Cosmic transparency and acceleration

Abstract

In this paper, by considering an absorption probability independent of photon wavelength, we show that current type Ia supernovae (SNe Ia) and gamma ray burst (GRBs) observations plus high-redshift measurements of the cosmic microwave background (CMB) radiation temperature support cosmic acceleration regardless of the transparent-universe assumption. Two flat scenarios are considered in our analyses: the $\Lambda$CDM model and a kinematic model. We consider $\tau(z)=2\ln(1+z)^{\varepsilon}$, where $\tau(z)$ denotes the opacity between an observer at $z=0$ and a source at $z$. This choice is equivalent to deforming the cosmic distance duality relation as $D_LD^{-1}_A = (1 + z)^{2+\varepsilon}$ and, if the absorption probability is independent of photon wavelength, the CMB temperature evolution law is $T_{CMB}(z)=T_0(1+z)^{1+2\varepsilon/3 }$. By marginalizing on the $\varepsilon$ parameter, our analyses rule out a decelerating universe at 99.99 \% c.l. for all scenarios considered. Interestingly, by considering only SNe Ia and GRBs observations, we obtain that a decelerated universe indicated by $\Omega_{\Lambda} \leq 0.33$ and $q_0 > 0$ is ruled out around 1.5$\sigma$ c.l. and 2$\sigma$ c.l., respectively, regardless of the transparent-universe assumption.