Matter instabilities in general Gauss-Bonnet gravity (2009)
De Felice, Antonio, Mota, David, Tsujikawa, Shinji
We study the evolution of matter density perturbations in f(G) gravity, where the Lagrangian density is the sum of a Ricci scalar R and an arbitrary function f in terms of a Gauss-Bonnet term G. We...
The universe is accelerating. Do we need a new mass scale? (2009)
Nesseris, Savvas, Piazza, Federico, Tsujikawa, Shinji
We try to address quantitatively the question whether a new mass is needed to fit current supernovae data. For this purpose, we consider an infra-red modification of gravity that does not contain any...
Ohashi, Junko, Tsujikawa, Shinji
Cosmological scaling solutions, which give rise to a scalar-field density proportional to a background fluid density during radiation and matter eras, are attractive to alleviate the energy scale...
Equilibrium thermodynamics in modified gravitational theories (2009)
Bamba, Kazuharu, Geng, Chao-Qiang, Tsujikawa, Shinji
We show that it is possible to obtain a picture of equilibrium thermodynamics on the apparent horizon in the expanding cosmological background for a wide class of modified gravity theories with the...
The dispersion of growth of matter perturbations in f(R) gravity (2009)
Tsujikawa, Shinji, Gannouji, Radouane, Moraes, Bruno, Polarski, David
We study the growth of matter density perturbations delta_m for a number of viable f(R) gravity models that satisfy both cosmological and local gravity constraints, where the Lagrangian density f is...
Solar system constraints on f(G) gravity models (2009)
De Felice, Antonio, Tsujikawa, Shinji
We discuss solar system constraints on f(G) gravity models, where f is a function of the Gauss-Bonnet term G. We focus on cosmologically viable f(G) models that can be responsible for late-time...
Ando, Masaki, Kawamura, Seiji, Sato, Shuichi, Nakamura, Takashi, Tsubono, Kimio, Araya, Akito, ...
DECIGO pathfinder (DPF) is a milestone satellite mission for DECIGO (DECi-hertz Interferometer Gravitational wave Observatory), which is a future space gravitational wave antenna. DECIGO is expected...
Chameleon scalar fields in relativistic gravitational backgrounds (2009)
Tsujikawa, Shinji, Tamaki, Takashi, Tavakol, Reza
We study the field profile of a scalar field $\phi$ that couples to a matter fluid (dubbed a chameleon field) in the relativistic gravitational background of a spherically symmetric spacetime....
Construction of cosmologically viable f(G) gravity models (2008)
De Felice, Antonio, Tsujikawa, Shinji
We derive conditions under which f(G) gravity models, whose Lagrangian densities f are written in terms of a Gauss-Bonnet term G, are cosmologically viable. The most crucial condition to be satisfied...
Revisiting chameleon gravity - thin-shells and no-shells with appropriate boundary conditions (2008)
Tamaki, Takashi, Tsujikawa, Shinji
We derive analytic solutions of a chameleon scalar field $\phi$ that couples to a non-relativistic matter in the weak gravitational background of a spherically symmetric body, paying particular...
Ando, Masaki, Kawamura, Seiji, Nakamura, Takashi, Tsubono, Kimio, Tanaka, Takahiro, Funaki, Ikkoh, ...
DECIGO pathfinder (DPF) is a milestone satellite mission for DECIGO (DECi-hertz Interferometer Gravitational wave Observatory) which is a future space gravitational wave antenna. DECIGO is expected...
The Japanese space gravitational wave antenna; DECIGO (2008)
Kawamura, Seiji, Ando, Masaki, Nakamura, Takashi, Tsubono, Kimio, Tanaka, Takahiro, Funaki, Ikkoh, ...
DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) is the future Japanese space gravitational wave antenna. DECIGO is expected to open a new window of observation for gravitational...
Tatekawa, Takayuki, Tsujikawa, Shinji
We study second-order cosmological perturbations in scalar-tensor models of dark energy that satisfy local gravity constraints, including f(R) gravity. We derive equations for matter fluctuations...
Bamba, Kazuharu, Ohta, Nobuyoshi, Tsujikawa, Shinji
We estimate the strength of large-scale magnetic fields produced during inflation in the framework of Dirac-Born-Infeld (DBI) theories. This analysis is sufficiently general in the sense that it...
The effect of modified gravity on weak lensing (2008)
Tsujikawa, Shinji, Tatekawa, Takayuki
We study the effect of modified gravity on weak lensing in a class of scalar-tensor theory that includes $f(R)$ gravity as a special case. These models are designed to satisfy local gravity...
