Union2 Data Research Articles

Exact solutions in teleparallel dark energy model

One of the most important challenges facing modern cosmology is to explain the phenomenon of dark energy. Many models have been developed to describe the nature of dark energy in the literature. The scalar field model in the framework of teleparallel gravity is one of the most interesting ones. We investigate the teleparallel dark energy model via the anharmonic oscillator method. We obtain the exact cosmological solutions of the teleparallel dark energy model for the different potentials, such as the general power-law and Higgs potentials. Using Type Ia Supernovae (SNIa) and Pantheon data, we examine the parameter space of the model and determine the allowed regions for the proposed potentials. We study the power-law potential utilizing the χ2 method and Union2 data. Also, we analyze the Higgs potential by applying the MCMC method according to Pantheon data. Although both models are in agreement with the observational data, it can be realized that the Higgs potential can better support the data by comparing the AIC and BIC criteria.

Friedmann equations with the generalized logarithmic modification of Barrow entropy and Tsallis entropy

Barrow, inspired by the Covid-19 characteristics and its effect on human cells, explained that quantum gravitational effects might create complex and fractal properties on the black hole horizon. In this perspective, the entropy of a black hole no longer follows the law of area, but can instead be represented by exponential raising of the area Δ, where 0≤Δ≤1, and it indicates the quantum gravitational deformation effects. In this paper, starting from the first law of thermodynamics dE=ThdSh+WdV, at apparent horizon of a FRW universe, and using the entropy of Tsallis, Sq=A4G+αlnq⁡A4G+β where lnq⁡x=x1−q−11−q, we derive modified Friedmann equations describing the dynamics of the universe. The Tsallis parameter q measures the nonextensivity, or extensivity, level of any specific system. In fact, we modify the cosmological field equations in order to include new additional expressions that act as an effective dark energy part. We also put constrain on the parameters of the model using observational data (Union2 and updated Pantheon data) For both cases; absence and presence of cosmological constant Λ. For the case where we do not consider an explicit cosmological constant the resulting cosmology is very interesting and in agreement with the observed behavior. The interesting feature of the model in this case is that modified geometry term of the Friedman equations caused by Barrow Tsallis entropy correction plays the role of dark energy and could explain the acceleration of the universe. For the case Λ≠0, the dark energy equation-of-state parameter deviates from ΛCDM cosmology at small redshifts, while lying always in the phantom regime, and at asymptotically large times the Universe moves to the dark-energy dominated, de Sitter phase. For the best fitted of the parameters in both cases, the model implying a universe which undergoes first a radiation dominated phase, followed by a matter dominated phase and allowing the intermediate matter epoch to happen slowly and finally reaching a dark energy dominated phase. This is exactly the expected behavior of the observed universe which is well modeled by a universe filled with radiation, dark matter and dark energy.

Cosmological bulk flow in the QCDM model: (in)consistency with ΛCDM

ABSTRACT We study the bulk flow of the local universe with Type Ia supernova data (a compilation of Union2 and Pantheon data) in the spatially flat homogeneous and isotropic space–time. In particular, we take the so-called QCDM models, which consist of cold dark matter (CDM) and a Q-component described by a scalar field with its self-interactions determined by an exponential potential. We use different cumulative redshift slices of the Union2 and Pantheon catalogues. A maximum-likelihood analysis of peculiar velocities confirms that, at low redshifts 0.015 < z < 0.1, the bulk flow is moving in the $l=272^{+17}_{-17}, b=33^{+12}_{-12}$, and $302^{+20}_{-20},3^{+10}_{-10}$ directions with $v _\mathrm{bulk} = 225^{+38}_{-35}$ and $246^{+64}_{-46}$ km s−1 for the Pantheon and Union2 data respectively, in good agreement with the direction of the cosmic microwave background dipole and with a number of previous studies at 1σ. However, for high redshifts 0.1 < z < 0.2, we get $v _\mathrm{bulk} = 708^{+110}_{-110}$ and $v_\mathrm{bulk}=1014^{+86}_{-114}\,\text{km\,s}^{-1}$ towards l = 318 ± 10°, b = −15 ± 9° and $l=254^{+16}_{-14},\ b=6^{+7}_{-10}$ for the Pantheon and Union2 data respectively. This indicates that for low redshifts our results are approximately consistent with the ΛCDM model; however, for high redshifts they disagree with ΛCDM and support the results of those studies that report a large bulk flow for the universe.

