117 research outputs found
Cosmological significance of one-loop effective gravity
We study the one-loop effective action for gravity in a cosmological setup to
determine possible cosmological effects of quantum corrections to Einstein
theory. By considering the effect of the universal non-local terms in a toy
model, we show that they can play an important role in the very early universe.
We find that during inflation, the non-local terms are significant, leading to
deviations from the standard inflationary expansion.Comment: 8 pages (REVTeX
Stability of the Einstein static universe in f(R) gravity
We analyze the stability of the Einstein static universe by considering
homogeneous scalar perturbations in the context of f(R) modified theories of
gravity. By considering specific forms of f(R), the stability regions of the
solutions are parameterized by a linear equation of state parameter w=p/rho.
Contrary to classical general relativity, it is found that in f(R) gravity a
stable Einstein cosmos with a positive cosmological constant does indeed exist.
Thus, we are lead to conclude that, in principle, modifications in f(R) gravity
stabilize solutions which are unstable in general relativity.Comment: 7 pages, 2 figures, 2 tables; references adde
Universality in a Class of Q-Ball Solutions: An Analytic Approach
The properties of Q-balls in the general case of a sixth order potential have
been studied using analytic methods. In particular, for a given potential, the
initial field value that leads to the soliton solution has been derived and the
corresponding energy and charge have been explicitly evaluated. The proposed
scheme is found to work reasonably well for all allowed values of the model
parameters.Comment: 9 Pages, 6 Figure
Spherically symmetric vacuum solutions of modified gravity theory in higher dimensions
In this paper we investigate spherically symmetric vacuum solutions of
gravity in a higher dimensional spacetime. With this objective we construct a
system of non-linear differential equations, whose solutions depend on the
explicit form assumed for the function . We explicit
show that for specific classes of this function exact solutions from the field
equations are obtained; also we find approximated results for the metric tensor
for more general cases admitting close to the unity.Comment: 14 pages, no figure. New version accepted for publication in EPJ
Constraining Newtonian stellar configurations in f(R) theories of gravity
We consider general metric theories of gravity by solving the field
equations in the presence of a spherical static mass distribution by analytical
perturbative means. Expanding the field equations systematically in \cO(G),
we solve the resulting set of equations and show that theories which
attempt to solve the dark energy problem very generally lead to
in the solar system. This excludes a large class of theories
as possible explanations of dark energy. We also present the first order
correction to and show that it cannot have a significant effect.Comment: 4 pages; v2: added references, modified abstract and introduction,
conclusions unchange
Velocity and Heat Flow in a Composite Two Fluid System
We describe the stress energy of a fluid with two unequal stresses and heat
flow in terms of two perfect fluid components. The description is in terms of
the fluid velocity overlap of the components, and makes no assumptions about
the equations of state of the perfect fluids. The description is applied to the
metrics of a conformally flat system and a black string.Comment: typos correcte
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