69 research outputs found
Quinstant Dark Energy Predictions for Structure Formation
We explore the predictions of a class of dark energy models, quinstant dark
energy, concerning the structure formation in the Universe, both in the linear
and non-linear regimes. Quinstant dark energy is considered to be formed by
quintessence and a negative cosmological constant. We conclude that these
models give good predictions for structure formation in the linear regime, but
fail to do so in the non-linear one, for redshifts larger than one.Comment: 9 pages, 14 figures, "Accepted for publication in Astrophysics &
Space Science
Distinguishing among Scalar Field Models of Dark Energy
We show that various scalar field models of dark energy predict degenerate
luminosity distance history of the Universe and thus cannot be distinguished by
supernovae measurements alone. In particular, models with a vanishing
cosmological constant (the value of the potential at its minimum) are
degenerate with models with a positive or negative cosmological constant whose
magnitude can be as large as the critical density. Adding information from CMB
anisotropy measurements does reduce the degeneracy somewhat but not
significantly. Our results indicate that a theoretical prior on the preferred
form of the potential and the field's initial conditions may allow to
quantitatively estimate model parameters from data. Without such a theoretical
prior only limited qualitative information on the form and parameters of the
potential can be extracted even from very accurate data.Comment: 15 pages, 5 figure
Probing Dark Energy with Supernovae : Bias from the time evolution of the equation of state
Observation of thousands of type Ia supernovae should offer the most direct
approach to probe the dark energy content of the universe. This will be
undertaken by future large ground-based surveys followed by a space mission
(SNAP/JDEM). We address the problem of extracting the cosmological parameters
from the future data in a model independent approach, with minimal assumptions
on the prior knowledge of some parameters. We concentrate on the comparison
between a fiducial model and the fitting function and adress in particular the
effect of neglecting (or not) the time evolution of the equation of state. We
present a quantitative analysis of the bias which can be introduced by the
fitting procedure. Such bias cannot be ignored as soon as the statistical
errors from present data are drastically improved.Comment: 22 pages, 10 figures, submitted to Phys. Rev.
Observational Bounds on Cosmic Doomsday
Recently it was found, in a broad class of models, that the dark energy
density may change its sign during the evolution of the universe. This may lead
to a global collapse of the universe within the time t_c ~ 10^{10}-10^{11}
years. Our goal is to find what bounds on the future lifetime of the universe
can be placed by the next generation of cosmological observations. As an
example, we investigate the simplest model of dark energy with a linear
potential V(\phi) =V_0(1+\alpha\phi). This model can describe the present stage
of acceleration of the universe if \alpha is small enough. However, eventually
the field \phi rolls down, V(\phi) becomes negative, and the universe
collapses. The existing observational data indicate that the universe described
by this model will collapse not earlier than t_c > 10 billion years from the
present moment. We show that the data from SNAP and Planck satellites may
extend the bound on the "doomsday" time to t_c > 40 billion years at the 95%
confidence level.Comment: 11 pages, 6 figures, revtex
Reconstruction of the Scalar-Tensor Lagrangian from a LCDM Background and Noether Symmetry
We consider scalar-tensor theories and reconstruct their potential U(\Phi)
and coupling F(\Phi) by demanding a background LCDM cosmology. In particular we
impose a background cosmic history H(z) provided by the usual flat LCDM
parameterization through the radiation (w_{eff}=1/3), matter (w_{eff}=0) and
deSitter (w_{eff}=-1) eras. The cosmological dynamical system which is
constrained to obey the LCDM cosmic history presents five critical points in
each era, one of which corresponding to the standard General Relativity (GR).
In the cases that differ from GR, the reconstructed coupling and potential are
of the form F(\Phi)\sim \Phi^2 and U(\Phi)\sim F(\Phi)^m where m is a constant.
This class of scalar tensor theories is also theoretically motivated by a
completely independent approach: imposing maximal Noether symmetry on the
scalar-tensor Lagrangian. This approach provides independently: i) the form of
the coupling and the potential as F(\Phi)\sim \Phi^2 and U(\Phi)\sim F(\Phi)^m,
ii) a conserved charge related to the potential and the coupling and iii)
allows the derivation of exact solutions by first integrals of motion.Comment: Added comments, discussion, references. 15 revtex pages, 5 fugure
The growth of matter perturbations in some scalar-tensor DE models
We consider asymptotically stable scalar-tensor dark energy (DE) models for
which the equation of state parameter tends to zero in the past. The
viable models are of the phantom type today, however this phantomness is milder
than in General Relativity if we take into account the varying gravitational
constant when dealing with the SNIa data. We study further the growth of matter
perturbations and we find a scaling behaviour on large redshifts which could
provide an important constraint. In particular the growth of matter
perturbations on large redshifts in our scalar-tensor models is close to the
standard behaviour , while it is substantially different
for the best-fit model in General Relativity for the same parametrization of
the background expansion. As for the growth of matter perturbations on small
redshifts, we show that in these models the parameter can take absolute values much larger than in models inside
General Relativity. Assuming a constant when is large
would lead to a poor fit of the growth function . This provides another
characteristic discriminative signature for these models.Comment: 13 pages, 7 figures, matches version published in JCA
Supernova / Acceleration Probe: A Satellite Experiment to Study the Nature of the Dark Energy
The Supernova / Acceleration Probe (SNAP) is a proposed space-based
experiment designed to study the dark energy and alternative explanations of
the acceleration of the Universe's expansion by performing a series of
complementary systematics-controlled measurements. We describe a
self-consistent reference mission design for building a Type Ia supernova
Hubble diagram and for performing a wide-area weak gravitational lensing study.
