Cosmic transparency and acceleration
  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.
