36 research outputs found
Decaying Dark Energy in Higher-Dimensional Gravity
We use data from observational cosmology to put constraints on
higher-dimensional extensions of general relativity in which the effective
four-dimensional dark-energy density (or cosmological "constant") decays with
time. In particular we study the implications of this decaying dark energy for
the age of the universe, large-scale structure formation, big-bang
nucleosynthesis and the magnitude-redshift relation for Type Ia supernovae. Two
of these tests (age and the magnitude-redshift relation) place modest lower
limits on the free parameter of the theory, a cosmological length scale L akin
to the de Sitter radius. These limits will improve if experimental
uncertainties on supernova magnitudes can be reduced around z=1.Comment: 11 pages, 5 figures, submitted to A&
Cosmic Acceleration With A Positive Cosmological Constant
We have considered a cosmological model with a phenomenological model for the
cosmological constant of the form \Lambda=\bt\fr{\ddot R}{R}, \bt is a
constant. For age parameter consistent with observational data the Universe
must be accelerating in the presence of a positive cosmological constant. The
minimum age of the Universe is , where is the present Hubble
constant. The cosmological constant is found to decrease as . Allowing
the gravitational constant to change with time leads to an ever increasing
gravitational constant at the present epoch. In the presence of a viscous fluid
this decay law for is equivalent to the one with () provided \alpha=\fr{\bt}{3(\bt-2)}. The
inflationary solution obtained from this model is that of the de-Sitter type.Comment: a more revised versio
Renormalization group improved gravitational actions: a Brans-Dicke approach
A new framework for exploiting information about the renormalization group
(RG) behavior of gravity in a dynamical context is discussed. The
Einstein-Hilbert action is RG-improved by replacing Newton's constant and the
cosmological constant by scalar functions in the corresponding Lagrangian
density. The position dependence of and is governed by a RG
equation together with an appropriate identification of RG scales with points
in spacetime. The dynamics of the fields and does not admit a
Lagrangian description in general. Within the Lagrangian formalism for the
gravitational field they have the status of externally prescribed
``background'' fields. The metric satisfies an effective Einstein equation
similar to that of Brans-Dicke theory. Its consistency imposes severe
constraints on allowed backgrounds. In the new RG-framework, and
carry energy and momentum. It is tested in the setting of homogeneous-isotropic
cosmology and is compared to alternative approaches where the fields and
do not carry gravitating 4-momentum. The fixed point regime of the
underlying RG flow is studied in detail.Comment: LaTeX, 72 pages, no figure
Cosmology with Self-Adjusting Vacuum Energy Density from a Renormalization Group Fixed Point
Cosmologies with a time dependent Newton constant and cosmological constant
are investigated. The scale dependence of and is governed by a
set of renormalization group equations which is coupled to Einstein's equation
in a consistent way. The existence of an infrared attractive renormalization
group fixed point is postulated, and the cosmological implications of this
assumption are explored. It turns out that in the late Universe the vacuum
energy density is automatically adjusted so as to equal precisely the matter
energy density, and that the deceleration parameter approaches . This
scenario might explain the data from recent observations of high redshift type
Ia Supernovae and the cosmic microwave background radiation without introducing
a quintessence field.Comment: v2: published version, two references update
Nucleosynthesis Constraints on Scalar-Tensor Theories of Gravity
We study the cosmological evolution of massless single-field scalar-tensor
theories of gravitation from the time before the onset of annihilation
and nucleosynthesis up to the present. The cosmological evolution together with
the observational bounds on the abundances of the lightest elements (those
mostly produced in the early universe) place constraints on the coefficients of
the Taylor series expansion of , which specifies the coupling of the
scalar field to matter and is the only free function in the theory. In the case
when has a minimum (i.e., when the theory evolves towards general
relativity) these constraints translate into a stronger limit on the
Post-Newtonian parameters and than any other observational
test. Moreover, our bounds imply that, even at the epoch of annihilation and
nucleosynthesis, the evolution of the universe must be very close to that
predicted by general relativity if we do not want to over- or underproduce
He. Thus the amount of scalar field contribution to gravity is very small
even at such an early epoch.Comment: 15 pages, 2 figures, ReVTeX 3.1, submitted to Phys. Rev. D1
Exact Black Hole and Cosmological Solutions in a Two-Dimensional Dilaton-Spectator Theory of Gravity
Exact black hole and cosmological solutions are obtained for a special
two-dimensional dilaton-spectator () theory of gravity. We show how
in this context any desired spacetime behaviour can be determined by an
appropriate choice of a dilaton potential function and a ``coupling
function'' in the action. We illustrate several black hole solutions
as examples. In particular, asymptotically flat double- and multiple- horizon
black hole solutions are obtained. One solution bears an interesting
resemblance to the string-theoretic black hole and contains the same
thermodynamic properties; another resembles the Reissner-Nordstrom
solution. We find two characteristic features of all the black hole solutions.
First the coupling constants in must be set equal to constants of
integration (typically the mass). Second, the spectator field and its
derivative both diverge at any event horizon. A test particle with
``spectator charge" ({\it i.e.} one coupled either to or ),
will therefore encounter an infinite tidal force at the horizon or an
``infinite potential barrier'' located outside the horizon respectively. We
also compute the Hawking temperature and entropy for our solutions. In
cosmology, two non-singular solutions which resemble two exact solutions
in string-motivated cosmology are obtained. In addition, we construct a
singular model which describes the standard non-inflationary big bang
cosmology (). Motivated by the
similaritiesbetween and gravitational field equations in
cosmology, we briefly discuss a special dilaton-spectator action
constructed from the bosonic part of the low energy heterotic string action andComment: 34 pgs. Plain Tex, revised version contains some clarifying comments
concerning the relationship between the constants of integration and the
coupling constants
Stars in five dimensional Kaluza Klein gravity
In the five dimensional Kaluza Klein (KK) theory there is a well known class
of static and electromagnetic--free KK--equations characterized by a naked
singularity behavior, namely the Generalized Schwarzschild solution (GSS). We
present here a set of interior solutions of five dimensional KK--equations.
These equations have been numerically integrated to match the GSS in the
vacuum. The solutions are candidates to describe the possible interior perfect
fluid source of the exterior GSS metric and thus they can be models for stars
for static, neutral astrophysical objects in the ordinary (four dimensional)
spacetime.Comment: 15 pages, 8 figures. To be published in EPJ
Evolution of the Scale Factor with a Variable Cosmological Term
Evolution of the scale factor a(t) in Friedmann models (those with zero
pressure and a constant cosmological term Lambda) is well understood, and
elegantly summarized in the review of Felten and Isaacman [Rev. Mod. Phys. 58,
689 (1986)]. Developments in particle physics and inflationary theory, however,
increasingly indicate that Lambda ought to be treated as a dynamical quantity.
We revisit the evolution of the scale factor with a variable Lambda-term, and
also generalize the treatment to include nonzero pressure. New solutions are
obtained and evaluated using a variety of observational criteria. Existing
arguments for the inevitability of a big bang (ie., an initial state with a=0)
are substantially weakened, and can be evaded in some cases with Lambda_0 (the
present value of Lambda) well below current experimental limits.Comment: 29 pages, 12 figures (not included), LaTeX, uses Phys Rev D style
files (revtex.cls, revtex.sty, aps.sty, aps10.sty, prabib.sty). To appear in
Phys Rev
Five Dimensional Cosmological Models in General Relativity
A Five dimensional Kaluza-Klein space-time is considered in the presence of a
perfect fluid source with variable G and . An expanding universe is
found by using a relation between the metric potential and an equation of
state. The gravitational constant is found to decrease with time as whereas the variation for the cosmological constant follows as
, and
where is the equation of state parameter and is the scale factor.Comment: 13 pages, 4 figures, accepted in Int. J. Theor. Phy
Modified gravity in a viscous and non-isotropic background
We study the dynamical evolution of an model of gravity in a viscous
and anisotropic background which is given by a Bianchi type-I model of the
Universe. We find viable forms of gravity in which one is exactly the
Einsteinian model of gravity with a cosmological constant and other two are
power law models. We show that these two power law models are stable
with a suitable choice of parameters. We also examine three potentials which
exhibit the potential effect of models in the context of scalar tensor
theory. By solving different aspects of the model and finding the physical
quantities in the Jordan frame, we show that the equation of state parameter
satisfy the dominant energy condition. At last we show that the two power law
models behave like quintessence model at late times and also the shear
coefficient viscosity tends to zero at late times.Comment: 7 pages, 2 figure