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Register a new userModel-independent cosmic acceleration and type Ia supernovae intrinsic luminosity redshift dependence
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The $Lambda$CDM model is the current standard model in cosmology thanks to its ability to reproduce the observations. Its first observational evidence appeared from the type Ia supernovae (SNIa) Hubble diagram. However, there has been some debate in the literature concerning the statistical treatment of SNIa. In this paper we relax the standard assumption that SNIa intrinsic luminosity is independent of the redshift, and we examine whether it may have an impact on the accelerated nature of the expansion of the Universe. In order to be as general as possible, we reconstruct the expansion rate of the Universe through a cubic spline interpolation fitting observations of different probes: SNIa, baryon acoustic oscillations (BAO), and the high-redshift information from the cosmic microwave background (CMB). We show that when SNIa intrinsic luminosity is not allowed to vary as a function of the redshift, cosmic acceleration is definitely proven in a model-independent approach. However, allowing for a redshift dependence, a non-accelerated reconstruction of the expansion rate is able to fit, as well as $Lambda$CDM, the combination of SNIa and BAO data, both treating the BAO standard ruler $r_d$ as a free parameter, or adding the recently published prior from CMB observations. We further extend the analysis by including the CMB data, and we show that a non-accelerated reconstruction is able to nicely fit this combination of low and high-redshift data. In this work we present a model-independent reconstruction of a non-accelerated expansion rate of the Universe that is able to nicely fit all the main background cosmological probes. However, the predicted value of $H_0$ is in tension with recent direct measurements. Our analysis points out that a final, reliable, and consensual value for $H_0$ would be critical to definitively prove the cosmic acceleration in a model-independent way. [Abridged]
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The standard model of cosmology is founded on the basis that the expansion rate of the universe is accelerating at present --- as was inferred originally from the Hubble diagram of Type Ia supernovae. There exists now a much bigger database of supernovae so we can perform rigorous statistical tests to check whether these standardisable candles indeed indicate cosmic acceleration. Taking account of the empirical procedure by which corrections are made to their absolute magnitudes to allow for the varying shape of the light curve and extinction by dust, we find, rather surprisingly, that the data are still quite consistent with a constant rate of expansion.
With the recent increase in precision of our cosmological datasets, measurements of $Lambda$CDM model parameter provided by high- and low-redshift observations started to be in tension, i.e., the obtained values of such parameters were shown to be significantly different in a statistical sense. In~this work we tackle the tension on the value of the Hubble parameter, $H_0$, and the weighted amplitude of matter fluctuations, $S_8$, obtained from local or low-redshift measurements and from cosmic microwave background (CMB) observations. We combine the main approaches previously used in the literature by extending the cosmological model and accounting for extra systematic uncertainties. With such analysis we aim at exploring non standard cosmological models, implying deviation from a cosmological constant driven acceleration of the Universe expansion, in the presence of additional uncertainties in measurements. In more detail, we reconstruct the Dark Energy equation of state as a function of redshift, while we study the impact of type-Ia supernovae (SNIa) redshift-dependent astrophysical systematic effects on these tensions. We consider a SNIa intrinsic luminosity dependence on redshift due to the star formation rate in its environment, or the metallicity of the progenitor. We find that the $H_0$ and $S_8$ tensions can be significantly alleviated, or~even removed, if we account for varying Dark Energy for SNIa and CMB data. However, the tensions remain when we add baryon acoustic oscillations (BAO) data into the analysis, even after the addition of extra SNIa systematic uncertainties. This points towards the need of either new physics beyond late-time Dark Energy, or other unaccounted systematic effects (particulary in BAO measurements), to fully solve the present tensions.
The first observational evidence for the cosmic acceleration appeared from the type Ia supernovae (SNe Ia) Hubble diagram from two different groups. However, the empirical treatment of SNe Ia and their ability to show cosmic acceleration have been the subject of some debate in the literature. In this work we probe the assumption of redshift-independent absolute magnitude $(M_{mathrm{B}})$ of SNe along with its correlation with spatial curvature ($Omega_{k0}$) and cosmic distance duality relation (CDDR) parameter ($eta(z)$). This work is divided into two parts. Firstly, we check the validity of CDDR which relates the luminosity distance ($d_L$) and angular diameter distance ($d_A$) via redshift. We use three different redshift-dependent parametrizations of the distance duality parameter $(eta(z))$. CDDR is fairly consistent for almost every parametrization within $95%$ confidence level in both flat and non-flat universe. However, one of the parametrizations does not validate CDDR in the case of a non-flat universe. In second part, we take the validity of CDDR for granted and emphasise on the variability of $M_{mathrm{B}}$ and its correlation with $Omega_{k0}$. We choose three different redshift-dependent parametrizations of $M_{mathrm{B}}$. The results indicate no evolution of $M_{mathrm{B}}$ at $99%$ confidence level but clearly indicates the inclination towards a non-flat open universe. We further extend our analysis and examine the dependence of the results on the choice of different priors for $H_0$.
In this study we present constraints on the deceleration (q) and jerk (j) parameters using the late time integrated Sachs-Wolfe effect, type Ia supernovae, and H(z) data . We first directly measure the deceleration and jerk parameters using the cosmic chronometers data with the Taylor series expression of H(z).However, due to the unusual variations in the deceleration parameter with slight changes in other parameters like snap (s) and lerk (l), we found that direct measurements using the series expression of the H(z) is not a suitable method for non-Lambda-CDM models and so we will need to derive the deceleration parameter after constraining density parameters and dark energy equation of state parameters. Then we present derived values of the deceleration parameter from Lambda CDM, WCDM and CPL models. We also discuss the transition redshift (zt) in relation with the deceleration parameter.
We have assembled a dataset of 165 low redshift, $z<$0.06, publicly available type Ia supernovae (SNe Ia). We produce maximum light magnitude ($M_{B}$ and $M_{V}$) distributions of SNe Ia to explore the diversity of parameter space that they can fill. Before correction for host galaxy extinction we find that the mean $M_{B}$ and $M_{V}$ of SNe Ia are $-18.58pm0.07$mag and $-18.72pm0.05$mag respectively. Host galaxy extinction is corrected using a new method based on the SN spectrum. After correction, the mean values of $M_{B}$ and $M_{V}$ of SNe Ia are $-19.10pm0.06$ and $-19.10pm0.05$mag respectively. After correction for host galaxy extinction, `normal SNeIa ($Delta m_{15}(B)<1.6$mag) fill a larger parameter space in the Width-Luminosity Relation (WLR) than previously suggested, and there is evidence for luminous SNe Ia with large $Delta m_{15}(B)$. We find a bimodal distribution in $Delta m_{15}(B)$, with a pronounced lack of transitional events at $Delta m_{15}(B)$=1.6 mag. We confirm that faster, low-luminosity SNe tend to come from passive galaxies. Dividing the sample by host galaxy type, SNe Ia from star-forming (S-F) galaxies have a mean $M_{B}=-19.20 pm 0.05$ mag, while SNe Ia from passive galaxies have a mean $M_{B}=-18.57 pm 0.24$ mag. Even excluding fast declining SNe, `normal ($M_{B}<-18$ mag) SNe Ia from S-F and passive galaxies are distinct. In the $V$-band, there is a difference of 0.4$ pm $0.13 mag between the median ($M_{V}$) values of the `normal SN Ia population from passive and S-F galaxies. This is consistent with ($sim 15 pm $10)% of `normal SNe Ia from S-F galaxies coming from an old stellar population.
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