397 research outputs found
The Self-Calibrating Hubble Diagram
As an increasing number of well measured type Ia supernovae (SNe Ia) become
available, the statistical uncertainty on w has been reduced to the same size
as the systematic uncertainty. The statistical error will decrease further in
the near future, and hence the improvement of systematic uncertainties needs to
be addressed, if further progress is to be made. We study how uncertainties in
the primary reference spectrum - which are a main contribution to the
systematic uncertainty budget - affect the measurement of the Dark Energy
equation of state parameter w from SNe Ia. The increasing number of SN
observations can be used to reduce the uncertainties by including perturbations
of the reference spectrum as nuisance parameters in a cosmology fit, thus
"self-calibrating" the Hubble diagram.
We employ this method to real SNe data for the first time and find the
perturbations of the reference spectrum consistent with zero at the 1%-level.
For future surveys we estimate that ~3500 SNe will be required for our method
to outperform the standard method of deriving the cosmological parameters.Comment: 17 pages, 8 figures, 1 table. Update to revised version accepted for
publication in JCA
Testing homogeneity with galaxy number counts : light-cone metric and general low-redshift expansion for a central observer in a matter dominated isotropic universe without cosmological constant
As an alternative to dark energy it has been suggested that we may be at the
center of an inhomogeneous isotropic universe described by a
Lemaitre-Tolman-Bondi (LTB) solution of Einstein's field equations. In order to
test this hypothesis we calculate the general analytical formula to fifth order
for the redshift spherical shell mass. Using the same analytical method we
write the metric in the light-cone by introducing a gauge invariant quantity
which together with the luminosity distance completely
determine the light-cone geometry of a LTB model.Comment: 13 page
Can the cosmological constant be mimicked by smooth large-scale inhomogeneities for more than one observable?
As an alternative to dark energy it has been suggested that we may be at the
center of an inhomogeneous isotropic universe described by a
Lemaitre-Tolman-Bondi (LTB) solution of Einstein's field equations. In order to
test such an hypothesis we calculate the low redshift expansion of the
luminosity distance and the redshift spherical shell mass density
for a central observer in a LTB space without cosmological constant and
show how they cannot fit the observations implied by a model if
the conditions to avoid a weak central singularity are imposed, i.e. if the
matter distribution is smooth everywhere. Our conclusions are valid for any
value of the cosmological constant, not only for as
implied by previous proofs that has to be positive in a smooth LTB
space, based on considering only the luminosity distance.
The observational signatures of smooth LTB matter dominated models are
fundamentally different from the ones of models not only because
it is not possible to reproduce a negative apparent central deceleration
, but because of deeper differences in their space-time geometry
which make impossible the inversion problem when more than one observable is
considered, and emerge at any redshift, not only for .Comment: 18 pages, corrected a typo in the definition of the energy density
which doesn't change the conclusion, references adde
Probing the cosmic acceleration from combinations of different data sets
We examine in some detail the influence of the systematics in different data
sets including type Ia supernova sample, baryon acoustic oscillation data and
the cosmic microwave background information on the fitting results of the
Chevallier-Polarski-Linder parametrization. We find that the systematics in the
data sets does influence the fitting results and leads to different evolutional
behavior of dark energy. To check the versatility of Chevallier-Polarski-Linder
parametrization, we also perform the analysis on the Wetterich parametrization
of dark energy. The results show that both the parametrization of dark energy
and the systematics in data sets influence the evolutional behavior of dark
energy.Comment: 15 pages, 5 figures and 1 table, major revision, delete bao a data,
main results unchanged. jcap in press
Observational constraints on finite scale factor singularities
We discuss the combined constraints on a Finite Scale Factor Singularity
(FSF) universe evolution scenario, which come from the shift parameter R,
baryon acoustic oscillations (BAO) A, and from the type Ia supernovae. We show
that observations allow existence of such singularities in the 2x10^9 years, in
future, at the 1{\sigma} CL, and that at the present moment of the cosmic
evolution, one cannot differentiate between cosmological scenario which allow
finite scale factor singularities and the standard dark energy models. We also
show that there is an allowed value of m = 2/3 within 1{\sigma} CL, which
