  Extinction by intergalactic gray dust introduces a magnitude redshift
dependent offset in the standard-candle relation of SN Ia. This leads to
overestimated luminosity distances compared to a dust-free universe.
Quantifying the amplitude of this systematic effect is crucial for an accurate
determination of the dark energy parameters. In this paper we model the gray
dust extinction in terms of the star-formation history of the universe and the
physical properties of the dust grains. We focus on a class of cosmic dust
models which satisfy current observational constraints. These can produce an
extinction as large as 0.08 mag at z=1.7 and potentially disrupt the dark
energy parameter inference from future SN surveys. In particular depending on
the dust model we find that an unaccounted extinction can bias the estimation
of a constant dark energy equation of state w by shifting its best fit value up
to 20% from its true value. Near-IR broadband photometry will hardly detect
this effect, while the induced decrement of the Balmer lines requires high
signal-to-noise spectra. Indeed IR-spectroscopy will be needed for high
redshift SNe. Cosmic dust extinction may also cause a detectable violation of
the distance-duality relation. A more comprehensive knowledge of the physics of
the IGM is necessary for an accurate modeling of intergalactic dust. Due to the
large magnitude dispersion current luminosity distance measurements are
insensitive to such possible extinction effects. In contrast these must be
taken into account if we hope to disclose the true nature of dark energy with
the upcoming generation of SN Ia surveys.
