6 research outputs found
Palatini formulation of the modified gravity with an additionally squared scalar curvature term
In this paper by deriving the Modified Friedmann equation in the Palatini
formulation of gravity, first we discuss the problem of whether in
Palatini formulation an additional term in Einstein's General Relativity
action can drive an inflation. We show that the Palatini formulation of
gravity cannot lead to the gravity-driven inflation as in the metric formalism.
If considering no zero radiation and matter energy densities, we obtain that
only under rather restrictive assumption about the radiation and matter energy
densities there will be a mild power-law inflation , which is
obviously different from the original vacuum energy-like driven inflation. Then
we demonstrate that in the Palatini formulation of a more generally modified
gravity, i.e., the model that intends to explain both the current
cosmic acceleration and early time inflation, accelerating cosmic expansion
achieved at late Universe evolution times under the model parameters satisfying
.Comment: 14 pages, accepted for publication by CQ
Modified Friedmann Equations in R-Modified Gravity
Recently, corrections to Einstein-Hilbert action that become important at
small curvature are proposed. We discuss the first order and second order
approximations to the field equations derived by the Palatini variational
principle. We work out the first and second order Modified Friedmann equations
and present the upper redshift bounds when these approximations are valid. We
show that the second order effects can be neglected on the cosmological
predictions involving only the Hubble parameter itself, e.g. the various
cosmological distances, but the second order effects can not be neglected in
the predictions involving the derivatives of the Hubble parameter. Furthermore,
the Modified Friedmann equations fit the SN Ia data at an acceptable level.Comment: 15 pages, 6 figures, v2: discussion added; v3: minor changes,
accepted by Class. and Quan. Gra
The Laser Astrometric Test of Relativity Mission
This paper discusses new fundamental physics experiment to test relativistic
gravity at the accuracy better than the effects of the 2nd order in the
gravitational field strength. The Laser Astrometric Test Of Relativity (LATOR)
mission uses laser interferometry between two micro-spacecraft whose lines of
sight pass close by the Sun to accurately measure deflection of light in the
solar gravity. The key element of the experimental design is a redundant
geometry optical truss provided by a long-baseline (100 m) multi-channel
stellar optical interferometer placed on the International Space Station. The
geometric redundancy enables LATOR to measure the departure from Euclidean
geometry caused by the solar gravity field to a very high accuracy. LATOR will
not only improve the value of the parameterized post-Newtonian (PPN) parameter
gamma to unprecedented levels of accuracy of 1 part in 1e8, it will also reach
ability to measure effects of the next post-Newtonian order (1/c^4) of light
deflection resulting from gravity's intrinsic non-linearity. The solar
quadrupole moment parameter, J2, will be measured with high precision, as well
as a variety of other relativistic. LATOR will lead to very robust advances in
the tests of fundamental physics: this mission could discover a violation or
extension of general relativity, or reveal the presence of an additional long
range interaction in the physical law. There are no analogs to the LATOR
experiment; it is unique and is a natural culmination of solar system gravity
experiments.Comment: 8 pages, 2 figures, invited talk given at the Second International
Conference on Particle and Fundamental Physics in Space (SpacePart'03), 10-12
December 2003, Washington, D
Can the dark energy equation-of-state parameter w be less than -1?
Models of dark energy are conveniently characterized by the equation-of-state
parameter w=p/\rho, where \rho is the energy density and p is the pressure.
Imposing the Dominant Energy Condition, which guarantees stability of the
theory, implies that w \geq -1. Nevertheless, it is conceivable that a
well-defined model could (perhaps temporarily) have w<-1, and indeed such
models have been proposed. We study the stability of dynamical models
exhibiting w<-1 by virtue of a negative kinetic term. Although naively
unstable, we explore the possibility that these models might be
phenomenologically viable if thought of as effective field theories valid only
up to a certain momentum cutoff. Under our most optimistic assumptions, we
argue that the instability timescale can be greater than the age of the
universe, but only if the cutoff is at or below 100 MeV. We conclude that it is
difficult, although not necessarily impossible, to construct viable models of
dark energy with w<-1; observers should keep an open mind, but the burden is on
theorists to demonstrate that any proposed new models are not ruled out by
rapid vacuum decay.Comment: 29 pages, 8 figures, minor corrections, reference adde