Recent Type Ia Supernovae Data Research Articles

Friedmann cosmology with decaying vacuum density in Brans\u2013Dicke theory

In this paper, we study Friedmann cosmology with time-varying vacuum energy density in the context of Brans–Dicke theory. We consider an isotropic and homogeneous flat space, filled with a matter-dominated perfect fluid and a dynamical cosmological term varLambda (t) , obeying the equation of state of the vacuum. As the exact nature of a possible time-varying vacuum is yet to be found, we explore varLambda (t) given by the phenomenological law varLambda (t)=lambda +sigma H, where lambda and sigma are positive constants. We solve the model and then focus on two different cases varLambda _{H1} and varLambda _{H2} by assuming varLambda =lambda and varLambda =sigma H, respectively. Notice that varLambda _{H1} is the analog of the standard varLambda CDM, but within the Brans–Dicke cosmology. We find the analytical solution of the main cosmological functions such as the Hubble parameter, the scale factor, deceleration and equation of state parameters for these models. In order to test the viability of the cosmological scenarios, we perform two sets of joint observational analyses of the recent Type Ia supernova data (Pantheon), observational measurements of Hubble parameter data, Baryon acoustic oscillation/Cosmic microwave background data and Local Hubble constant for each model. For the sake of comparison, the same data analysis is performed for the varLambda CDM model. Each model shows a transition from decelerated phase to accelerated phase and can be viewed as an effective quintessence behavior. Using the model selection criteria AIC and BIC to distinguish from existing dark energy models, we find that the Brans–Dicke analog of the varLambda -cosmology (i.e. our model varLambda _{H1}) performs at a level comparable to the standard varLambda CDM, whereas varLambda _{H2} is less favoured.

Cosmology using long gamma-ray bursts: statistical analysis of errors in calibrated data

We investigate direction dependence and non-Gaussian features in high-z cosmological data using Δχ2 and Δχ statistics and the Kolmogorov–Smirnov test. These techniques are applied on a set of calibrated long gamma-ray bursts (GRBs) and its combination with recent Type Ia supernovae data (Union2). Our statistical analysis shows a weak but consistent direction dependence in both the data sets. The analysis also indicates a non-Gaussian nature of errors in both data sets.

Gauging the cosmic acceleration with recent type Ia supernovae data sets

We revisit a model-independent estimator for cosmic acceleration based on type Ia supernovae distance measurements. This approach does not rely on any specific theory for gravity, energy content or parameterization for the scale factor or deceleration parameter and is based on falsifying the {\it null hypothesis} that the Universe never expanded in an accelerated way. By generating mock catalogues of known cosmologies we test the robustness of this estimator establishing its limits of applicability. We detail the pros and cons of such approach. For example, we find that there are specific counterexamples in which the estimator wrongly provides evidence against acceleration in accelerating cosmologies. The dependence of the estimator on the $H_0$ value is also discussed. Finally, we update the evidence for acceleration using the recent UNION2.1 and JLA samples. Contrary to recent claims, available data strongly favors an accelerated expansion of the Universe in complete agreement with the standard $\Lambda$CDM model.

Analysis of recent type Ia supernova data based on evolving dark energy models

We study characters of recent type Ia supernova (SNIa) data using evolving dark energy models with changing equation of state parameter w. We consider sudden-jump approximation of w for some chosen redshift spans with double transitions, and constrain these models based on Markov Chain Monte Carlo (MCMC) method using the SNIa data (Constitution, Union, Union2) together with baryon acoustic oscillation A parameter and cosmic microwave background shift parameter in a flat background. In the double-transition model the Constitution data shows deviation outside 1 sigma from LCDM model at low (z < 0.2) and middle (0.2 < z < 0.4) redshift bins whereas no such deviations are noticeable in the Union and Union2 data. By analyzing the Union members in the Constitution set, however, we show that the same difference is actually due to different calibration of the same Union sample in the Constitution set, and is not due to new data added in the Constitution set. All detected deviations are within 2 sigma from the LCDM world model. From the LCDM mock data analysis, we quantify biases in the dark energy equation of state parameters induced by insufficient data with inhomogeneous distribution of data points in the redshift space and distance modulus errors. We demonstrate that location of peak in the distribution of arithmetic means (computed from the MCMC chain for each mock data) behaves as an unbiased estimator for the average bias, which is valid even for non-symmetric likelihood distributions.

Inhomogeneous Structure Formation May Alleviate Need for Accelerating Universe~!2010-04-21~!2010-05-19~!2010-08-13~!