Constraints on scalar-tensor models of dark energy from observational and local gravity tests (2008)
Tsujikawa, Shinji, Uddin, Kotub, Mizuno, Shuntaro, Tavakol, Reza, Yokoyama, Jun'ichi
We construct a family of viable scalar-tensor models of dark energy (DE) which possess a phase of late-time acceleration preceded by a standard matter era, while at the same time satisfying the local...
Solar system and equivalence principle constraints on f(R) gravity by chameleon approach (2007)
Capozziello, Salvatore, Tsujikawa, Shinji
We study constraints on f(R) dark energy models from solar system experiments combined with experiments on the violation of equivalence principle. When the mass of an equivalent scalar field degree...
Density perturbations in f(R) gravity theories in metric and Palatini formalisms (2007)
Tsujikawa, Shinji, Uddin, Kotub, Tavakol, Reza
We make a detailed study of matter density perturbations in both metric and Palatini formalisms in theories whose Lagrangian density is a general function, f(R), of the Ricci scalar. We derive these...
We discuss observational consequences of f(R) dark energy scenarios that satisfy local gravity constraints (LGC) as well as conditions of the cosmological viability. The model we study is given by...
Prospects of inflation in delicate D-brane cosmology (2007)
Panda, Sudhakar, Sami, M., Tsujikawa, Shinji
We study D-brane inflation in a warped conifold background that includes brane-position dependent corrections for the nonperturbative superpotential. Instead of stabilizing the volume modulus chi at...
We derive the equation of matter density perturbations on sub-horizon scales for a general Lagrangian density f(R, phi, X) that is a function of a Ricci scalar R, a scalar field phi and a kinetic...
Amendola, Luca, Tsujikawa, Shinji
We identify the class of f(R) dark energy models which have a viable cosmology, i.e. a matter dominated epoch followed by a late-time acceleration. The deviation from a LambdaCDM model (f=R-Lambda)...
UV stable, Lorentz-violating dark energy with transient phantom era (2007)
Libanov, Maxim, Papantonopoulos, Eleftherios, Rubakov, Valery, Sami, M., Tsujikawa, Shinji
Phantom fields with negative kinetic energy are often plagued by the vacuum quantum instability in the ultraviolet region. We present a Lorentz-violating dark energy model free from this problem and...
Probing the Coupling between Dark Components of the Universe (2007)
Guo, Zong-Kuan, Ohta, Nobuyoshi, Tsujikawa, Shinji
We place observational constraints on a coupling between dark energy and dark matter by using 71 Type Ia supernovae (SNe Ia) from the first year of the five-year Supernova Legacy Survey (SNLS), the...
Fay, Stephane, Tavakol, Reza, Tsujikawa, Shinji
We make a systematic study of the cosmological dynamics for a number of f(R) gravity theories in Palatini formalism. We find a number of interesting results: (i) models based on theories of the type...
Conditions for the cosmological viability of f(R) dark energy models (2006)
Amendola, Luca, Gannouji, Radouane, Polarski, David, Tsujikawa, Shinji
We derive the conditions under which dark energy models whose Lagrangian densities f are written in terms of the Ricci scalar R are cosmologically viable. We show that the cosmological behavior of...
Realizing Scale-invariant Density Perturbations in Low-energy Effective String Theory (2006)
Guo, Zong-Kuan, Ohta, Nobuyoshi, Tsujikawa, Shinji
We discuss the realization of inflation and resulting cosmological perturbations in the low-energy effective string theory. In order to obtain nearly scale-invariant spectra of density perturbations...
The Gauss-Bonnet (GB) curvature invariant coupled to a scalar field $\phi$ can lead to an exit from a scaling matter-dominated epoch to a late-time accelerated expansion, which is attractive to...
Cosmologies from higher-order string corrections (2006)
We study cosmologies based on low-energy effective string theory with higher-order string corrections to a tree-level action and with a modulus scalar field (dilaton or compactification modulus). In...
Avoidance of future singularities in loop quantum cosmology (2006)
Sami, M., Singh, Parampreet, Tsujikawa, Shinji
We consider the fate of future singularities in the effective dynamics of loop quantum cosmology. Non-perturbative quantum geometric effects which lead to $\rho^2$ modification of the Friedmann...
Challenges for scaling cosmologies (2006)
Amendola, Luca, Quartin, Miguel, Tsujikawa, Shinji, Waga, Ioav
A cosmological model that aims at solving the coincidence problem should show that dark energy and dark matter follow the same scaling solution from some time onward. At the same time, the model...
