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 ...

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...

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...

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...

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...

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...

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...

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 , which is larger than the previous study. We have recognized that there ...

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...

The breaking of 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...

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...

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 ...

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...

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...

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...

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. ...

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}...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...