Constraints on Energy–Momentum Squared Gravity from cosmic chronometers and Supernovae Type Ia data

In this work we perform an observational data analysis on the energy–momentum squared gravity model. Possible solutions for matter density are obtained from the model and their cosmological implications are studied. Some recent observational data is used to constrain model parameters using statistical techniques. We have used the cosmic chronometer and SNe Type-Ia Riess (292) H(z)−z data-sets in our study. Along with the data-sets we have also used baryon acoustic oscillation (BAO) peak parameter and cosmic microwave background (CMB) peak parameter to obtain bounds on the model parameters. Joint analysis of the data with the above mentioned parameters has been performed to obtain better results. For the statistical analysis we have used the minimization technique of the χ2 statistic. Using this tool we have constrained the free parameters of the model. Confidence contours have been generated for the predicted values of the free parameters at the 66%, 90% and 99% confidence levels. Finally we have compared our analysis with the union2 data sample presented by Amanullah et al. (2010) and the recently published Pantheon data sample. Finally a multi-component model is investigated by adding dust to a general cosmological fluid with equation of state w=−1∕3. The density parameters were studied and their values were found to comply with the observational results.

Are Type Ia supernova powerful tool to detect anisotropic expansion of the Universe?

In this paper, we present a detailed analysis of the dark energy dipole using Union2, Pantheon and GRB dataset in Chameleon and Teleparallel dark energy models, in comparison with Lambda CDM. Both models are extensively studied in recent years and our result shows that with Union2 and Pantheon data, the preferred direction of the anisotropy in both models are very close to each other as well as with those obtained in some studies for Lambda CDM. However, when the models fitted with a combination of Union 2 and GRB, the statistical analysis slightly favors the Chameleon cosmology over Teleparallel gravity, with the maximum anisotropic direction of (l = 330^{+30}_{-28}, b = -15^{+23}_{-25}) in galactic coordinate system, comparable with alpha -dipole result in Keck-VLT data and LCDM.

Observational constraints on extended Chaplygin gas cosmologies

We investigate cosmological models with extended Chaplygin gas (ECG) as a candidate for dark energy and determine the equation of state parameters using observed data namely, observed Hubble data, baryon acoustic oscillation data and cosmic microwave background shift data. Cosmological models are investigated considering cosmic fluid which is an extension of Chaplygin gas, however, it reduces to modified Chaplygin gas (MCG) and also to generalized Chaplygin gas (GCG) in special cases. It is found that in the case of MCG and GCG, the best-fit values of all the parameters are positive. The distance modulus agrees quite well with the experimental Union2 data. The speed of sound obtained in the model is small, necessary for structure formation. We also determine the observational constraints on the constants of the ECG equation.

Test of the cosmic evolution using Gaussian processes

Much focus was on the possible slowing down of cosmic acceleration under the dark energy parametrization. In the present paper, we investigate this subject using the Gaussian processes (GP), without resorting to a particular template of dark energy. The reconstruction is carried out by abundant data including luminosity distance from Union2, Union2.1 compilation and gamma-ray burst, and dynamical Hubble parameter. It suggests that slowing down of cosmic acceleration cannot be presented within 95% C.L., in considering the influence of spatial curvature and Hubble constant. In order to reveal the reason of tension between our reconstruction and previous parametrization constraint for Union2 data, we compare them and find that slowing down of acceleration in some parametrization is only a ``mirage". Although these parameterizations fits well with the observational data, their tension can be revealed by high order derivative of distance D. Instead, GP method is able to faithfully model the cosmic expansion history.

Redshift parametrizations of dark energy and observational constraint on their parameters: Galileon gravity as background

In this work, Friedmann–Robertson–Walker (FRW) universe filled with dark matter (DM) (perfect fluid with negligible pressure) along with dark energy (DE) in the background of Galileon gravity is considered. Four DE models with different equation of state (EoS) parametrizations have been employed namely, linear, Chevallier–Polarski–Lindler (CPL), Jassal–Bagla–Padmanabhan (JBP) and logarithmic parametrizations. From Stern, Stern+Baryonic Acoustic Oscillation (BAO) and Stern+BAO+Cosmic Microwave Background (CMB) joint data analysis, we have obtained the bounds of the arbitrary parameters [Formula: see text] and [Formula: see text] by minimizing the [Formula: see text] test. The best fit values and bounds of the parameters are obtained at 66%, 90% and 99% confidence levels which are shown by closed confidence contours in the figures. For the logarithmic model unbounded confidence contours are obtained and hence the model parameters could not be finitely constrained. The distance modulus [Formula: see text](z) against redshift [Formula: see text] has also been plotted for our predicted theoretical models for the best fit values of the parameters and compared with the observed Union2 data sample and SNe Type Ia 292 data and we have shown that our predicted theoretical models permits the observational datasets. From the data fitting it is seen that at lower redshifts [Formula: see text] the SNe Type Ia 292 data gives a better fit with our theoretical models compared to the Union2 data sample. So, from the data analysis, SNe Type Ia 292 data is the more favored data sample over its counterpart given the present choice of free parameters. From the study, it is also seen that the logarithmic parametrization model is less supported by the observational data. Finally, we have generated the plot for the deceleration parameter against the redshift parameter for all the theoretical models and compared the results with the work of Farooq et al., (2013).