When taking the real, inhomogeneous and anisotropic matter distribution in the semi-local universe into account, there may be no need to postulate an accelerating expansion of the universe despite recent type Ia supernova data. Local curvatures must be integrated (over all space) to obtain the global curvature of the universe, which seems to be very close to zero from cosmic microwave background data. As gravitational structure formation creates bound regions of positive curvature, the regions in between become negatively curved in order to comply with a vanishing global curvature. The actual dynamics of the universe is altered due to the self-induced inhomogeneities, again more prominently so as structure formation progresses. Furthermore, this negative curvature will increase as a function of time as structure formation proceeds, which mimics the effect of "dark energy" with negative pressure. Hence, the "acceleration" may be merely a mirage. We make a qualitative and semi-quantitative analysis, using newtonian gravity corrected for special relativistic effects, which works surprisingly well, to corroborate and illustrate/visualize these statements. This article may be seen as a plea to start taking seriously the observed inhomogeneous distribution and the nonlinearities of nonperturbative general relativity, and their impact on the dynamics and behavior of the cosmos.

Crossingw=−1by a single scalar on a Dvali-Gabadadze-Porrati brane

Recent type Ia supernovae data seem to favor a dark energy model whose equation of state $w(z)$ crosses $\ensuremath{-}1$, which is a much more amazing problem than the acceleration of the universe. Either the case that $w(z)$ evolves from above $\ensuremath{-}1$ to below $\ensuremath{-}1$ or the case that $w(z)$ runs from below $\ensuremath{-}1$ to above $\ensuremath{-}1$, is consistent with present data. In this paper we show that it is possible to realize the crossing behaviors of both of the two cases by only a single scalar field in frame of Dvali-Gabadadze-Porrati braneworld. At the same time we prove that there does not exist scaling solution in a universe with dust.

Supernova constraints on decaying vacuum cosmology

There is mounting observational evidence that the expansion of our Universe is undergoing a late-time acceleration. Among many proposals to describe this phenomenon, the cosmological constant ($\ensuremath{\Lambda}$) seems to be the simplest and the most natural explanation. However, despite its observational successes, such a possibility exacerbates the well known $\ensuremath{\Lambda}$ problem, requiring a natural explanation for its small, but nonzero, value. In this paper we consider a cosmological scenario driven by a varying cosmological term, in which the vacuum energy density decays linearly with the Hubble parameter, $\ensuremath{\Lambda}\ensuremath{\propto}H$. We show that this $\ensuremath{\Lambda}(t)\mathrm{CDM}$ model is indistinguishable from the standard one ($\ensuremath{\Lambda}\mathrm{CDM}$) in that the early radiation phase is followed by a long dust-dominated era, and only recently the varying $\ensuremath{\Lambda}$ term becomes dominant, accelerating the cosmic expansion. In order to test the viability of this scenario, we have used the most recent type Ia supernova data, i.e., the High-Z SN Search Team and the Supernova Legacy Survey (SNLS) Collaboration data. In particular, for the SNLS sample we have found $0.27\ensuremath{\le}{\ensuremath{\Omega}}_{\mathrm{m}}\ensuremath{\le}0.37$ and $0.68\ensuremath{\le}{H}_{0}\ensuremath{\le}0.72$ (at $2\ensuremath{\sigma}$), which is in good agreement with the currently accepted estimates for these parameters.

CONSTRAINTS ON A NEW ALTERNATIVE MODEL TO DARK ENERGY

The recent type Ia supernova data suggest that the Universe is accelerating now and had decelerated in recent past. This may provide the evidence that the standard Friedmann equation needs to be modified. We analyze in detail a new model in the context of modified Friedmann equation using the supernova data published by the High-z Supernova Search Team and the Supernova Cosmology Project. The new model explains recent acceleration and past deceleration. Furthermore, the acceleration of the expansion of the Universe is almost zero in the future.

Extended Born-Infeld dynamics and cosmology

We introduce an extension of the Born-Infeld action for a scalar field and show that it can act as unifying dark matter, providing an explanation for both structure formation and the accelerated expansion of the universe. We investigate the cosmological dynamics of this theory in a particular case, referred to as the ``Milne-Born-Infeld'' (MBI) Lagrangian. We show that this model, whose equation of state has effectively a single free parameter, is consistent with recent type Ia supernovae data, providing a fit as good as for the $\ensuremath{\Lambda}\mathrm{C}\mathrm{D}\mathrm{M}$ model with the same number of degrees of freedom. Furthermore, this parameter is tightly constrained by current data, making the model easily testable with other observables. Contrary to previous candidates for unifying dark matter, the sound velocity of the MBI model is vanishing both close to the dark-matter state as well as near the cosmological constant state. This could avoid the problems on the matter power spectrum that were present in previous adiabatic dark-matter/dark-energy unification models. We also present a short discussion on the causal propagation in nonlinear scalar field theories such as the one proposed here.

Latest supernova data in the framework of the generalized Chaplygin gas model

We use the most recent Type Ia supernova data in order to study the dark energy–dark matter (XCDM) unification approach in the context of the generalized Chaplygin gas (GCG) model. Rather surprisingly, we find that data allow models with α > 1. We have studied how the GCG adjusts flat and non-flat models, and our results show that GCG is consistent with a flat case up to 68 per cent confidence level. Actually this holds even if one relaxes the flat prior assumption. We have also analysed what one should expect from a future experiment such as the Supernova/Acceleration Probe (SNAP). We find that there is a degeneracy between the GCG model and an XCDM model with a phantom-like dark energy component.