Exploring the potentiality of standard sirens to probe cosmic opacity at high redshifts

In this work, using the Gaussian process, we explore the potentiality of future gravitational wave (GW) measurements to probe cosmic opacity at high redshifts through comparing its opacity-free luminosity distance (LD) with the opacity-dependent one from the combination of Type Ia supernovae (SNIa) and gamma-ray bursts (GRBs). The GW data, SNIa and GRB data are simulated from the measurements of the future Einstein Telescope, the actual Pantheon compilation and the latest observation of GRBs compiled by L. Amati {\it et al}, respectively. A nonparametric method is proposed to probe the spatial homogeneity of cosmic transparency at high redshift by comparing the LD reconstructed from the GW data with that reconstructed from the Pantheon and GRB data. In addition, the cosmic opacity is tested by using the parametrization for the optical depth, and the results show that the constraints on cosmic opacity are more stringent than the previous ones. It shows that the future GW measurements may be used as an important tool to probe the cosmic opacity in the high redshift region.Comment: 21pages, 3 figures accepted by EPJC. arXiv admin note: text overlap with arXiv:1912.0232

Testing the FLRW metric with the Hubble and transversal BAO measurements

The cosmological principle is one of the fundamental assumptions of the standard model of Cosmology (SCM), and it allow us to describe cosmic distances and clocks by using the Friedmann-Lema\rm{\hat{{\i}}}tre-Roberton-Walker (FLRW) metric. Thus, it is essential to test the FLRW metric with cosmological observations to verify the validity of the SCM. In this work, we perform tests of the FLRW metric by comparing the observational comoving angles between the Hubble $H(z)$ and angular Baryon Acoustic Oscillation (BAO) measurements. The Gaussian process is employed to reconstruct the Hubble $H(z)$ measurements and the angular diameter distance (ADD) from the transversal BAO data. A non-parametric method is adopted to probe the possible deviations from the FLRW metric at any redshift by comparing the comoving distances from the reconstructed Hubble $H(z)$ measurements with the ADD reconstructed from the transversal BAO data. Then, we propose two types of parameterizations for the deviations from the FLRW metric, and test the FLRW metric by using the priors of specific sound horizon scales. To avoid the bias caused by the prior of a specific sound horizon scale, we perform the consistency test with a flat prior of the sound horizon scale. We find that there a concordance between the FLRW metric and the observational data by using parametric and non-parametric methods, and the parameterizations can be employed to test the FLRW metric in a new way independent of the sound horizon scale.Comment: Submitted to PRD and received comments from referee

Cold quark matter in a quasiparticle model: thermodynamic consistency and stellar properties

The strong coupling in the effective quark mass was usually taken as a constant in a quasiparticle model while it is, in fact, running with an energy scale. With a running coupling, however, the thermodynamic inconsistency problem appears in the conventional treatment. We show that the renormalization subtraction point should be taken as a function of the summation of the biquadratic chemical potentials if the quark's current masses vanish, in order to ensure full thermodynamic consistency. Taking the simplest form, we study the properties of up-down ($ud$) quark matter, and confirm that the revised quasiparticle model fulfills the quantitative criteria for thermodynamic consistency. Moreover, we find that the maximum mass of an $ud$ quark star can be larger than two times the solar mass, reaching up to $2.31M_{\odot}$, for reasonable model parameters. However, to further satisfy the upper limit of tidal deformability $\tilde{\Lambda}_{1.4}\leq 580$ observed in the event GW170817, the maximum mass of an $ud$ quark star can only be as large as $2.08M_{\odot}$, namely $M_{\text{max}}\lesssim2.08M_{\odot}$. In other words, our results indicate that the measured tidal deformability for event GW170817 places an upper bound on the maximum mass of $ud$ quark stars, but which does not rule out the possibility of the existence of quark stars composed of $ud$ quark matter, with a mass of about two times the solar mass.Comment: 10 pages, 8 figure

A parametrization for the growth index of linear matter perturbations

We propose a parametrization for the growth index of the linear matter perturbations, $\gamma(z)=\gamma_0+\frac{z}{1+z}\gamma_1$. The growth factor of the perturbations parameterized as $\Omega_m^{\gamma}$ is analyzed for both the $w$CDM model and the DGP model with our proposed form for $\gamma$. We find that $\gamma_1$ is negative for the $w$CDM model but is positive for the DGP model. Thus it provides another signature to discriminate them. We demonstrate that $\Omega_m^{\gamma}$ with $\gamma$ taking our proposed form approximates the growth factor very well both at low and high redshfits for both kinds of models. In fact, the error is below 0.03% for the $\Lambda$CDM model and 0.18% for the DGP model for all redshifts when $\Omega_{m0}=0.27$. Therefore, our parametrization may be robustly used to constrain the growth index of different models with the observational data which include points for redshifts ranging from 0.15 to 3.8, thus providing discriminative signatures for different models.Comment: 14 pages, 6 figures; Added reference

Density pertubation of unparticle dark matter in the flat Universe

The unparticle has been suggested as a candidate of dark matter. We investigated the growth rate of the density perturbation for the unparticle dark matter in the flat Universe. First, we consider the model in which unparticle is the sole dark matter and find that the growth factor can be approximated well by $f=(1+3\omega_u)\Omega^{\gamma}_u$, where $\omega_u$ is the equation of state of unparticle. Our results show that the presence of $\omega_u$ modifies the behavior of the growth factor $f$. For the second model where unparticle co-exists with cold dark matter, the growth factor has a new approximation $f=(1+3\omega_u)\Omega^{\gamma}_u+\alpha \Omega_m$ and $\alpha$ is a function of $\omega_u$. Thus the growth factor of unparticle is quite different from that of usual dark matter. These information can help us know more about unparticle and the early evolution of the Universe.Comment: 6pages, 4 figures, accepted for publication in Eur. Phys. J.

The growth of linear perturbations in the DGP model

We study the linear growth of matter perturbations in the DGP model with the growth index $\gamma$ as a function of redshift. At the linear approximation: $\gamma(z)\approx\gamma_0+\gamma_0^\prime z$, we find that, for $0.2\leq\Omega_{m,0}\leq0.35$, $\gamma_0$ takes the value from 0.658 to 0.671, and $\gamma_0^\prime$ ranges from 0.035 to 0.042. With three low redshift observational data of the growth factor, we obtain the observational constraints on $\gamma_0$ and $\gamma_0'$ for the $\Lambda CDM$ and DGP models and find that the observations favor the $\Lambda CDM$ model but at the $1\sigma$ confidence level both the $\Lambda CDM$ and DGP models are consistent with the observations.Comment: 12 pages, 4 figuers, to appear in PL