Study of some parameters of modified Chaplygin gas in Galileon gravity theory from observational perspective

We have assumed the Friedmann–Robertson–Walker model of the universe in Galileon gravity, which is filled with dark matter and modified Chaplygin gas (MCG) type dark energy. We present the Hubble parameter in terms of some unknown parameters and observational parameters with the redshift z. Some cosmological parameters are reconstructed and plots are generated to study the nature of the model and its viability. It is seen that the model is perfectly consistent with the present cosmic acceleration. From observed Hubble data (OHD) set or Stern data set of 12 points, we have obtained the bounds of the arbitrary parameters (A, B) and (A, C) by minimizing the χ2 test. Next because of joint analysis of OHD + baryonic acoustic oscillation (BAO) and OHD+BAO+CMB observations, we have also obtained the best fit values and the bounds of the parameters (A, B) and (A, C) by fixing some other parameters. The best-fit values and bounds of the parameters are obtained with 66%, 90%, and 99% confidence levels for OHD, OHD+BAO, and OHD+BAO+CMB joint analysis. Next we have also taken type Ia supernovae data set (union2 data set with 557 data points). The distance modulus μ(z) against redshift z for our theoretical MCG model in Galileon gravity have been tested for the best fit values of the parameters and the observed type Ia supernovae union2 data sample and from this, we have concluded that our model is in agreement with the union2 sample data.

Comparison between hemisphere comparison method and dipole-fitting method in tracing the anisotropic expansion of the Universe use the Union2 data set

Type-Ia supernovae (SNe Ia) are often used as the standard candles to probe the anisotropic expansion of the Universe. In this paper, we make a comprehensive comparison between the hemisphere comparison (HC) method and dipole-fitting (DF) method in searching for the cosmological preferred direction using the Union2 dataset, a compilation of 557 well-calibrated SNe Ia. We find that the directions of the faintest SNe Ia derived from these two methods are approximately opposite. Monte Carlo simulations show that the results of the HC method strongly depend on the distribution of the data points in the sky. The coincidence that the HC method and DF method give two completely opposite directions may be due to the extremely nonuniform distribution of the Union2 dataset.

Observational Constraints of Red-shift Parametrization Parameters of Dark Energy in Horava-Lifshitz Gravity

We have assumed that the FRW universe filled with baryonic matter, radiation and dark matter along with dark energy in the frame-work of Horava-Lifshitz gravity. Here three parameterizations like Linear, CPL and JBP for the dark energy have been assumed for the variations of EOS parameter w(z). The observational data analysis by χ2 minimum test have been analyzed for our models. From Stern, Stern+BAO and Stern+BAO+CMB joint data analysis, we have obtained the bounds of the arbitrary parameters w0 and w1. The best fit values of the parameters w0 and w1 for these three models and the minimum values of χ2 have been obtained by observational data analysis. Also the bounds of the parameters w0 and w1 are obtained by 66 %, 90 % and 99 % confidence levels for linear, CPL and JBP models. Next red shift-magnitude observational data points from type Ia supernovae have been considered and which contains 557 data points. From this observation, the distance modulus μ(z) against red shift z has been investigated for our predicted theoretical model (three DE models) for the best fit values of the parameters and the observed SNe Ia Union2 data sample. Finally, we have investigated that our predicted theoretical three models permitted the observational data sets.

Observational Constraints of Modified Chaplygin Gas in Chern-Simons Gravity

We have considered a five-dimensional FRW model of the universe in Chern-Simons gravity filled with dark matter and Modified Chaplygin gas (MCG) type dark energy. We present the Hubble parameter in terms of the unknown parameters A,B,C,α,λ,l with the redshift z. From Stern data set (12 points), we have obtained the bounds of the arbitrary parameters by minimizing the χ 2 test. Next due to joint analysis of BAO and CMB observations, we have also obtained the best fit values and the bounds of the parameters (B,C) by fixing some other parameters. The best-fit values and bounds of the parameters are obtained by 66 %, 90 % and 99 % confidence levels for Stern, Stern +BAO and Stern +BAO +CMB analysis. We have shown that the best fit values of B and C for our chosen values of α are lie between 0 and 1. The distance modulus μ(z) against redshift z for our theoretical model of the MCG in Chern-Simons gravity model for the best fit values of the parameters and the observed SNe Ia union2 data sample.

