Pathological behaviour of the scalar graviton in Ho\v{r}ava-Lifshitz gravity (2009)
Koyama, Kazuya, Arroja, Frederico
We confirm the claim by Blas et al. [arXiv:0909.3525] that, in the infrared limit of Ho\v{r}ava-Lifshitz gravity, the scalar graviton becomes a ghost if the sound speed squared is positive on the...
Weak lensing predictions for modified gravities at non-linear scales (2009)
Beynon, Emma, Bacon, David J., Koyama, Kazuya
We present a set of predictions for weak lensing correlation functions in the context of modified gravity models, including a prescription for the impact of the nonlinear power spectrum regime in...
Self-Accelerating Universe in Galileon Cosmology (2009)
Silva, Fabio P, Koyama, Kazuya
We present a cosmological model with a solution that self-accelerates at late-times without signs of ghost instabilities on small scales. The model is a natural extension of the Brans-Dicke (BD)...
Cosmological matching conditions for gravitational waves at second order (2009)
Arroja, Frederico, Assadullahi, Hooshyar, Koyama, Kazuya, Wands, David
We compute the second-order matching conditions for tensor metric perturbations at an abrupt change in the equation of state. For adiabatic perturbations on large scales the matching hypersurface...
On the full quantum trispectrum in multi-field DBI inflation (2009)
Mizuno, Shuntaro, Arroja, Frederico, Koyama, Kazuya
We compute the leading order connected four-point function of the primordial curvature perturbation coming from the four-point function of the fields in multi-field DBI inflation models. We confirm...
Velocities as a probe of dark sector interactions (2009)
Koyama, Kazuya, Maartens, Roy, Song, Yong-Seon
Dark energy in General Relativity is typically non-interacting with other matter. However, it is possible that the dark energy interacts with the dark matter, and in this case, the dark matter can...
Lorentz boost and non-Gaussianity in multi-field DBI-inflation (2009)
Mizuno, Shuntaro, Arroja, Frederico, Koyama, Kazuya, Tanaka, Takahiro
We show that higher-order actions for cosmological perturbations in the multi-field DBI-inflation model are obtained by a Lorentz boost from the rest frame of the brane to the frame where the brane...
On the full trispectrum in single field DBI-inflation (2009)
Arroja, Frederico, Mizuno, Shuntaro, Koyama, Kazuya, Tanaka, Takahiro
We compute the tree-level connected four-point function of the primordial curvature perturbation for a fairly general minimally coupled single field inflationary model, where the inflaton's...
Non-linear Evolution of Matter Power Spectrum in Modified Theory of Gravity (2009)
Koyama, Kazuya, Taruya, Atsushi, Hiramatsu, Takashi
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...
Ghosts in asymmetric brane gravity and the decoupled stealth limit (2009)
Koyama, Kazuya, Padilla, Antonio, Silva, Fabio P
We study the spectrum of gravitational perturbations around a vacuum de Sitter brane in a 5D asymmetric braneworld model, with induced curvature on the brane. This generalises the stealth...
Limits on Isocurvature Perturbations from Non-Gaussianity in WMAP Temperature Anisotropy (2008)
Hikage, Chiaki, Koyama, Kazuya, Matsubara, Takahiko, Takahashi, Tomo, Yamaguchi, Masahide
We study the effect of primordial isocurvature perturbations on non-Gaussian properties in CMB temperature anisotropies. We consider generic forms of the non-linearity of isocurvature perturbations...
Signature of primordial non-Gaussianity on the matter power spectrum (2008)
Taruya, Atsushi, Koyama, Kazuya, Matsubara, Takahiko
Employing the perturbative treatment of gravitational clustering, we discuss possible effects of primordial non-Gaussianity on the matter power spectrum. As gravitational clustering develops, the...
Signature of Primordial Non-Gaussianity on Matter Power Spectrum (2008)
Taruya, Atsushi, Koyama, Kazuya, Matsubara, Takahiko
Employing the perturbative treatment of gravitational clustering, we discuss possible effects of primordial non-Gaussianity on the matter power spectrum. As gravitational clustering develops, the...
Non-gaussianity from the bispectrum in general multiple field inflation (2008)
Arroja, Frederico, Mizuno, Shuntaro, Koyama, Kazuya
We study the non-gaussianity from the bispectrum in multi-field inflation models with a general kinetic term. The models include the multi-field K-inflation and the multi-field Dirac-Born-Infeld...