Power-laws f(R) theories are cosmologically unacceptable (2006)
Amendola, Luca, Polarski, David, Tsujikawa, Shinji
In a recent paper [1] (PRL 98,131302,2007) we have shown that f(R)=R + mu R^{n} modified gravity dark energy models are not cosmologically viable because during the matter era that precedes the...
Are f(R) dark energy models cosmologically viable ? (2006)
Amendola, Luca, Polarski, David, Tsujikawa, Shinji
All $f(R)$ modified gravity theories are conformally identical to models of quintessence in which matter is coupled to dark energy with a strong coupling. This coupling induces a cosmological...
Dynamics of dark energy (2006)
Copeland, Edmund J., Sami, M., Tsujikawa, Shinji
In this paper we review in detail a number of approaches that have been adopted to try and explain the remarkable observation of our accelerating Universe. In particular we discuss the arguments for...
We study general properties of attractors for scalar-field dark energy scenarios which possess cosmological scaling solutions. In all such models there exists a scalar-field dominant solution with an...
Inflation from D3-brane motion in the background of D5-branes (2006)
Panda, Sudhakar, Sami, M., Tsujikawa, Shinji, Ward, John
We study inflation arising from the motion of a BPS D3-brane in the background of a stack of k parallel D5-branes. There are two scalar fields in this set up-- (i) the radion field R, a real scalar...
Inflation and dark energy arising from geometrical tachyons (2005)
Panda, Sudhakar, Sami, M., Tsujikawa, Shinji
We study the motion of a BPS D3-brane in the NS5-brane ring background. The radion field becomes tachyonic in this geometrical set up. We investigate the potential of this geometrical tachyon in the...
Reconstruction of general scalar-field dark energy models (2005)
The reconstruction of scalar-field dark energy models is studied for a general Lagrangian density $p(\phi, X)$, where $X$ is a kinematic term of a scalar field $\phi$. We implement the coupling $Q$...
Inflation Dynamics and Reheating (2005)
Bassett, Bruce A., Tsujikawa, Shinji, Wands, David
We review the theory of inflation with single and multiple fields paying particular attention to the dynamics of adiabatic and entropy/isocurvature perturbations which provide the primary means of...
Phantom damping of matter perturbations (2005)
Amendola, Luca, Tsujikawa, Shinji, Sami, M.
Cosmological scaling solutions are particularly important in solving the coincidence problem of dark energy. We derive the equations of sub-Hubble linear matter perturbations for a general...
Dynamics of assisted quintessence (2005)
Kim, Soo A, Liddle, Andrew R, Tsujikawa, Shinji
We explore the dynamics of assisted quintessence, where more than one scalar field is present with the same potential. For potentials with tracking solutions, the fields naturally approach the same...
Dark energy and cosmological solutions in second-order string gravity (2005)
Calcagni, Gianluca, Tsujikawa, Shinji, Sami, M
We study the cosmological evolution based upon a $D$-dimensional action in low-energy effective string theory in the presence of second-order curvature corrections and a modulus scalar field (dilaton...
The fate of (phantom) dark energy universe with string curvature corrections (2005)
Sami, M., Toporensky, Alexey, Tretjakov, Peter V., Tsujikawa, Shinji
We study the evolution of (phantom) dark energy universe by taking into account the higher-order string corrections to Einstein-Hilbert action with a fixed dilaton. While the presence of a...
Coupled dark energy: Towards a general description of the dynamics (2005)
Gumjudpai, Burin, Naskar, Tapan, Sami, M., Tsujikawa, Shinji
In dark energy models of scalar-field coupled to a barotropic perfect fluid, the existence of cosmological scaling solutions restricts the Lagrangian of the field $\vp$ to $p=X g(Xe^{\lambda \vp})$,...
Properties of singularities in (phantom) dark energy universe (2005)
Nojiri, Shin'ichi, Odintsov, Sergei D., Tsujikawa, Shinji
The properties of future singularities are investigated in the universe dominated by dark energy including the phantom-type fluid. We classify the finite-time singularities into four classes and...
Constraints on Dirac-Born-Infeld type dark energy models from varying alpha (2004)
Garousi, Mohammad R., Sami, M., Tsujikawa, Shinji
We study the variation of the effective fine structure constant alpha for Dirac-Born-Infeld (DBI) type dark energy models. The DBI action based on string theory naturally gives rise to a coupling...
What is needed of a tachyon if it is to be the dark energy? (2004)
Copeland, Edmund J., Garousi, Mohammad R., Sami, M., Tsujikawa, Shinji
We study a dark energy scenario in the presence of a tachyon field $\phi$ with potential $V(\phi)$ and a barotropic perfect fluid. The cosmological dynamics crucially depends on the asymptotic...
