The case for dynamical dark energy revisited

Journal of Cosmology and Astroparticle Physics, 2007

We reconstruct dark energy properties from two complementary supernova datasets-the newly released Gold+HST sample and SNLS. The results obtained are consistent with standard ΛCDM model within 2σ error bars although the Gold+HST data favour evolving dark energy slightly more than SNLS. Using complementary data from baryon acoustic oscillations and the cosmic microwave background to constrain dark energy, we find that our results in this case are strongly dependent on the present value of the matter density Ω0m. Consequently, no firm conclusions regarding constancy or variability of dark energy density can be drawn from these data alone unless the value of Ω0m is known to an accuracy of a few percent. However, possible variability is significantly restricted if this data is used in conjunction with supernova data.

Physical Review D, 2005

We consider how well the dark energy equation of state w as a function of red shift z will be measured using current and anticipated experiments. We use a procedure which takes fair account of the uncertainties in the functional dependence of w on z, as well as the parameter degeneracies, and avoids the use of strong prior constraints. We apply the procedure to current data from WMAP, SDSS, and the supernova searches, and obtain results that are consistent with other analyses using different combinations of data sets. The effects of systematic experimental errors and variations in the analysis technique are discussed. Next, we use the same procedure to forecast the dark energy constraints achieveable by the end of the decade, assuming 8 years of WMAP data and realistic projections for ground-based measurements of supernovae and weak lensing. We find the 2σ constraints on the current value of w to be ∆w0(2σ) = 0.20, and on dw/dz (between z = 0 and z = 1) to be ∆w1(2σ) = 0.37. Finally, we compare these limits to other projections in the literature. Most show only a modest improvement; others show a more substantial improvement, but there are serious concerns about systematics. The remaining uncertainty still allows a significant span of competing dark energy models. Most likely, new kinds of measurements, or experiments more sophisticated than those currently planned, are needed to reveal the true nature of dark energy. PACS numbers: 98.80.Es,98.65.Dx,98.62.Sb z 0 dz ′ /H(z ′ ) depends on Ω m and the dark energy

2006

A model-independent method to study the possible evolution of dark energy is presented. Optimal estimates of the dark energy equation of state w are obtained from current supernovae data from [A. G. Riess et al., Astrophys. J. 607, 665 (2004).] following a principal components approach. We assess the impact of varying the number of piecewise constant w estimates Np using a model selection method, the Bayesian information criterion, and compare the most favored models with some parametrizations commonly used in the literature. Although data seem to prefer a cosmological constant, some models are only moderately disfavored by our selection criterion: a constant w, w ∝ a, w ∝ z and the two-parameter models introduced here. Among these, the models we find by optimization are slightly preferred. However, current data do not allow us to draw a conclusion on the possible evolution of dark energy. Interestingly, the best fits for all varying-w models exhibit a w < −1 at low redshifts.

Astrophys Space Sci 369, 17, 2024

Measurements of the current expansion rate of the Universe, H0, using standard candles, disagree with those derived from observations of the Cosmic Microwave Background (CMB). This discrepancy, known as the Hubble tension, is substantial and suggests the possibility of revisions to the standard cosmological model (Cosmological constant Λ and cold dark matter-ΛCDM). Dynamic dark energy (DE) models that introduce deviations in the expansion history relative to ΛCDM could potentially explain this tension. We used Type Ia supernovae (SNe) data to test a dynamic DE model consisting of an equation of state that varies linearly with the cosmological scale factor a. To evaluate this model, we developed a new statistic (the Tα statistic) used in conjunction with an optimization code that minimizes its value to obtain model parameters. The Tα statistic reduces bias errors (in comparison to the χ 2 statistic) because it retains the sign of the residuals, which is meaningful in testing the dynamic DE model as the deviations in the expansion history introduced by this model act asymmetrically in redshift space. The DE model fits the SNe data reasonably well, but the available SNe data lacks the statistical power to discriminate between ΛCDM and alternative models. To further assess the model using CMB data, we computed the distance to the last scattering surface and compared the results with that derived from the Planck observations. Although the simple dynamic DE model tested does not completely resolve the tension, it is not ruled out by the data and could still play a role alongside other physical effects.