Detectability of a phantom-like braneworld model with the integrated Sachs-Wolfe effect (2008)
Giannantonio, Tommaso, Song, Yong-Seon, Koyama, Kazuya
We study a braneworld model in which a phantom-like behaviour occurs with only cold dark matter and a cosmological constant, due to a large distance modification of gravity. With the addition of...
Induced gravity on intersecting brane-worlds Part II: Cosmology (2008)
Corradini, Olindo, Koyama, Kazuya, Tasinato, Gianmassimo
We explore cosmology of intersecting braneworlds with induced gravity on the branes. We find the cosmological equations that control the evolution of a moving codimension one brane and a codimension...
Induced gravity on intersecting brane-worlds. Part II: Cosmology (2008)
Corradini, Olindo, Koyama, Kazuya, Tasinato, Gianmassimo
19 pages, no figures.
Induced gravity on intersecting brane-worlds. Part I: Maximally symmetric solutions (2008)
Corradini, Olindo, Koyama, Kazuya, Tasinato, Gianmassimo
Submitted on 3 Dec 2007 (v1), last revised 12 Mar 2008 (this version, v2).-- 16 pages.-- Version to appear in Phys Rev D.
Consistency test of general relativity from large scale structure of the Universe (2008)
Song, Yong-Seon, Koyama, Kazuya
We construct a consistency test of General Relativity (GR) on cosmological scales. This test enables us to distinguish between the two alternatives to explain the late-time accelerated expansion of...
Non-gaussianity from the trispectrum in general single field inflation (2008)
Arroja, Frederico, Koyama, Kazuya
We compute the fourth order action in perturbation theory for scalar and second order tensor perturbations for a minimally coupled single field inflationary model, where the inflaton's lagrangian is...
Induced gravity on intersecting brane-worlds Part I: Maximally symmetric solutions (2007)
Corradini, Olindo, Koyama, Kazuya, Tasinato, Gianmassimo
We explore models of intersecting brane-worlds with induced gravity terms on codimension one branes and on their intersection. Maximally symmetric solutions for the branes and the intersection are...
Spherical collapse in modified gravity with the Birkhoff-theorem (2007)
Schaefer, Bjoern Malte, Koyama, Kazuya
We study structure formation in a phenomenological model of modified gravity which interpolates between LambdaCDM and phenomenological DGP-gravity. Generalisation of spherical collapse by using the...
Cosmological perturbations in the DGP braneworld: numeric solution (2007)
Cardoso, Antonio, Koyama, Kazuya, Seahra, Sanjeev S., Silva, Fabio P.
We solve for the behaviour of cosmological perturbations in the Dvali-Gabadadze-Porrati (DGP) braneworld model using a new numerical method. Unlike some other approaches in the literature, our method...
Low energy effective theory on a regularized brane in 6D gauged chiral supergravity (2007)
Arroja, Frederico, Kobayashi, Tsutomu, Koyama, Kazuya, Shiromizu, Tetsuya
We derive the low energy effective theory on a brane in six-dimensional chiral supergravity. The conical 3-brane singularities are resolved by introducing cylindrical codimension one 4-branes whose...
Ghosts in the self-accelerating universe (2007)
The self-accelerating universe realizes the accelerated expansion of the universe at late times by large-distance modification of general relativity without a cosmological constant. The...
Brane-world inflation: slow-roll corrections to the spectral index (2007)
Koyama, Kazuya, Mennim, Andrew, Wands, David
We quantify the slow-roll corrections to primordial density perturbations arising from inflation driven by a four-dimensional scalar field with a monomial potential in a five-dimensional non-compact...
Non-Gaussianities from ekpyrotic collapse with multiple fields (2007)
Koyama, Kazuya, Mizuno, Shuntaro, Vernizzi, Filippo, Wands, David
We compute the non-Gaussianity of the curvature perturbation generated by ekpyrotic collapse with multiple fields. The transition from the multi-field scaling solution to a single-field dominated...
Bubbles in the Self-Accelerating Universe (2007)
Izumi, Keisuke, Koyama, Kazuya, Pujolas, Oriol, Tanaka, Takahiro
We revisit the issue of the stability in the Dvali-Gabadadze-Porrati model, by considering the nucleation of bubbles of the conventional branch within the self-accelerating branch. We construct an...
The cosmological constant and dark energy in braneworlds (2007)
We review recent attempts to address the cosmological constant problem and the late-time acceleration of the Universe based on braneworld models. In braneworld models, the way in which the vacuum...
