Takashi Hiramatsu

J-GLOBAL         Last updated: May 4, 2019 at 22:09
Takashi Hiramatsu
The University of Tokyo
Institute for Cosmic Ray Research

Research Areas


Academic & Professional Experience

Sep 2016
Postdoctoral fellow, Department of Physics, Collage of Science, Rikkyo University
Apr 2016
Aug 2016
Research fellow, Yukawa Institute for Theoretical Physics, University of Kyoto
Apr 2011
Mar 2016
Program-specific Researcher, Yukawa Institute for Theoretical Physics, University of Kyoto
Apr 2010
Mar 2011
Research fellow, Yukawa Institute for Theoretical Physics, University of Kyoto
Apr 2008
Mar 2010
Research fellow, Institute for Cosmic Ray Research, The University of Tokyo


Apr 2004
Mar 2007
Department of Physics, Doctor's course, Graduate school of Science, The University of Tokyo
Apr 2002
Mar 2004
Department of Physics, Master's cource, Graduate school of Science, The University of Tokyo
Apr 2000
Mar 2002
Department of Physics, School of Science, The University of Tokyo
Apr 1998
Mar 2000
Natural Sciences I, Collage of Arts and Sciences, The University of Tokyo

Published Papers

Takashi Hiramatsu, Shuichiro Yokoyama, Tomohiro Fujita, Ippei Obata
D98 083522   Oct 2018   [Refereed]
We investigate a possibility of constraining statistical anisotropies of the
primordial tensor perturbations by using future observations for the Cosmic
Microwave Background (CMB) B-mode polarization. By parameterizing a
statistically-anisotropic ...
Tsutomu Kobayashi, Takashi Hiramatsu
Physical Review   D97 104012   May 2018   [Refereed]
We study relativistic stars in degenerate higher-order scalar-tensor theories
that evade the constraint on the speed of gravitational waves imposed by
GW170817. It is shown that the exterior metric is given by the usual
Schwarzschild solution if t...
Takashi Hiramatsu, Eiichiro Komatsu, Masashi Hazumi, Misao Sasaki
Physical Review   D97 123511   Jun 2018   [Refereed]
Given observations of B-mode polarization power spectrum of the cosmic microwave background (CMB), we can reconstruct power spectra of primordial tensor modes from the early Universe without assuming their functional form such as a power-law spect...
Hiromu Ogawa, Takashi Hiramatsu, Tsutomu Kobayashi
   Feb 2018   [Refereed]
The Vainshtein mechanism is known as an efficient way of screening the fifth force around a matter source in modified gravity. This has been verified mainly in highly symmetric matter configurations. To study how the Vainshtein mechanism works in ...
D. A. Steer, Marc Lilley, Daisuke Yamauchi, Takashi Hiramatsu
Phys. Rev. D 97, 023507 (2018)      Oct 2017   [Refereed]
Under certain conditions the collision and intercommutation of two cosmic strings can result in the formation of a third string, with the three strings then remaining connected at Y-junctions. The kinematics and dynamics of collisions of this type...
Guillem Dom?nech, Takashi Hiramatsu, Chunshan Lin, Misao Sasaki, Maresuke Shiraishi, Yi Wang
   Jan 2017   [Refereed]
We consider a cosmological model in which the tensor mode becomes massive during inflation, and study the Cosmic Microwave Background (CMB) temperature and polarization bispectra arising from the mixing between the scalar mode and the massive tens...
Sachiko Kuroyanagi, Takashi Hiramatsu, Jun'ichi Yokoyama
JCAP 02 (2016) 023      Sep 2015   [Refereed]
We investigate the imprint of reheating on the gravitational wave spectrum produced by self-ordering of multi-component scalar fields after a global phase transition. The equation of state of the Universe during reheating, which usually has differ...
Takashi Hiramatsu, Yuhei Miyamoto, Jun'ichi Yokoyama
   Dec 2014   [Refereed]
The mechanism of thermal inflation, a relatively short period of accelerated expansion after primordial inflation, is a desirable ingredient for a certain class of particle physics models if they are not to be in contention with the cosmology of t...
Ryo Saito, Atsushi Naruko, Takashi Hiramatsu, Misao Sasaki
JCAP10(2014)051      Sep 2014   [Refereed]
In this paper, we introduce a new approach to a treatment of the gravitational effects (redshift, time delay and lensing) on the observed cosmic microwave background (CMB) anisotropies based on the Boltzmann equation. From the Liouville's theorem ...
Atsushi Taruya, Takahiro Nishimichi, Francis Bernardeau, Takashi Hiramatsu, Kazuya Koyama
Phys. Rev. D 90, 123515 (2014)      Aug 2014   [Refereed]
Based on the multi-point propagator expansion, we present resummed perturbative calculations for cosmological power spectra and correlation functions in the context of modified gravity. In a wide class of modified gravity models that have a screen...