Chaotic dynamics in preheating after inflation (2004)
Jin, Yoshida, Tsujikawa, Shinji
We study chaotic dynamics in preheating after inflation in which an inflaton $\phi$ is coupled to another scalar field $\chi$ through an interaction $(1/2)g^2\phi^2\chi^2$. We first estimate the size...
A unified approach to scaling solutions in a general cosmological background (2004)
Our ignorance about the source of cosmic acceleration has stimulated study of a wide range of models and modifications to gravity. Cosmological scaling solutions in any of these theories are...
Testing for double inflation with WMAP (2004)
Parkinson, David, Tsujikawa, Shinji, Bassett, Bruce A., Amendola, Luca
With the WMAP data we can now begin to test realistic models of inflation involving multiple scalar fields. These naturally lead to correlated adiabatic and isocurvature (entropy) perturbations with...
Observational constraints on patch inflation in noncommutative spacetime (2004)
Calcagni, Gianluca, Tsujikawa, Shinji
We study constraints on a number of patch inflationary models in noncommutative spacetime using a compilation of recent high-precision observational data. In particular, the four-dimensional General...
Observational constraints on braneworld inflation: the effect of a Gauss-Bonnet term (2004)
Tsujikawa, Shinji, Sami, M., Maartens, Roy
High-energy modifications to general relativity introduce changes to the perturbations generated during inflation, and the latest high-precision cosmological data can be used to place constraints on...
Dilatonic ghost condensate as dark energy (2004)
Piazza, Federico, Tsujikawa, Shinji
We explore a dark energy model with a ghost scalar field in the context of the runaway dilaton scenario in low-energy effective string theory. We address the problem of vacuum stability by...
Garousi, Mohammad R., Sami, M., Tsujikawa, Shinji
We study the generation of electromagnetic fields in a string-inspired scenario associated with a rolling massive scalar field $\phi$ on the anti-D3 branes of KKLT de Sitter vacua. The 4-dimensional...
Cosmology from Rolling Massive Scalar Field on the anti-D3 Brane of de Sitter Vacua (2004)
Garousi, Mohammad R., Sami, M., Tsujikawa, Shinji
We investigate a string-inspired scenario associated with a rolling massive scalar field on D-branes and discuss its cosmological implications. In particular, we discuss cosmological evolution of the...
Tsujikawa, Shinji, Gumjudpai, Burin
We study cosmological perturbations in generalized Einstein scenarios and show the equivalence of inflationary observables both in the Jordan frame and the Einstein frame. In particular the...
Inflation in string-inspired cosmology and suppression of CMB low multipoles (2003)
Piao, Yun-Song, Tsujikawa, Shinji, Zhang, Xinmin
We construct a string-inspired nonsingular cosmological scenario in which an inflaton field is driven up the potential before the graceful exit by employing a low-energy string effective action with...
Constraints on braneworld inflation from CMB anisotropies (2003)
Tsujikawa, Shinji, Liddle, Andrew R.
We obtain observational constraints on Randall--Sundrum type II braneworld inflation using a compilation of data including WMAP, the 2dF and latest SDSS galaxy redshift surveys. We place constraints...
Loop quantum gravity effects on inflation and the CMB (2003)
Tsujikawa, Shinji, Singh, Parampreet, Maartens, Roy
In loop quantum cosmology, the universe avoids a big bang singularity and undergoes an early and short super-inflation phase. During super-inflation, non-perturbative quantum corrections to the...
Non-commutative inflation and the CMB (2003)
Tsujikawa, Shinji, Maartens, Roy, Brandenberger, Robert
Non-commutative inflation is a modification of standard general relativity inflation which takes into account some effects of the space-time uncertainty principle motivated by ideas from string...
Cosmological density perturbations from perturbed couplings (2003)
The density perturbations generated when the inflaton decay rate is perturbed by a light scalar field $\chi$ are studied. By explicitly solving the perturbation equations for the system of two scalar...
Introductory review of cosmic inflation (2003)
These lecture notes provide an introduction to cosmic inflation. In particular I will review the basic concepts of inflation, generation of density perturbations, and reheating after inflation.
Construction of nonsingular cosmological solutions in string theories (2003)
We study nonsingular cosmological scenarios in a general $D$-dimensional string effective action of the dilaton-modulus-axion system in the presence of the matter source. In the standard dilatonic...
Aspects of String-Gas Cosmology at Finite Temperature (2003)
Bassett, Bruce A., Borunda, Monica, Serone, Marco, Tsujikawa, Shinji
We study string-gas cosmology in dilaton gravity, inspired by the fact that it naturally arises in a string theory context. Our main interest is the thermodynamical treatment of the string-gas and...
