3,070 research outputs found
Reconstruction of general scalar-field dark energy models
The reconstruction of scalar-field dark energy models is studied for a
general Lagrangian density , where is a kinematic term of a
scalar field . We implement the coupling between dark energy and dark
matter and express reconstruction equations using two observables: the Hubble
parameter and the matter density perturbation . This allows us to
determine the structure of corresponding theoretical Lagrangian together with
the coupling 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
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
A new null diagnostic customized for reconstructing the properties of dark energy from BAO data
Baryon Acoustic Oscillations (BAO) provide an important standard ruler which
can be used to probe the recent expansion history of our universe. We show how
a simple extension of the Om diagnostic, which we call Om3, can combine
standard ruler information from BAO with standard candle information from type
Ia supernovae (SNIa) to yield a powerful novel null diagnostic of the
cosmological constant hypothesis. A unique feature of Om3 is that it requires
minimal cosmological assumptions since its determination does not rely upon
prior knowledge of either the current value of the matter density and the
Hubble constant, or the distance to the last scattering surface. Observational
uncertainties in these quantities therefore do not affect the reconstruction of
Om3. We reconstruct Om3 using the Union 2.1 SNIa data set and BAO data from
SDSS, WiggleZ and 6dFGS. Our results are consistent with dark energy being the
cosmological constant. We show how Om and Om3 can be used to obtain accurate
model independent constraints on the properties of dark energy from future data
sets such as BigBOSS.Comment: 9 pages, 4 figures, discussions extended, results unchanged, matches
the final version published in PR
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
CMB acoustic scale in the entropic-like accelerating universe
We consider generalizations of the entropic accelerating universe recently
proposed in Ref. [4,5] and show that their background equations can be made
equivalent to a model with a dark energy component with constant parameter of
state , where is related to the
coefficients of the new terms in the Friedman equations. After discussing all
the Friedman equations for an arbitrary , we show how to recover the
standard scalings for dust and radiation. The acoustic scale , related
to the peak positions in the pattern of the angular power spectrum of the
Cosmic Microwave Background anisotropies, is also computed and yields the
stringent bound . We then argue that future data might be able
to distinguish this model from pure CDM (corresponding to ).Comment: 6 pages, 2 figures. Accepted for publication in Physical Review
Is cosmic acceleration slowing down?
We investigate the course of cosmic expansion in its `recent past' using the
Constitution SN Ia sample (which includes CfA data at low redshifts), jointly
with signatures of baryon acoustic oscillations (BAO) in the galaxy
distribution and fluctuations in the cosmic microwave background (CMB). Earlier
SN Ia data sets could not address this issue because of a paucity of data at
low redshifts. Allowing the equation of state of dark energy (DE) to vary, we
find that a coasting model of the universe (q_0=0) fits the data about as well
as LCDM. This effect, which is most clearly seen using the recently introduced
`Om' diagnostic, corresponds to an increase of Om(z) and q(z) at redshifts z
\lleq 0.3. In geometrical terms, this suggests that cosmic acceleration may
have already peaked and that we are currently witnessing its slowing down. The
case for evolving DE strengthens if a subsample of the Constitution set
consisting of SNLS+ESSENCE+CfA SN Ia data is analysed in combination with
BAO+CMB using the same statistical methods. The effect we observe could
correspond to DE decaying into dark matter (or something else). A toy model
which mimics this process agrees well with the combined SN Ia+BAO+CMB data.Comment: 6 pages, 5 figures, presentation expanded, results for a new
subsample of the Constitution set are added, new BAO data are accounted for,
main results unchange
Background cosmological dynamics in gravity and observational constraints
In this paper, we carry out a study of viable cosmological models in
-gravity at the background level. We use observable parameters like
and to form autonomous system of equations and show that the
models under consideration exhibit two different regimes in their time
evolution, namely, a phantom phase followed by a quintessence like behavior. We
employ statefinder parameters to emphasize a characteristic discriminative
signature of these models.Comment: 6 pages, Latex style, 9 eps figures, replaced versions with new
references added, Submitted to Phys.Rev.
Two new diagnostics of dark energy
We introduce two new diagnostics of dark energy (DE). The first, Om, is a
combination of the Hubble parameter and the cosmological redshift and provides
a "null test" of dark energy being a cosmological constant. Namely, if the
value of Om(z) is the same at different redshifts, then DE is exactly
cosmological constant. The slope of Om(z) can differentiate between different
models of dark energy even if the value of the matter density is not accurately
known. For DE with an unevolving equation of state, a positive slope of Om(z)
is suggestive of Phantom (w < -1) while a negative slope indicates Quintessence
(w > -1). The second diagnostic, "acceleration probe"(q-probe), is the mean
value of the deceleration parameter over a small redshift range. It can be used
to determine the cosmological redshift at which the universe began to
accelerate, again without reference to the current value of the matter density.
We apply the "Om" and "q-probe" diagnostics to the Union data set of type Ia
supernovae combined with recent data from the cosmic microwave background
(WMAP5) and baryon acoustic oscillations.Comment: 14 pages, 9 figures. Some new results and an additional reference.
Main conclusions unchanged. Matches published versio
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