Atsushi Taruya, Kazuya Koyama, Takashi Hiramatsu, Akira Oka
Phys. Rev. D 89, 043509 (2014)      Sep 2013   [Refereed]
Redshift-space distortions (RSD) offer an attractive method to measure the growth of cosmic structure on large scales, and combining with the measurement of the cosmic expansion history, it can be used as cosmological tests of gravity. With the ad...
Takashi Hiramatsu, Masahiro Kawasaki, Ken'ichi Saikawa
JCAP02(2014)031      Sep 2013   [Refereed]
We revisit the production of gravitational waves from unstable domain walls analyzing their spectrum by the use of field theoretic lattice simulations with grid size Tex, which is larger than the previous study. We have recognized that there ...
Takashi Hiramatsu, Yuuiti Sendouda, Keitaro Takahashi, Daisuke Yamauchi, Chul-Moon Yoo
Phys. Rev. D 88, 085021 (2013)      Jul 2013   [Refereed]
We study the network of Type-I cosmic strings using the field-theoretic numerical simulations in the Abelian-Higgs model. For Type-I strings, the gauge field plays an important role, and thus we find that the correlation length of the strings is s...
Takashi Hiramatsu, Minoru Eto, Kohei Kamada, Tatsuo Kobayashi, Yutaka Ookouchi
   Apr 2013   [Refereed]
The breaking of Tex symmetry plays a crucial role in modeling the breaking of supersymmetry (SUSY). In the models that possess both SUSY preserving and SUSY breaking vacua, tube-like cosmic strings called R-tubes, whose surfaces are constitut...
Takashi Hiramatsu, Wayne Hu, Kazuya Koyama, Fabian Schmidt
   Sep 2012   [Refereed]
In massive gravity, galileon, and braneworld explanations of cosmic acceleration, force modifications are screened by nonlinear derivative self-interactions of the scalar field mediating that force. Interactions between the field of a central body...
Takashi Hiramatsu, Masahiro Kawasaki, Ken'ichi Saikawa, Toyokazu Sekiguchi
JCAP01(2013)001      Jul 2012   [Refereed]
We investigate the cosmological constraints on axion models where the domain wall number is greater than one. In these models, multiple domain walls attached to strings are formed, and they survive for a long time. Their annihilation occurs due to...
Takashi Hiramatsu, Masahiro Kawasaki, Ken'ichi Saikawa, Toyokazu Sekiguchi
Phys. Rev. D 85, 105020 (2012); Phys. Rev. D 86, 089902(E) (2012)      Feb 2012   [Refereed]
We analyze the spectrum of axions radiated from collapse of domain walls, which have received less attention in the literature. The evolution of topological defects related to the axion models is investigated by performing field-theoretic lattice ...
Takashi Hiramatsu, Masahiro Kawasaki, Toyokazu Sekiguchi, Masahide Yamaguchi, Jun'ichi Yokoyama
Phys.Rev.D83:123531, 2011      Dec 2010   [Refereed]
Cosmological evolution of axionic string network is analyzed in terms of field-theoretic simulations in a box of 512^3 grids, which are the largest ever, using a new and more efficient identification scheme of global strings. The scaling parameter...
Takashi Hiramatsu, Masahiro Kawasaki, Ken'ichi Saikawa
JCAP 1108:030,2011      Dec 2010   [Refereed]
We study the cosmological evolution of domain walls bounded by strings which arise naturally in axion models. If we introduce a bias in the potential, walls become metastable and finally disappear. We perform two dimensional lattice simulations of...
Takashi Hiramatsu, Masahiro Kawasaki, Fuminobu Takahashi
JCAP 1006:008,2010      Mar 2010   [Refereed]
We study Q-ball formation in the expanding universe on 1D, 2D and 3D lattice simulations. We obtain detailed Q-ball charge distributions, and find that the distribution is peaked at Q^{3D}_{peak} \simeq 1.9\times 10^{-2} (|\Phi_{in}|/m)^2, which i...
Takashi Hiramatsu, Masahiro Kawasaki, Ken'ichi Saikawa
JCAP 1005:032,2010      Feb 2010   [Refereed]
We study the production of gravitational waves from cosmic domain walls created during phase transition in the early universe. We investigate the process of formation and evolution of domain walls by running three dimensional lattice simulations. ...
Atsushi Taruya, Takahiro Nishimichi, Shun Saito, Takashi Hiramatsu
   Jun 2009   [Refereed]
We study the non-linear evolution of baryon acoustic oscillations in the matter power spectrum and correlation function from the improved perturbation theory (PT). Based on the framework of renormalized PT, we apply the {\it closure approximation}...
Takashi Hiramatsu, Atsushi Taruya
Phys.Rev.D79:103526,2009      Feb 2009   [Refereed]
We present a new numerical scheme to treat the non-linear evolution of cosmological power spectra. Governing equations for matter power spectra have been previously derived by a non-perturbative technique with closure approximation. Solutions of t...