Diagrammatic approach to non-Gaussianity from inflation (2007)
Byrnes, Christian T., Koyama, Kazuya, Sasaki, Misao, Wands, David
We present Feynman type diagrams for calculating the n-point function of the primordial curvature perturbation in terms of scalar field perturbations during inflation. The diagrams can be used to...
Scalar perturbations in braneworld cosmology (2007)
Cardoso, Antonio, Hiramatsu, Takashi, Koyama, Kazuya, Seahra, Sanjeev S.
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...
Curvature perturbations from ekpyrotic collapse with multiple fields (2007)
Koyama, Kazuya, Mizuno, Shuntaro, Wands, David
A scale-invariant spectrum of isocurvature perturbations is generated during collapse in the ekpyrotic scaling solution in models where multiple fields have steep negative exponential potentials. The...
Ekpyrotic collapse with multiple fields (2007)
A scale invariant spectrum of isocurvature perturbations is generated during collapse in the scaling solution in models where two or more fields have steep negative exponential potentials. The scale...
Non-linear interactions in a cosmological background in the DGP braneworld (2007)
Koyama, Kazuya, Silva, Fabio P
We study quasi-static perturbations in a cosmological background in the Dvali-Gabadadze-Porrati (DGP) braneworld model. We identify the Vainshtein radius at which the non-linear interactions of the...
Primordial perturbations from slow-roll inflation on a brane (2007)
Koyama, Kazuya, Mennim, Andrew, Rubakov, V. A., Wands, David, Hiramatsu, Takashi
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...
Coupled bulk and brane fields about a de Sitter brane (2006)
Cardoso, Antonio, Koyama, Kazuya, Mennim, Andrew, Seahra, Sanjeev S., Wands, David
We consider the evolution of a bulk scalar field in anti-de Sitter (AdS) spacetime linearly coupled to a scalar field on a de Sitter boundary brane. We present results of a spectral analysis of the...
Unexorcized ghost in DGP brane world (2006)
Izumi, Keisuke, Koyama, Kazuya, Tanaka, Takahiro
The braneworld model of Dvali-Gabadadze-Porrati realizes the self-accelerating universe. However, it is known that this cosmological solution contains a spin-2 ghost. We study the possibility of...
On the 4D effective theory in warped compactifications with fluxes and branes (2006)
Koyama, Kazuya, Koyama, Kayoko, Arroja, Frederico
We present a systematic way to derive the four-dimensional effective theories for warped compactifications with fluxes and branes in the ten-dimensional type IIB supergravity. The ten-dimensional...
Numerical study of curvature perturbations in a brane-world inflation at high-energies (2006)
Hiramatsu, Takashi, Koyama, Kazuya
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...
Inflaton perturbations in brane-world cosmology with induced gravity (2006)
Koyama, Kazuya, Mizuno, Shuntaro
We study cosmological perturbations in the brane models with an induced Einstein-Hilbert term on a brane. We consider an inflaton confined to a de Sitter brane in a five-dimensional Minkowski...
Moduli instability in warped compactification - 4D effective theory approach (2006)
Arroja, Frederico, Koyama, Kazuya
We consider a 5D BPS dilatonic two brane model which reduces to the Randall-Sundrum model or the Horava-Witten theory for a particular choice of parameters. Recently new dynamical solutions were...
Structure formation in modified gravity models alternative to dark energy (2006)
We study structure formation in phenomenological models in which the Friedmann equation receives a correction of the form $H^{\alpha}/r_c^{2-\alpha}$, which realize an accelerated expansion without...
More on ghosts in DGP model (2005)
Gorbunov, Dmitry, Koyama, Kazuya, Sibiryakov, Sergei
It is shown by an explicit calculation that the excitations about the self-accelerating cosmological solution of the Dvali--Gabadaze--Porrati model contain a ghost mode. This raises serious doubts...
Structure formation in the DGP cosmological model (2005)
The DGP brane-world model provides an alternative to the standard LCDM cosmology, in which the late universe accelerates due to a modification of gravity rather than vacuum energy. The cosmological...
Enhanced gravitational scattering from large extra dimensions (2005)
Koyama, Kazuya, Piazza, Federico, Wands, David
We investigate whether enhanced gravitational scattering on small scales (< 0.1mm), which becomes possible in models with large extra dimensions, can establish statistical equilibrium between...
Regular collision of dilatonic inflating branes (2005)
Leeper, Emily, Koyama, Kazuya, Maartens, Roy
We demonstrate that a two brane system with a bulk scalar field driving power-law inflation on the branes has an instability in the radion. We solve for the resulting trajectory of the brane, and...