2020

Abstract. We apply a parameterization-independent approach to fitting the dark energy equation-of-state (EOS). Utilizing the latest type Ia supernova data, combined with results from the cosmic microwave background and baryon acoustic oscillations, we find that the dark energy is consistent with a cosmological constant. We establish independent estimates of the evolution of the dark energy EOS by diagonalizing the covariance matrix. We find three independent constraints, which taken together imply that the equation of state is more negative than -0.2 at the 68% confidence level in the redshift range 0 &lt; z &lt; 1.8, independent of the flat universe assumption. Our estimates argue against previous claims of dark energy &quot;metamorphosis,&quot; where the EOS was found to be strongly varying at low redshifts. Our results are inconsistent with extreme models of dynamical dark energy, both in the form of &quot;freezing&quot; models where the dark energy EOS begins with a value greate...

2013

Cosmological distances inferred from supernova Ia observations constitute the most direct and solid evidence for the recently detected accelerated expansion of the universe. In this contribution, we show some inconsistencies between two of the main light-curve fitters used for the elaboration of supernova Ia data sets, opening new observational challenges regarding the use of these luminosity distances when combined with CMB and BAO data. We also mention ongoing analysis related to alternative models. The resolution of these challenges will be crucial for XXI century cosmology. Resumen. Las distancias cosmológicas inferidas a partir de observaciones de supernovas del tipo Ia constituyen la evidencia más directa y sólida de la recientemente detectada expansión acelerada del universo. En esta contribución, se muestran algunas inconsistencias entre las dos maneras más usadas para procesar las curvas de luz de las supernovas Ia, abriendo nuevos desafíos observacionales referidos al uso de estas distancias luminosas cuando son combinadas con datos del CMB y BAO. También se mencionan análisis en curso, relacionados con modelos alternativos. La resolución de estos desafíos será crucial para la cosmología del siglo XXI.

International Journal of Modern Physics D, 2006

In this paper we review in detail a number of approaches that have been adopted to try and explain the remarkable observation of our accelerating Universe. In particular we discuss the arguments for and recent progress made towards understanding the nature of dark energy. We review the observational evidence for the current accelerated expansion of the universe and present a number of dark energy models in addition to the conventional cosmological constant, paying particular attention to scalar field models such as quintessence, K-essence, tachyon, phantom and dilatonic models. The importance of cosmological scaling solutions is emphasized when studying the dynamical system of scalar fields including coupled dark energy. We study the evolution of cosmological perturbations allowing us to confront them with the observation of the Cosmic Microwave Background and Large Scale Structure and demonstrate how it is possible in principle to reconstruct the equation of state of dark energy by also using Supernovae Ia observational data. We also discuss in detail the nature of tracking solutions in cosmology, particle physics and braneworld models of dark energy, the nature of possible future singularities, the effect of higher order curvature terms to avoid a Big Rip singularity, and approaches to modifying gravity which leads to a late-time accelerated expansion without recourse to a new form of dark energy.

Physical Review D, 2006

We compute the Bayesian evidences for one-and two-parameter models of evolving dark energy, and compare them to the evidence for a cosmological constant, using current data from Type Ia supernova, baryon acoustic oscillations, and the cosmic microwave background. We use only distance information, ignoring dark energy perturbations. We find that, under various priors on the dark energy parameters, ΛCDM is currently favoured as compared to the dark energy models. We consider the parameter constraints that arise under Bayesian model averaging, and discuss the implication of our results for future dark energy projects seeking to detect dark energy evolution. The model selection approach complements and extends the figure-of-merit approach of the Dark Energy Task Force in assessing future experiments, and suggests a significantly-modified interpretation of that statistic.

2005

We constrain the evolution of the dark energy density Ω d from Cosmic Microwave Background, Large Scale Structure, and Supernovae Ia measurements. While Supernovae Ia are most sensitive to the equation of state w0 of dark energy today, the Cosmic Microwave Background and Large Scale Structure data best constrains the dark energy evolution at earlier times. For the parametrization used in our models, we find w0 < −0.8 and the dark energy fraction at very high redshift Ω e d < 0.03 at 95 per cent confidence level.

Physics Letters B, 2008

Determining the mechanism behind the current cosmic acceleration constitutes a major question nowadays in theoretical physics. If the dark energy route is taken, this problem may potentially bring to light new insights not only in Cosmology but also in high energy physics theories. Following this approach, we explore in this paper some cosmological consequences of a new time-dependent parameterization for the dark energy equation of state (EoS), which is a well behaved function of the redshift z over the entire cosmological evolution, i.e., z ∈ [−1, ∞). This parameterization allows us to divide the parametric plane (w0, w1) in defined regions associated to distinct classes of dark energy models that can be confirmed or excluded from a confrontation with current observational data. A statistical analysis involving the most recent observations from type Ia supernovae, baryon acoustic oscillation peak, Cosmic Microwave Background shift parameter and Hubble evolution H(z) is performed to check the observational viability of the EoS parameterization here proposed.