Kazuya Koyama, Atsushi Taruya, Takashi Hiramatsu
Phys.Rev.D79:123512,2009      Feb 2009   [Refereed]
We present a formalism to calculate the non-linear matter power spectrum in modified gravity models that explain the late-time acceleration of the Universe without dark energy. Any successful modified gravity models should contain a mechanism to r...
Atsushi Taruya, Takashi Hiramatsu
   Aug 2007   [Refereed]
We apply a non-linear statistical method in turbulence to the cosmological perturbation theory and derive a closed set of evolution equations for matter power spectra. The resultant closure equations consistently recover the one-loop results of st...
Antonio Cardoso, Takashi Hiramatsu, Kazuya Koyama, Sanjeev S. Seahra
JCAP 0707:008,2007      May 2007   [Refereed]
We study the behaviour of scalar perturbations in the radiation-dominated era of Randall-Sundrum braneworld cosmology by numerically solving the coupled bulk and brane master wave equations. We find that density perturbations with wavelengths less...
Kazuya Koyama, Andrew Mennim, V. A. Rubakov, David Wands, Takashi Hiramatsu
JCAP 0704:001,2007      Jan 2007   [Refereed]
In this paper we quantise scalar perturbations in a Randall-Sundrum-type model of inflation where the inflaton field is confined to a single brane embedded in five-dimensional anti-de Sitter space-time. In the high energy regime, small-scale infla...
Takashi Hiramatsu, Kazuya Koyama
JCAP 0612:009,2006      Jul 2006   [Refereed]
We study the evolution of scalar curvature perturbations in a brane-world inflation model in a 5D Anti-de Sitter spacetime. The inflaton perturbations are confined to a 4D brane but they are coupled to the 5D bulk metric perturbations. We numerica...
Yoshiaki Himemoto, Atsushi Taruya, Hideaki Kudoh, Takashi Hiramatsu
Phys.Rev.D75:022003,2007      Jul 2006   [Refereed]
We discuss a robust data analysis method to detect a stochastic background of gravitational waves in the presence of non-Gaussian noise. In contrast to the standard cross-correlation (SCC) statistic frequently used in the stochastic background sea...
Takashi Hiramatsu
Phys.Rev.D73:084008,2006      Jan 2006   [Refereed]
We discuss the cosmological evolution of the inflationary gravitational wave background (IGWB) in the Randall-Sundrum single-brane model. In the braneworld cosmology, in which three-dimensional space-like hypersurface that we live in is embedded i...
Hideaki Kudoh, Atsushi Taruya, Takashi Hiramatsu, Yoshiaki Himemoto
Phys.Rev. D73 (2006) 064006      Nov 2005   [Refereed]
Future missions of gravitational-wave astronomy will be operated by space-based interferometers, covering very wide range of frequency. Search for stochastic gravitational-wave backgrounds (GWBs) is one of the main targets for such missions, and w...
Takashi Hiramatsu, Kei Kotake, Hideaki Kudoh, Atsushi Taruya
Mon.Not.Roy.Astron.Soc.364:1063-1068,2005      Sep 2005   [Refereed]
We discuss the gravitational wave background (GWB) from a cosmological population of gamma-ray bursts (GRBs). Among various emission mechanisms for the gravitational waves (GWs), we pay a particular attention to the vast anisotropic neutrino emiss...
Takashi Hiramatsu, Kazuya Koyama, Atsushi Taruya
Phys.Lett. B609 (2005) 133-142      Oct 2004   [Refereed]
We discuss the cosmological evolution of gravitational waves (GWs) after inflation in a brane-world cosmology embedded in five-dimensional anti-de Sitter (AdS_5) bulk spacetime. In a brane-world scenario, the evolution of GWs is affected by the no...
Takashi Hiramatsu, Kazuya Koyama, Atsushi Taruya
Phys.Lett. B578 (2004) 269-275      Aug 2003   [Refereed]
We study the evolution of gravitational waves(GWs) after inflation in a brane-world cosmology embedded in five-dimensional anti-de Sitter spacetime. Contrary to the standard four-dimensional results, the GWs at the high-energy regime in brane-worl...

Conference Activities & Talks

Oscillons after inflation and gravitational waves
Takashi Hiramatsu
74th Annual Meeting of The Physical Society of Japan   14 Mar 2019   
Hunting for Statistical Anisotropy in Tensor Modes with B-mode Observations
Takashi Hiramatsu
Accelerating Universe in the Dark   5 Mar 2019   
Hunting for Statistical Anisotropy in Tensor Modes with B-mode Observations
Takashi Hiramatsu
2019 YITP Asian-Pacific Winter School and Workshop on Gravitation and Cosmology   11 Feb 2019   
Hunting for Statistical Anisotropy in Tensor Modes with B-mode observations
Takashi Hiramatsu
Workshop on observational cosmology   12 Nov 2018   
CMB Bispectra induced by lensing
Takashi Hiramatsu
28th JGRG   5 Nov 2018   

Association Memberships