Constraining the Parameters of Modified Chaplygin Gas in Einstein-Aether Gravity

We have assumed FRW model of the universe in Einstein-Aether gravity filled with dark matter and modified Chaplygin gas (MCG) type dark energy. We present the Hubble parameter in terms of some unknown parameters and observational parameters with the redshiftz. From observed Hubble data (OHD) set (12 points), we have obtained the bounds of the arbitrary parameters(A,B)of MCG by minimizing theχ2test. Next due to joint analysis of BAO and CMB observations, we have also obtained the best fit values and the bounds of the parameters(A,B)by fixing some other parameters. We have also taken type Ia supernovae data set (union 2 data set with 557 data points). Next due to joint analysis with SNe, we have obtained the best fit values of parameters. The best fit values and bounds of the parameters are obtained by 66%, 90%, and 99% confidence levels for OHD, OHD + BAO, OHD + BAO + CMB, and OHD + BAO + CMB + SNe joint analysis. The distance modulusμzagainst redshiftzfor our theoretical MCG model in Einstein-Aether gravity has been tested for the best fit values of the parameters and the observed SNe Ia union2 data sample.

ΛCDM model with a scalar perturbation vs. preferred direction of the universe

We present a scalar perturbation for the ΛCDM model, which breaks the isotropic symmetry of the universe. Based on the Union2 data, the least-χ 2 fit of the scalar perturbed ΛCDM model shows that the universe has a preferred direction (l,b)=(287∘±25∘,11∘±22∘). The magnitude of scalar perturbation is about −2.3×10−5. The scalar perturbation for the ΛCDM model implies a peculiar velocity, which is perpendicular to the radial direction. We show that the maximum peculiar velocities at redshift z=0.15 and z=0.015 equal to 73±28 km s−1 and 1099±427 km s−1, respectively. They are compatible with the constraints on the peculiar velocity given by the Planck Collaboration.

Observational constraints on modified Chaplygin gas in Horava–Lifshitz gravity with dark radiation

We present Cosmological models with modified Chaplygin gas (MCG) in the framework of Horava-Lifshitz (HL) theory of gravity both with and without detailed balance. The equation of state (EOS) for a MCG contains three unknown parameters namely, $A$, $\alpha$, $B$. The allowed values of some of these parameters of the EOS are determined using the recent astrophysical and cosmological observational data. Using observational data from $H(z)-z$, BAO peak parameter, CMB shift parameter we study cosmologies in detailed-balance and beyond detailed-balance scenario. In this paper we take up the beyond detailed-balance scenario in totality and contribution of dark radiation in the case of detailed-balance scenario on the parameters of the EOS. We explore the effect of dark radiation on the whole range the of effective neutrino parameter to constrain matter contributing parameter $B$ in both the detailed-balance and the beyond-detailed balance scenario. It has been observed that greater the dark radiation less the matter contribution in the MCG in both the scenario considered here. In order to check the validity of beyond detailed balance scenario we plot supernovae magnitudes ($\mu$) with redshift of Union2 data and then the variation of state parameter with redshift is studied. It has been observed that beyond detailed balance scenario is equally suitable in HL gravity with MCG.

Testing X-ray measurements of galaxy cluster gas mass fraction using the cosmic distance-duality relation

We propose a consistency test for some recent X-ray gas mass fraction (fgas) measurements in galaxy clusters, using the cosmic distance-duality relation, ηtheory = DL(1 + z)−2/DA, with luminosity distance (DL) data from the Union2 compilation of type Ia supernovae. We set ηtheory ≡ 1, instead of assigning any redshift parameterizations to it, and constrain the cosmological information preferred by fgas data along with supernova observations. We adopt a new binning method in the reduction of the Union2 data, in order to minimize the statistical errors. Four data sets of X-ray gas mass fraction, which are reported by Allen et al. (two samples), LaRoque et al. and Ettori et al., are analyzed in detail in the context of two theoretical models of fgas. The results from the analysis of Allen et al.'s samples demonstrate the feasibility of our method. It is found that the preferred cosmology by LaRoque et al.'s sample is consistent with its reference cosmology within the 1σ confidence level. However, for Ettori et al.'s fgas sample, the inconsistency can reach more than a 3σ confidence level and this dataset shows special preference to an ΩΛ = 0 cosmology.