corresponds to a dust-filled Einstein-de-Sitter universe limit of the early
time evolution.Comment: 6 pages, some misprints correcte
Late-time acceleration in Higher Dimensional Cosmology
We investigate late time acceleration of the universe in higher dimensional
cosmology. The content in the universe is assumed to exert pressure which is
different in the normal and extra dimensions. Cosmologically viable solutions
are found to exist for simple forms of the equation of state. The parameters of
the model are fixed by comparing the predictions with supernovae data. While
observations stipulate that the matter exerts almost vanishing pressure in the
normal dimensions, we assume that, in the extra dimensions, the equation of
state is of the form . For appropriate choice
of parameters, a late time acceleration in the universe occurs with and
being approximately -0.46 and 0.76 respectively.Comment: 10 pages, 5 figure
Regularizing cosmological singularities by varying physical constants
Varying physical constant cosmologies were claimed to solve standard
cosmological problems such as the horizon, the flatness and the
-problem. In this paper, we suggest yet another possible application
of these theories: solving the singularity problem. By specifying some examples
we show that various cosmological singularities may be regularized provided the
physical constants evolve in time in an appropriate way.Comment: 9 pages, 6 figures, Revtex4-1, an improved version to appear in JCA
A minimal set of invariants as a systematic approach to higher order gravity models: Physical and Cosmological Constraints
We compare higher order gravity models to observational constraints from
magnitude-redshift supernova data, distance to the last scattering surface of
the CMB, and Baryon Acoustic Oscillations. We follow a recently proposed
systematic approach to higher order gravity models based on minimal sets of
curvature invariants, and select models that pass some physical acceptability
conditions (free of ghost instabilities, real and positive propagation speeds,
and free of separatrices). Models that satisfy these physical and observational
constraints are found in this analysis and do provide fits to the data that are
very close to those of the LCDM concordance model. However, we find that the
limitation of the models considered here comes from the presence of
superluminal mode propagations for the constrained parameter space of the
models.Comment: 12 pages, 6 figure
Investigating dark energy experiments with principal components
We use a principal component approach to contrast different kinds of probes
of dark energy, and to emphasize how an array of probes can work together to
constrain an arbitrary equation of state history w(z). We pay particular
attention to the role of the priors in assessing the information content of
experiments and propose using an explicit prior on the degree of smoothness of
w(z) that is independent of the binning scheme. We also show how a figure of
merit based on the mean squared error probes the number of new modes
constrained by a data set, and use it to examine how informative various
experiments will be in constraining the evolution of dark energy.Comment: A significantly expanded version with an added PCA for weak lensing,
a new detailed discussion of the correlation prior proposed in this work, and
a new discussion outlining the differences between the Bayesian and the
frequentist approaches to reconstructing w(z). Matches the version accepted
to JCAP. 8 pages, 2 figure
Bayesian Analysis and Constraints on Kinematic Models from Union SNIa
The kinematic expansion history of the universe is investigated by using the
307 supernovae type Ia from the Union Compilation set. Three simple model
parameterizations for the deceleration parameter (constant, linear and abrupt
transition) and two different models that are explicitly parametrized by the
cosmic jerk parameter (constant and variable) are considered. Likelihood and
Bayesian analyses are employed to find best fit parameters and compare models
among themselves and with the flat CDM model. Analytical expressions
and estimates for the deceleration and cosmic jerk parameters today ( and
) and for the transition redshift () between a past phase of cosmic
deceleration to a current phase of acceleration are given. All models
characterize an accelerated expansion for the universe today and largely
indicate that it was decelerating in the past, having a transition redshift
around 0.5. The cosmic jerk is not strongly constrained by the present
supernovae data. For the most realistic kinematic models the
confidence limits imply the following ranges of values: ,
and , which are compatible with the
CDM predictions, , and .
We find that even very simple kinematic models are equally good to describe the
data compared to the concordance CDM model, and that the current
observations are not powerful enough to discriminate among all of them.Comment: 13 pages. Matches published versio
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