Modified gravity and the stability of de Sitter space

Within the context of modified gravity and dark energy scenarios of the accelerating universe, we study the stability of de Sitter space with respect to inhomogeneous perturbations using a gauge-independent formalism. In modified gravity the stability condition is exactly the same that one obtains from a homogeneous perturbation analysis, while the stability condition in scalar-tensor gravity is more restrictive.Comment: 5 pages, no figures, RevTeX, to appear in Phys. Rev. D (Rapid Communications section

Solving for the dynamics of the universe

A new method of solving the Einstein-Friedmann dynamical equations of a spatially homogeneous and isotropic universe is presented. The method is applicable when the equation of state of the material content assumes the form P=(g -1) rho, g = constant. The solution for the expansion factor is commonly given only for g = 0, 1 and 4/3 when the curvature index K is 1 or -1. The proposed procedure is valid for general g and K and it allows for ease of derivation of the solutions. This alternative method is useful pedagogically to introduce basic cosmology.Comment: 8 pages, LaTeX, no figures. To appear in American Journal of Physic

Lemaitre model and cosmic mass

The mass of a sphere of simmetry in the Lemaitre universe is discussed using the Hawking-Hayward quasi-local energy and clarifying existing ambiguities. A covariantly conserved current introduced by Cahill and McVittie is shown to be a multiple of the Kodama energy current.Comment: 10 pages, no figures, to appear in Gen. Rel. Gravi

Massive spin zero fields in cosmology and the tail-free property

Fields of spin $s \geq 1/2$ satisfying wave equations in a curved space obey the Huygens principle under certain conditions clarified by a known theorem. Here this theorem is generalized to spin zero and applied to an inflaton field in de Sitter-like space, showing that tails of scalar radiation are an unavoidable physical feature. Requiring the absence of tails, on the contrary, necessarily implies an unnatural tuning between cosmological constant, scalar field mass, and coupling constant to the curvature.Comment: To appear in "Cosmology and Quantum Vacuum", special issue of Symmetry edited by E. Elizald