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Search Results: 1 - 10 of 1122 matches for " Shinji Tsujikawa "
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Observational signatures of f(R) dark energy models that satisfy cosmological and local gravity constraints
Tsujikawa, Shinji
High Energy Physics - Phenomenology , 2007, DOI: 10.1103/PhysRevD.77.023507
Abstract: 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 m(r)=C(-r-1)^p (C>0, p>1) with m=Rf_{,RR}/f_{,R} and r=-Rf_{,R}/f, which cover viable f(R) models proposed so far in a high-curvature region designed to be compatible with LGC. The equation of state of dark energy exhibits a divergence at a redshift z_c that can be as close as a few while satisfying sound horizon constraints of Cosmic Microwave Background (CMB). We study the evolution of matter density perturbations in details and place constraints on model parameters from the difference of spectral indices of power spectra between CMB and galaxy clustering. The models with p>5 can be consistent with those observational constraints as well as LGC. We also discuss the evolution of perturbations in the Ricci scalar R and show that an oscillating mode (scalaron) can easily dominate over a matter-induced mode as we go back to the past. This violates the stability of cosmological solutions, thus posing a problem about how the over-production of scalarons should be avoided in the early universe.
Quintessence: A Review
Tsujikawa, Shinji
High Energy Physics - Phenomenology , 2013,
Abstract: Quintessence is a canonical scalar field introduced to explain the late-time cosmic acceleration. The cosmological dynamics of quintessence is reviewed, paying particular attention to the evolution of the dark energy equation of state w. For the field potentials having tracking and thawing properties, the evolution of w can be known analytically in terms of a few model parameters. Using the analytic expression of w, we constrain quintessence models from the observations of supernovae type Ia, cosmic microwave background, and baryon acoustic oscillations. The tracking freezing models are hardly distinguishable from the LCDM model, whereas in thawing models the today's field equation of state is constrained to be w_0<-0.7 (95 % CL). We also derive an analytic formula for the growth rate of matter density perturbations in dynamical dark energy models, which allows a possibility to put further bounds on w from the measurement of redshift-space distortions in the galaxy power spectrum. Finally we review particle physics models of quintessence- such as those motivated by supersymmetric theories. The field potentials of thawing models based on a pseudo-Nambu-Goldstone boson or on extended supergravity theories have a nice property that a tiny mass of quintessence can be protected against radiative corrections.
Matter density perturbations and effective gravitational constant in modified gravity models of dark energy
Tsujikawa, Shinji
High Energy Physics - Phenomenology , 2007, DOI: 10.1103/PhysRevD.76.023514
Abstract: 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 term X=-(nabla phi)^2/2. This is useful to constrain modified gravity dark energy models from observations of large-scale structure and weak lensing. We obtain the solutions for the matter perturbation delta_m as well as the gravitational potential Phi for some analytically solvable models. In a f(R) dark energy model with the Lagrangian density f(R)=alpha R^{1+m}-Lambda, the growth rates of perturbations exhibit notable differences from those in the standard Einstein gravity unless m is very close to 0. In scalar-tensor models with the Lagrangian density f=F(phi)R+2p(phi,X) we relate the models with coupled dark energy scenarios in the Einstein frame and reproduce the equations of perturbations known in the current literature by making a conformal transformation. We also estimate the evolution of perturbations in both Jordan and Einstein frames when the energy fraction of dark energy is constant during the matter-dominated epoch.
Dark energy: investigation and modeling
Shinji Tsujikawa
Physics , 2010, DOI: 10.1007/978-90-481-8685-3_8
Abstract: Constantly accumulating observational data continue to confirm that about 70% of the energy density today consists of dark energy responsible for the accelerated expansion of the Universe. We present recent observational bounds on dark energy constrained by the type Ia supernovae, cosmic microwave background, and baryon acoustic oscillations. We review a number of theoretical approaches that have been adopted so far to explain the origin of dark energy. This includes the cosmological constant, modified matter models (such as quintessence, k-essence, coupled dark energy, unified models of dark energy and dark matter), modified gravity models (such as f(R) gravity, scalar-tensor theories, braneworlds), and inhomogeneous models. We also discuss observational and experimental constraints on those models and clarify which models are favored or ruled out in current observations.
Introductory review of cosmic inflation
Shinji Tsujikawa
Physics , 2003,
Abstract: 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.
