Ryotaro Kase

Matter perturbations in Galileon cosmology

We study the evolution of matter density perturbations in Galileon cosmology where the late-time cosmic acceleration can be realized by a field kinetic energy. We obtain full perturbation equations at linear order in the presence of five covariant Lagrangians L{sub i} (i=1,{center_dot}{center_dot}{center_dot},5) satisfying the Galileon symmetry {partial_derivative}{sub {mu}}{phi}{yields}{partial_derivative}{sub {mu}}{phi}{sup +}b{sub {mu}} in the flat space-time. The equations for a matter perturbation as well as an effective gravitational potential are derived under a quasistatic approximation on subhorizon scales. This approximation can reproduce full numerical solutions with high accuracy for the wavelengths relevant to large-scale structures. For the model parameters constrained by the background expansion history of the Universe, the growth rate of matter perturbations is larger than that in the {Lambda}-cold dark matter model, with the growth index {gamma} today typically smaller than 0.4. We also find that, even on very large scales associated with the integrated-Sachs-Wolfe effect in cosmic microwave background temperature anisotropies, the effective gravitational potential exhibits a temporal growth during the transition from the matter era to the epoch of cosmic acceleration. These properties are useful to distinguish the Galileon model from the {Lambda}-cold dark matter model in future high-precision observations.

Vainshtein mechanism in second-order scalar-tensor theories

In second-order scalar-tensor theories we study how the Vainshtein mechanism works in a spherically symmetric background with a matter source. In the presence of the field coupling

Beyond generalized Proca theories

F(\phi)=e^{-2Q\phi}

Effective gravitational couplings for cosmological perturbations in generalized Proca theories

with the Ricci scalar

Cosmology in generalized Horndeski theories with second-order equations of motion

R

Effective field theory approach to modified gravity including Horndeski theory and Hořava–Lifshitz gravity

we generally derive the Vainshtein radius within which the General Relativistic behavior is recovered even for the coupling

Cosmology in generalized Proca theories

Q

Screening fifth forces in generalized Proca theories

of the order of unity. Our analysis covers the models such as the extended Galileon and Brans-Dicke theories with a dilatonic field self-interaction. We show that, if these models are responsible for the cosmic acceleration today, the corrections to gravitational potentials are generally small enough to be compatible with local gravity constraints.

Screening the fifth force in the Horndeski's most general scalar-tensor theories

We consider higher-order derivative interactions beyond second-order generalized Proca theories that propagate only the three desired polarizations of a massive vector field besides the two tensor polarizations from gravity. These new interactions follow the similar construction criteria to those arising in the extension of scalar–tensor Horndeski theories to Gleyzes–Langlois–Piazza–Vernizzi (GLPV) theories. On the isotropic cosmological background, we show the existence of a constraint with a vanishing Hamiltonian that removes the would-be Ostrogradski ghost. We study the behavior of linear perturbations on top of the isotropic cosmological background in the presence of a matter perfect fluid and find the same number of propagating degrees of freedom as in generalized Proca theories (two tensor polarizations, two transverse vector modes, and two scalar modes). Moreover, we obtain the conditions for the avoidance of ghosts and Laplacian instabilities of tensor, vector, and scalar perturbations. We observe key differences in the scalar sound speed, which is mixed with the matter sound speed outside the domain of generalized Proca theories.

Black holes in vector-tensor theories

We consider the finite interactions of the generalized Proca theory including the sixth-order Lagrangian and derive the full linear perturbation equations of motion on the flat Friedmann-Lemaitre-Robertson-Walker background in the presence of a matter perfect fluid. By construction, the propagating degrees of freedom (besides the matter perfect fluid) are two transverse vector perturbations, one longitudinal scalar, and two tensor polarizations. The Lagrangians associated with intrinsic vector modes neither affect the background equations of motion nor the second-order action of tensor perturbations, but they do give rise to non-trivial modifications to the no-ghost condition of vector perturbations and to the propagation speeds of vector and scalar perturbations. We derive the effective gravitational coupling