Slow-roll corrections to inflaton fluctuations on a brane (2005)
Koyama, Kazuya, Mizuno, Shuntaro, Wands, David
Quantum fluctuations of an inflaton field, slow-rolling during inflation are coupled to metric fluctuations. In conventional four dimensional cosmology one can calculate the effect of scalar metric...
Gravitational backreaction of anti-D branes in the warped compactification (2005)
Koyama, Kayoko, Koyama, Kazuya
We derive a low-energy effective theory for gravity with anti-D branes, which are essential to get de Sitter solutions in the type IIB string warped compactification, by taking account of...
Coupled boundary and bulk fields in anti-de Sitter (2005)
Koyama, Kazuya, Mennim, Andrew, Wands, David
We investigate the dynamics of a boundary field coupled to a bulk field with a linear coupling in an anti-de Sitter bulk spacetime bounded by a Minkowski (Randall-Sundrum) brane. An instability...
Are there ghosts in the self-accelerating brane universe? (2005)
We study the spectrum of gravitational perturbations about a vacuum de Sitter brane with the induced 4D Einstein-Hilbert term, in a 5D Minkowski spacetime (DGP model). We consider solutions that...
Brane induced gravity from asymmetric warped compactification (2005)
Koyama, Kazuya, Koyama, Kayoko
We show that brane induced gravity can be realized as a low energy effective theory of brane worlds with asymmetric warped compactification. A self-accelerating universe without cosmological constant...
Quantization of scalar perturbations in brane-world inflation (2004)
Yoshiguchi, Hiroyuki, Koyama, Kazuya
We consider a quantization of scalar perturbations about a de Sitter brane in a 5-dimensional anti-de Sitter (AdS) bulk spacetime. We first derive the second order action for a master variable...
Evolution of gravitational waves in the high-energy regime of brane-world cosmology (2004)
Hiramatsu, Takashi, Koyama, Kazuya, Taruya, Atsushi
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...
Late time behavior of cosmological perturbations in a single brane model (2004)
We present solutions for the late time evolution of cosmological tensor and scalar perturbations in a single-Randall-Sundrum brane world model. Assuming that the bulk is anti-de Sitter spacetime, the...
Spacetime dynamics and baryogenesis in braneworld (2004)
Shiromizu, Tetsuya, Koyama, Kazuya
We point out that the effective theory for the Randall-Sundrum braneworld models with bulk fields contains the baryon number violation process depending on the spacetime dynamics. Combining to the...
Bulk gravitational field and dark radiation on the brane in dilatonic brane world (2004)
Yoshiguchi, Hiroyuki, Koyama, Kazuya
We discuss the connection between the dark radiation on the brane and the bulk gravitational field in a dilatonic brane world model proposed by Koyama and Takahashi where the exact solutions for the...
Low energy effective action on a self-gravitating D-brane (2003)
Onda, Sumitada, Shiromizu, Tetsuya, Koyama, Kazuya, Hayakawa, Shoko
Recently the study of braneworld on the self-gravitating D-brane has been initiated and derived the gravitational equation on the brane by holographic and geometrical projection methods....
Hiramatsu, Takashi, Koyama, Kazuya, Taruya, Atsushi
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,...
Cosmological constant and gravitational theory on D-brane (2003)
Shiromizu, Tetsuya, Koyama, Kazuya, Torii, Takashi
In a toy model we derive the gravitational equation on a self-gravitating curved D-brane. The effective theory on the brane is drastically changed from the ordinal Einstein equation. The net...
Exactly solvable model for cosmological perturbations in dilatonic brane worlds (2003)
Koyama, Kazuya, Takahashi, Keitaro
We construct a model where cosmological perturbations are analytically solved based on dilatonic brane worlds. A bulk scalar field has an exponential potential in the bulk and an exponential coupling...
Can we live on a D-brane? -- Effective theory on a self-gravitating D-brane -- (2003)
Shiromizu, Tetsuya, Koyama, Kazuya, Onda, Sumitada, Torii, Takashi
We consider a D-brane coupled with gravity in type IIB supergravity on S^5 and derive the effective theory on the D-brane in two different ways, that is, holographic and geometrical projection...
Cosmic Microwave Radiation Anisotropies in Brane Worlds (2003)
We propose a new formulation to calculate the Cosmic Microwave Background (CMB) spectrum in the Randall Sundrum two-branes model based on recent progresses in solving the bulk geometry using a low...
