinos and what we should do theoretically for the next galactic supernovae which will occur in the near future. In detail, we calculate the realistic neutrino luminosities and spectra for all flavors and estimate the number of events at the terrestrial neutrino detectors.

reference:

**21-Jun-2018**

Takasumi Maruyama「Light Sterile Neutrino: is there any good pictures?」

Abstract:

In this seminar, the status of the light sterile neutrino searches and whether there is good and reasonable physics pictures for the neutrinos are discussed. The light sterile neutrino is non-weak interactive neutrino, and it was introduced to explain the results of the LSND experiment. The idea to exist the particle which is sensitive to gravity but not to strong, electromagnetic and weak forces are interesting, however, currently only LSND and MiniBooNE experiments provided positive results. This seminar reviews the status of the sterile neutrino searches in the world, which includes the JSNS2 experiment (J-PARC Sterile Neutrino Search at the J-PARC Spallation Neutron Source). The experiment aims the direct confirmation or refute the LSND results. It will be started from the end of JFY2018.

reference:

**14-Jun-2018**

Koutarou Kyutoku「Initial data of compact object binaries for numerical relativity」

Abstract:

Because nonlinear gravity and hydrodynamics play a crucial role, numerical-relativity simulations are necessary to understand accurately the merger stage of compact binary coalescences. In general relativity, valid initial data have to satisfy Hamiltonian and momentum constraints (reminiscent of divergence equations in Maxwell theory). Furthermore, astrophysically realistic time-evolution simulations can be performed only with astrophysically realistic initial data. These facts require us to solve constraint equations imposing realistic conditions. In this seminar, I will review the method to derive desirable initial data of compact object binaries for numerical relativity with a blackboad.

reference:

E. Gourgoulhon [gr-qc/0703035] (review)

**31-May-2018**

Nagisa Hiroshima「Modeling evolution of Dark Matter substructure and annihilation boost」

Abstract:

We study evolution of dark matter substructures, especially how they lose the mass and change density profile after they fall in gravitational potential of larger host halos. We develop an analytical prescription that models the subhalo mass evolution and calibrate it to results of N-body numerical simulations of various scales from very small (Earth size) to large (galaxies to clusters) halos. We then combine the results with halo accretion histories, and calculate the subhalo mass function that is physically motivated down to Earth-mass scales. Our results --- valid for arbitrary host masses and redshifts --- show reasonable agreement with those of numerical simulations at resolved scales. Our analytical model also enables self-consistent calculations of the boost factor of dark matter annhilation, which we find to increase from tens of percent at the smallest (Earth) and intermediate (dwarfs) masses to a factor of several at galaxy size, and to become as large as a factor of ∼10 for the largest halos (clusters) at small redshifts. Our analytical approach can accommodate substructures in the subhalos (sub-subhalos) in a consistent framework, which we find to give up to a factor of a few enhancement to the annihilation boost. Presence of the subhalos enhances the intensity of the isotropic gamma-ray background by a factor of a few, and as the result, the measurement by Fermi Large Area Telescope excludes the annihilation cross section greater than ∼4×10−26 cm3 s−1 for dark matter masses up to ∼200 GeV.

reference:

arXiv 1803.07691、1403.6827

**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: