2,399 research outputs found

### The Statefinder hierarchy: An extended null diagnostic for concordance cosmology

We show how higher derivatives of the expansion factor can be developed into
a null diagnostic for concordance cosmology (LCDM). It is well known that the
Statefinder -- the third derivative of the expansion factor written in
dimensionless form, a^{(3)}/aH^3, equals unity for LCDM. We generalize this
result to higher derivatives of the expansion factor and demonstrate that the
hierarchy, a^{(n)}/aH^n, can be converted to a form that stays pegged at unity
in concordance cosmology. This remarkable property of the Statefinder hierarchy
enables it to be used as an extended null diagnostic for the cosmological
constant. The Statefinder hierarchy combined with the growth rate of matter
perturbations defines a composite null diagnostic which can distinguish
evolving dark energy from LCDM.Comment: 6 pages, 6 figures; to appear in Phys. Rev.

### General analytic formulae for attractor solutions of scalar-field dark energy models and their multi-field generalizations

We study general properties of attractors for scalar-field dark energy
scenarios which possess cosmological scaling solutions. In all such models
there exists a scalar-field dominant solution with an energy fraction
\Omega_{\phi}=1 together with a scaling solution. A general analytic formula is
given to derive fixed points relevant to dark energy coupled to dark matter. We
investigate the stability of fixed points without specifying the models of dark
energy in the presence of non-relativistic dark matter and provide a general
proof that a non-phantom scalar-field dominant solution is unstable when a
stable scaling solution exists in the region \Omega_{\phi}<1. A phantom
scalar-field dominant fixed point is found to be classically stable. We also
generalize the analysis to the case of multiple scalar fields and show that for
a non-phantom scalar field assisted acceleration always occurs for all
scalar-field models which have scaling solutions. For a phantom field the
equation of state approaches that of cosmological constant as we add more
scalar fields.Comment: 11 pages, no figures, version to appear in Physical Review

### Possible use of self-calibration to reduce systematic uncertainties in determining distance-redshift relation via gravitational radiation from merging binaries

By observing mergers of compact objects, future gravity wave experiments
would measure the luminosity distance to a large number of sources to a high
precision but not their redshifts. Given the directional sensitivity of an
experiment, a fraction of such sources (gold plated -- GP) can be identified
optically as single objects in the direction of the source. We show that if an
approximate distance-redshift relation is known then it is possible to
statistically resolve those sources that have multiple galaxies in the beam. We
study the feasibility of using gold plated sources to iteratively resolve the
unresolved sources, obtain the self-calibrated best possible distance-redshift
relation and provide an analytical expression for the accuracy achievable. We
derive lower limit on the total number of sources that is needed to achieve
this accuracy through self-calibration. We show that this limit depends
exponentially on the beam width and give estimates for various experimental
parameters representative of future gravitational wave experiments DECIGO and
BBO.Comment: 6 pages, 2 figures, accepted for publication in PR

### New Vistas in Braneworld Cosmology

Traditionally, higher-dimensional cosmological models have sought to provide
a description of the fundamental forces in terms of a unifying geometrical
construction. In this essay we discuss how, in their present incarnation,
higher-dimensional `braneworld' models might provide answers to a number of
cosmological puzzles including the issue of dark energy and the nature of the
big-bang singularity.Comment: Honorable mention in the 2002 Essay Competition of the Gravity
Research Foundation. 10 pages, 2 figure

### Reconstruction of general scalar-field dark energy models

The reconstruction of scalar-field dark energy models is studied for a
general Lagrangian density $p(\phi, X)$, where $X$ is a kinematic term of a
scalar field $\phi$. We implement the coupling $Q$ between dark energy and dark
matter and express reconstruction equations using two observables: the Hubble
parameter $H$ and the matter density perturbation $\delta_m$. This allows us to
determine the structure of corresponding theoretical Lagrangian together with
the coupling $Q$ from observations. We apply our formula to several forms of
Lagrangian and present concrete examples of reconstruction by using the recent
Gold dataset of supernovae measurements. This analysis includes a generalized
ghost condensate model as a way to cross a cosmological-constant boundary even
for a single-field case.Comment: 8 pages, 2 figure

### The cosmic coincidence in Brans-Dicke cosmologies

Among the suggested solutions to the cosmological constant problem, we find
the idea of a dynamic vacuum, with an energy density decaying with the universe
expansion. We investigate the possibility of a variation in the gravitational
constant as well, induced, at the cosmological scale, by the vacuum decay. We
consider an effective Brans-Dicke theory in the spatially flat FLRW spacetime,
finding late time solutions characterized by a constant ratio between the
matter and vacuum energy densities. By using the observed limits for the
universe age, we fix the only free parameter of our solutions, obtaining a
relative matter density in the range 0.25-0.4. In particular, for Ht = 1 we
obtain a relative matter density equals to 1/3. This constitutes a possible
explanation for another problem related to the cosmological term, the cosmic
coincidence problem.Comment: This essay received an "honorable mention" in the 2005 Essay
Competition of the Gravity Research Foundatio

### Observational signatures of f(R) dark energy models that satisfy cosmological and local gravity constraints

We discuss observational consequences of f(R) dark energy scenarios that
satisfy local gravity constraints (LGC) as well as conditions of the
cosmological viability. The model we study is given by m(r)=C(-r-1)^p (C>0,
p>1) with m=Rf_{,RR}/f_{,R} and r=-Rf_{,R}/f, which cover viable f(R) models
proposed so far in a high-curvature region designed to be compatible with LGC.
The equation of state of dark energy exhibits a divergence at a redshift z_c
that can be as close as a few while satisfying sound horizon constraints of
Cosmic Microwave Background (CMB). We study the evolution of matter density
perturbations in details and place constraints on model parameters from the
difference of spectral indices of power spectra between CMB and galaxy
clustering. The models with p>5 can be consistent with those observational
constraints as well as LGC. We also discuss the evolution of perturbations in
the Ricci scalar R and show that an oscillating mode (scalaron) can easily
dominate over a matter-induced mode as we go back to the past. This violates
the stability of cosmological solutions, thus posing a problem about how the
over-production of scalarons should be avoided in the early universe.Comment: 13 pages, 7 figures, version to appear in Physical Review

### Entropy of Microwave Background Radiation in Observable Universe

We show that the cosmological constant at late time places a bound on the
entropy of microwave background radiation deposited in the future event horizon
of a given observer, $S\leq S_{\Lambda_0}^{3/4}$. This bound is independent of
the energy scale of reheating and the FRW evolution after reheating. We also
discuss why the entropy of microwave background in our observable universe has
its present value.Comment: 3 pages, no figures, revised version to publish in PRD brief repor

### Anisotropic Cosmological Constant and the CMB Quadrupole Anomaly

There are evidences that the cosmic microwave background (CMB) large-angle
anomalies imply a departure from statistical isotropy and hence from the
standard cosmological model. We propose a LCDM model extension whose dark
energy component preserves its nondynamical character but wield anisotropic
vacuum pressure. Exact solutions for the cosmological scale factors are
presented, upper bounds for the deformation parameter are evaluated and its
value is estimated considering the elliptical universe proposal to solve the
quadrupole anomaly. This model can be constructed from a Bianchi I cosmology
with cosmological constant from two different ways: i) a straightforward
anisotropic modification of the vacuum pressure consistently with
energy-momentum conservation; ii) a Poisson structure deformation between
canonical momenta such that the dynamics remain invariant under scale factors
rescalings.Comment: 8 pages, 2 columns, 1 figure. v2: figure improved, added comments on
higher eccentricity powers and references. v3: typos corrected, version to
appear in PR

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