A 2-m wide-field telescope feeds a focal plane consisting of a 0.7
square-degree imager tiled with equal areas of optical CCDs and near infrared
sensors, and a high-efficiency low-resolution integral field spectrograph. The
SNAP mission will obtain high-signal-to-noise calibrated light-curves and
spectra for several thousand supernovae at redshifts between z=0.1 and 1.7. A
wide-field survey covering one thousand square degrees resolves ~100 galaxies
per square arcminute. If we assume we live in a cosmological-constant-dominated
Universe, the matter density, dark energy density, and flatness of space can
all be measured with SNAP supernova and weak-lensing measurements to a
systematics-limited accuracy of 1%. For a flat universe, the
density-to-pressure ratio of dark energy can be similarly measured to 5% for
the present value w0 and ~0.1 for the time variation w'. The large survey area,
depth, spatial resolution, time-sampling, and nine-band optical to NIR
photometry will support additional independent and/or complementary dark-energy
measurement approaches as well as a broad range of auxiliary science programs.
(Abridged)Comment: 40 pages, 18 figures, submitted to PASP, http://snap.lbl.go
Reduction of Cosmological Data for the Detection of Time-varying Dark Energy Density
We present a method for reducing cosmological data to constraints on the
amplitudes of modes of the dark energy density as a function of redshift. The
modes are chosen so that (1) one of them has constant density and (2) the
others are non-zero only if there is time-variation in the dark energy density
and (3) the amplitude errors for the time-varying modes are uncorrelated with
each other. We apply our method to various combinations of three-year WMAP
data, baryon acoustic oscillation data, the 'Gold' supernova data set, and the
Supernova Legacy Survey data set. We find no significant evidence for a
time-varying dark energy density or for non-zero mean curvature. Although by
some measure the limits on four of the time-varying mode amplitudes are quite
tight, they are consistent with the expectation that the dark energy density
does not vary on timescales shorter than a Hubble time. Since we do not expect
detectable time variation in these modes, our results should be viewed as a
systematic error test which the data have passed. We discuss a procedure to
identify modes with maximal signal-to-noise ratio.Comment: 13 pages, 12 figures; Version accepted for publication by JCAP;
Updated with three-year WMAP data; added discussion on systematic error
detectio
Constraining the dark energy dynamics with the cosmic microwave background bispectrum
We consider the influence of the dark energy dynamics at the onset of cosmic
acceleration on the Cosmic Microwave Background (CMB) bispectrum, through the
weak lensing effect induced by structure formation. We study the line of sight
behavior of the contribution to the bispectrum signal at a given angular
multipole : we show that it is non-zero in a narrow interval centered at a
redshift satisfying the relation , where the
wavenumber corresponds to the scale entering the non-linear phase, and is
the cosmological comoving distance. The relevant redshift interval is in the
range 0.1\lsim z\lsim 2 for multipoles 1000\gsim\ell\gsim 100; the signal
amplitude, reflecting the perturbation dynamics, is a function of the
cosmological expansion rate at those epochs, probing the dark energy equation
of state redshift dependence independently on its present value. We provide a
worked example by considering tracking inverse power law and SUGRA Quintessence
scenarios, having sensibly different redshift dynamics and respecting all the
present observational constraints. For scenarios having the same present
equation of state, we find that the effect described above induces a projection
feature which makes the bispectra shifted by several tens of multipoles, about
10 times more than the corresponding effect on the ordinary CMB angular power
spectrum.Comment: 15 pages, 7 figures, matching version accepted by Physical Review D,
one figure improve
Recommended from our members
The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: cosmological implications of the large-scale two-point correlation function
We obtain constraints on cosmological parameters from the spherically
averaged redshift-space correlation function of the CMASS Data Release 9 (DR9)
sample of the Baryonic Oscillation Spectroscopic Survey (BOSS). We combine this
information with additional data from recent CMB, SN and BAO measurements. Our
results show no significant evidence of deviations from the standard
flat-Lambda CDM model, whose basic parameters can be specified by Omega_m =
0.285 +- 0.009, 100 Omega_b = 4.59 +- 0.09, n_s = 0.96 +- 0.009, H_0 = 69.4 +-
0.8 km/s/Mpc and sigma_8 = 0.80 +- 0.02. The CMB+CMASS combination sets tight
constraints on the curvature of the Universe, with Omega_k = -0.0043 +- 0.0049,
and the tensor-to-scalar amplitude ratio, for which we find r < 0.16 at the 95
per cent confidence level (CL). These data show a clear signature of a
deviation from scale-invariance also in the presence of tensor modes, with n_s
<1 at the 99.7 per cent CL. We derive constraints on the fraction of massive
neutrinos of f_nu < 0.049 (95 per cent CL), implying a limit of sum m_nu < 0.51
eV. We find no signature of a deviation from a cosmological constant from the
combination of all datasets, with a constraint of w_DE = -1.033 +- 0.073 when
this parameter is assumed time-independent, and no evidence of a departure from
this value when it is allowed to evolve as w_DE(a) = w_0 + w_a (1 - a). The
achieved accuracy on our cosmological constraints is a clear demonstration of
the constraining power of current cosmological observations.Comment: 26 pages, 15 figures. Minor changes to match version accepted by
MNRA
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