Primordial fluctuations in bulk inflaton model (2003)
Koyama, Kazuya, Takahashi, Keitaro
An inflationary brane model driven by a bulk inflaton with exponential potential is proposed. We find a family of exact solutions that describe power-law inflation on the brane. These solutions...
Project Report (M.P.S.(Agriculture & Life Sciences))--Cornell University, August 2003.
Effective theory for close limit of two branes (2002)
Shiromizu, Tetsuya, Koyama, Kazuya, Takahashi, Keitaro
We discuss the effective theory for the close limit of two branes in a covariant way. To do so we solve the five dimensional Einstein equation along the direction of the extra dimension. Using the...
Low energy effective theory for two branes system (2002)
Shiromizu, Tetsuya, Koyama, Kazuya
We derive the low energy effective theory for two branes system solving the bulk geometry formally in the covariant curvature formalism developed by Shiromizu, Maeda and Sasaki. As expected, the...
Cosmology with Radion and Bulk Scalar Field in Two Branes Model (2002)
Kobayashi, Shinpei, Koyama, Kazuya
We investigate cosmological evolutions of the bulk scalar field $\phi(t)$ and the radion $d(t)$ in five-dimensional dilatonic two branes model. The bulk potential for the scalar field is taken as the...
Cosmological Perturbations in Brane World - Brane view v.s. Bulk view - (2002)
First, we will study the cosmological perturbations from the brane point of view. It turns out that two types of the extra data are necessary to know the evolution of the system. To fix these data,...
Radion and Large Scale Anisotropy on the Brane (2002)
We investigate the effect of the radion on cosmological perturbations in the brane world. The S^1/Z_2 compactified 5D Anti-de Sitter spacetime bounded by positive and negative tension branes is...
Bulk Gravitational Field and Cosmological Perturbations on the Brane (2001)
We investigate the effect of the bulk gravitational field on the cosmological perturbations on a brane embedded in the 5D Anti-de Sitter (AdS) spacetime. The effective 4D Einstein equations for the...
Thick Brane Worlds and Their Stability (2001)
Kobayashi, Shinpei, Koyama, Kazuya, Soda, Jiro
Three types of thick branes, i.e., Poincar\'{e}, de Sitter and Anti-de Sitter brane are considered. They are realized as the non-singular solutions of the Einstein equations with the non-trivial...
Strongly Coupled CFT in FRW Universe from AdS/CFT Correspondence (2001)
We develop a formalism to calculate the effective action of the strongly coupled conformal field theory (CFT) in curved spacetime. The effective action of the CFT is obtained from AdS/CFT...
Quantum Fluctuations of Bulk Inflaton in Inflationary Brane World (2000)
Kobayashi, Shinpei, Koyama, Kazuya, Soda, Jiro
The inflationary scenario for the brane world driven by the bulk inflaton is proposed. The quantum fluctuations of the inflaton is calculated and compared to those of the conventional 4-dimensional...
Evolution of Cosmological Perturbations in the Brane World (2000)
The evolution of the cosmological perturbations is studied in the context of the Randall-Sundrum brane world scenario, in which our universe is realized on a three-brane in the five dimensional...
Birth of the Brane World (2000)
Birth of the brane world is studied using the Hamiltonian approach. It is shown that an inflating brane world can be created from nothing. The wave function of the universe obtained from the Wheeler...
Creation of an Open Universe from Valley Bounce (2000)
Koyama, Kazuya, Maeda, Kayoko, Soda, Jiro
It appears difficult to construct a simple model for an open universe based on the one bubble inflationary scenario. The reason is that one needs a large mass to avoid the tunneling via the Hawking...
An Open Universe from Valley Bounce (1999)
Koyama, Kazuya, Maeda, Kayoko, Soda, Jiro
It appears difficult to construct a simple model for an open universe based on the one bubble inflationary scenario. The reason is that one needs a large mass to avoid the tunneling via the Hawking...
Constraints on a non-gaussian ($\chi_m^2$) CDM model (1999)
Koyama, Kazuya, Soda, Jiro, Taruya, Atsushi
We consider constraints on the structure formation model based on non-Gaussian fluctuations generated during inflation, which have $\chi_m^2$ distributions. Using three data sets, the abundance of...
Baryon Isocurvature Perturbation in the Affleck-Dine Baryogenesis mechanism (1998)
We propose a primeval baryon isocurvature model in the context of the Affleck-Dine (AD) baryogenesis mechanism. Quantum fluctuations of the AD field during inflation produce fluctuations in the...