Is the Universe Transparent?
  We present our study on cosmic opacity, which relates to changes in photon
number as photons travel from the source to the observer. Cosmic opacity may be
caused by absorption/scattering due to matter in the universe, or by
extragalactic magnetic fields that can turn photons into unobserved particles
(e.g. light axions, chameleons, gravitons, Kaluza-Klein modes), and it is
crucial to correctly interpret astronomical photometric measurements like type
Ia supernovae observations. On the other hand, the expansion rate at different
epochs, i.e. the observational Hubble parameter data $H(z)$, are obtained from
differential ageing of passively evolving galaxies or from baryon acoustic
oscillations and thus are not affected by cosmic opacity. In this work, we
first construct opacity-free luminosity distances from $H(z)$ determinations,
taking correlations between different redshifts into consideration for our
error analysis. Moreover, we let the light-curve fitting parameters, accounting
for distance estimation in type Ia supernovae observations, free to ensure that
our analysis is authentically cosmological-model-independent and gives a robust
result. Any non-zero residuals between these two kinds of luminosity distances
can be deemed as an indication of the existence of cosmic opacity. While a
transparent universe is currently consistent with the data, our results show
that strong constraints on opacity (and consequently on physical mechanisms
that could cause it) can be obtained in a cosmological-model-independent
fashion.