An interacting dark energy model in a non-flat universe

In this paper, an interacting dark energy model in a non-flat universe is studied, with taking interaction form $$C=\alpha H\rho _{de}$$ . And in this study a property for the mysterious dark energy is aforehand assumed, i.e. its equation of state $$w_{\Lambda }=-1$$ . After several derivations, a power-law form of dark energy density is obtained $$\rho _{\Lambda } \propto a^{-\alpha }$$ , here $$a$$ is the cosmic scale factor, $$\alpha $$ is a constant parameter introducing to describe the interaction strength and the evolution of dark energy. By comparing with the current cosmic observations, the combined constraints on the parameter $$\alpha $$ is investigated in a non-flat universe. For the used data they include: the Union2 data of type Ia supernova, the Hubble data at different redshifts including several new published datapoints, the baryon acoustic oscillation data, the cosmic microwave background data, and the observational data from cluster X-ray gas mass fraction. The constraint results on model parameters are $$\Omega _{K}=0.0024\,(\pm 0.0053)^{+0.0052+0.0105}_{-0.0052-0.0103}, \alpha =-0.030\,(\pm 0.042)^{+0.041+0.079}_{-0.042-0.085}$$ and $$\Omega _{0m}=0.282\,(\pm 0.011)^{+0.011+0.023}_{-0.011-0.022}$$ . According to the constraint results, it is shown that small constraint values of $$\alpha $$ indicate that the strength of interaction is weak, and at $$1\sigma $$ confidence level the non-interacting cosmological constant model can not be excluded.

Constraining the anisotropic expansion of the Universe

We study the possibly existing anisotropy in the accelerating expansion universe with the Union2 Type Ia supernovae data and Gamma-ray burst data. We construct a direction-dependent dark energy model and constrain the anisotropy direction and strength of modulation. We find that the maximum anisotropic deviation direction is $(l,\,b)=(126^{\circ},\,13^{\circ})$ (or equivalently $(l,\,b)=(306^{\circ},\,-13^{\circ})$), and the current anisotropy level is $g_0=0.030_{+0.010}^{-0.030}$ ($1\sigma$ confidence level with Union2 data). Our results do not show strong evidence for the anisotropic dark energy model. We also discuss potential methods that may distinguish the peculiar velocity field from the anisotropic dark energy model.

Constraining redshift parametrization parameters of dark energy: loop quantum gravity as back ground

We have assumed that the FRW universe filled with dark matter (perfect fluid with negligible pressure) along with dark energy in LQC gravity where three parameterizations have been proposed for the variations of EOS parameter ω(z) and they are Linear, CPL and JBP parameterizations. From Stern, Stern+BAO and Stern+BAO+CMB joint data analysis, we have obtained the bounds of the arbitrary parameters ω 0 and ω 1 by minimizing the χ 2 test. The best-fit values and bounds of the parameters are obtained by 66 %, 90 % and 99 % confidence levels which are shown in (i) Figs. 1a, 1b, 1c for Stern data analysis, (ii) Figs. 2a, 2b, 2c for Stern+BAO joint data analysis and (iii) Figs. 3a, 3b, 3c for Stern+BAO+CMB joint data analysis for linear, CPL and JBP models. The distance modulus μ(z) against redshift z has been drawn in Figs. 4a, 4b, 4c for our predicted theoretical model of the three models for the best fit values of the parameters and the observed SNe Ia Union2 data sample and we have shown that our predicted theoretical three models permitted the observational data sets.

Modified holographic Ricci dark energy coupled to interacting dark matter and a non-interacting baryonic component

We examine a Friedmann-Robertson-Walker universe filled with interacting dark matter, modified holographic Ricci dark energy (MHRDE), and a decoupled baryonic component.The estimations of the cosmic parameters with Hubble data lead to an age of the universe of $13.17 {\rm Gyr}$ and show that the MHRDE is free from the cosmic-age problem at low redshift ($0 \leq z \leq 2$) in contrast to holographic Ricci dark energy (HRDE) case. We constrain the parameters with the Union2 data set and contrast with the Hubble data. We also study the behavior of dark energy at early times by taking into account the severe bounds found at recombination era and/or at big bang nucleosynthesis. The inclusion of a non interacting baryonic matter forces that the amount of dark energy at $z_{t} \sim {\cal O}(1)$ changes abruptly implying that $\Omega_x (z\simeq 1100) =0.03$, so the bounds reported by the forecast of Planck and CMBPol experiments are more favored for the MHRDE model than in the case of HRDE cutoff. For the former model, we also obtain that at high redshift the fraction of dark energy varies from 0.006 to 0.002, then the amount of $\Omega_x $ at the big bang nucleosynthesis era does not disturb the observed Helium abundance in the universe provided that the bound $\Omega_{x}(z\simeq 10^{10})<0.21$ is hold.