Matter density perturbations and effective gravitational constant in modified gravity models of dark energy
Shinji Tsujikawa
Physics , 2007, DOI: 10.1103/PhysRevD.76.023514
Abstract: 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 term X=-(nabla phi)^2/2. This is useful to constrain modified gravity dark energy models from observations of large-scale structure and weak lensing. We obtain the solutions for the matter perturbation delta_m as well as the gravitational potential Phi for some analytically solvable models. In a f(R) dark energy model with the Lagrangian density f(R)=alpha R^{1+m}-Lambda, the growth rates of perturbations exhibit notable differences from those in the standard Einstein gravity unless m is very close to 0. In scalar-tensor models with the Lagrangian density f=F(phi)R+2p(phi,X) we relate the models with coupled dark energy scenarios in the Einstein frame and reproduce the equations of perturbations known in the current literature by making a conformal transformation. We also estimate the evolution of perturbations in both Jordan and Einstein frames when the energy fraction of dark energy is constant during the matter-dominated epoch.
Construction of nonsingular cosmological solutions in string theories
Shinji Tsujikawa
Physics , 2003, DOI: 10.1088/0264-9381/20/11/303
Abstract: 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 Brans-Dicke parameter ($\omega=-1$) with radiation, we analytically obtain singularity-free bouncing solutions where the universe starts out in a state with a finite curvature and evolves toward the weakly coupled regime. We apply this analytic method to the string-gas cosmology including the massive state in addition to the leading massless state (radiation), with and without the axion. We numerically find bouncing solutions which asymptotically approach an almost radiation-dominant universe with a decreasing curvature irrespective of the presence of the axion, implying that inclusion of the matter source is crucial for the existence of such solutions for $\omega=-1$. In the theories with $\omega \ne -1$, it is possible to obtain complete regular bouncing solutions with a finite dilaton and curvature in both past and future asymptotics for the general dimension, $D$. We also discuss the case where dilatonic higher-order corrections are involved to the tree-level effective action and demonstrate that the presence of axion/modulus fields and the matter source does not significantly affect the dynamics of the dilaton-driven inflation and the subsequent graceful exit.
Cosmologies from higher-order string corrections
Shinji Tsujikawa
Physics , 2006, DOI: 10.1002/andp.200510189
Abstract: 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 the presence of such corrections it is possible to construct nonsingular cosmological solutions in the context of Pre-Big-Bang and Ekpyrotic universes. We review the construction of nonsingular bouncing solutions and resulting density perturbations in Pre-Big-Bang and Ekpyrotic models. We also discuss the effect of higher-order string corrections on dark energy universe and show several interesting possibilities of the avoidance of future singularities.
Reconstruction of general scalar-field dark energy models
Shinji Tsujikawa
Physics , 2005, DOI: 10.1103/PhysRevD.72.083512
Abstract: 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 $Q$ between dark energy and dark matter and express reconstruction equations using two observables: the Hubble parameter $H$ and the matter density perturbation $\delta_m$. This allows us to determine the structure of corresponding theoretical Lagrangian together with the coupling $Q$ from observations. We apply our formula to several forms of Lagrangian and present concrete examples of reconstruction by using the recent Gold dataset of supernovae measurements. This analysis includes a generalized ghost condensate model as a way to cross a cosmological-constant boundary even for a single-field case.
Distinguishing between inflationary models from cosmic microwave background
Shinji Tsujikawa
Physics , 2014,
Abstract: In this paper, inflationary cosmology is reviewed, paying particular attention to its observational signatures associated with large-scale density perturbations generated from quantum fluctuations. In the most general scalar-tensor theories with second-order equations of motion, we derive the scalar spectral index $n_s$, the tensor-to-scalar ratio $r$, and the nonlinear estimator $f_{\rm NL}$ of primordial non-Gaussianities to confront models with observations of Cosmic Microwave Background (CMB) temperature anisotropies. Our analysis includes models such as potential-driven slow-roll inflation, k-inflation, Starobinsky inflation, and Higgs inflation with non-minimal/derivative/Galileon couplings. We constrain a host of inflationary models by using the Planck data combined with other measurements to find models most favored observationally in the current literature. We also study anisotropic inflation based on a scalar coupling with a vector (or, two-form) field and discuss its observational signatures appearing in the two-point and three-point correlation functions of scalar and tensor perturbations.
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