Testing cosmic opacity from SNe Ia and Hubble parameter through three
  cosmological-model-independent methods
  We use the newly published 28 observational Hubble parameter data ($H(z)$)
and current largest SNe Ia samples (Union2.1) to test whether the universe is
transparent. Three cosmological-model-independent methods (nearby SNe Ia
method, interpolation method and smoothing method) are proposed through
comparing opacity-free distance modulus from Hubble parameter data and
opacity-dependent distance modulus from SNe Ia . Two parameterizations,
$\tau(z)=2\epsilon z$ and $\tau(z)=(1+z)^{2\epsilon}-1$ are adopted for the
optical depth associated to the cosmic absorption. We find that the results are
not sensitive to the methods and parameterizations. Our results support a
transparent universe.