Aspects of string-gas cosmology at finite temperature (2003)
Bassett, Bruce A., Borunda, Monica, Serone, Marco, Tsujikawa, Shinji
We study string-gas cosmology in dilaton gravity, inspired by the fact that it naturally arises in a string theory context. Our main interest is the thermodynamical treatment of the string-gas and...
Correlation-consistency cartography of the double inflation landscape (2002)
Tsujikawa, Shinji, Parkinson, David, Bassett, Bruce A.
We show explicitly some exciting features of double-inflation: (i) it can often lead to strongly correlated adiabatic and entropy (isocurvature) power spectra. (ii) The two-field slow-roll...
Tsujikawa, Shinji, Brandenberger, Robert, Finelli, Fabio
We consider the construction of nonsingular Pre-Big-Bang and Ekpyrotic type cosmological models realized by the addition to the action of specific higher-order terms stemming from quantum...
When can preheating affect the CMB? (2002)
Tsujikawa, Shinji, Bassett, Bruce A.
We discuss the principles governing the selection of inflationary models for which preheating can affect the CMB. This is a (fairly small) subset of those models which have non-negligible...
Nature of singularities in anisotropic string cosmology (2002)
Toporensky, Alexey, Tsujikawa, Shinji
We study nature of singularities in anisotropic string-inspired cosmological models in the presence of a Gauss-Bonnet term. We analyze two string gravity models-- dilaton-driven and modulus-driven...
Fermion production from preheating-amplified metric perturbations (2002)
Bassett, Bruce A., Peloso, Marco, Sorbo, Lorenzo, Tsujikawa, Shinji
We study gravitational creation of light fermions in the presence of classical scalar metric perturbations about a flat Friedmann-Lemaitre- Robertson-Walker (FLRW) background. These perturbations can...
Density perturbations in the Ekpyrotic Universe and string-inspired generalizations (2001)
We study density perturbations in several cosmological models motivated by string theory. The evolution and the spectra of curvature perturbations ${\cal R}$ are analyzed in the Ekpyrotic scenario...
Fermion production from preheating-amplified metric perturbations (2001)
Bassett, Bruce A., Peloso, Marco, Sorbo, Lorenzo, Tsujikawa, Shinji
We study gravitational creation of light fermions in the presence of classical scalar metric perturbations about a flat Friedmann-Lemaitre- Robertson-Walker (FLRW) background. These perturbations can...
Cosmological perturbations from multi-field inflation in generalized Einstein theories (2001)
Starobinsky, Alexei A., Tsujikawa, Shinji, Yokoyama, Jun'ichi
We study cosmological perturbations generated from quantum fluctuations in multi-field inflationary scenarios in generalized Einstein theories, taking both adiabatic and isocurvature modes into...
Massive fermion production in nonsingular superstring cosmology (2001)
Tsujikawa, Shinji, Yajima, Hiroki
We study massive spin-1/2 fermion production in nonsingular superstring cosmology, taking into account one-loop quantum corrections to a superstring effective action with dilaton and modulus fields....
Tsujikawa, Shinji, Maeda, Kei-ichi, Mizuno, Shuntaro
We study brane-world preheating in massive chaotic inflationary scenario where scalar fields are confined on the 3-brane. Assuming that quadratic contribution in energy densities dominates the Hubble...
Preheating - cosmic magnetic dynamo ? (2000)
Bassett, Bruce A., Pollifrone, Giuseppe, Tsujikawa, Shinji, Viniegra, Fermin
We study the amplification of large-scale magnetic fields during preheating and inflation in several different models. Preheating can resonantly amplify seed fields on cosmological scales. In the...
Inflationary preheating and primordial black holes (2000)
Bassett, Bruce A., Tsujikawa, Shinji
Preheating after inflation may over-produce primordial black holes (PBH's) in many regions of parameter space. As an example we study two-field models with a massless self-interacting inflaton,...
New constraints on multi-field inflation with nonminimal coupling (2000)
Tsujikawa, Shinji, Yajima, Hiroki
We study the dynamics and perturbations during inflation and reheating in a multi-field model where a second scalar field $\chi$ is nonminimally coupled to the scalar curvature $(\frac12 \xi...
Multi-field fermionic preheating (2000)
Tsujikawa, Shinji, Bassett, Bruce A., Viniegra, Fermin
Fermion creation during preheating in the presence of multiple scalar fields exhibits a range of interesting behaviour relevant to estimating post-inflation gravitino abundances. We present...