Upper Limit on Dimming of Cosmological Sources by Intergalactic Grey
  Dust from the Soft X-ray Background
  Active Galactic Nuclei (AGN) produce a dominant fraction (~80%) of the Soft
X-ray background (SXB) at photon energies 0.5<E<2 keV. If dust pervaded
throughout the intergalactic medium, its scattering opacity would have produced
diffuse X-ray halos around AGN. Taking account of known galaxies and galaxy
clusters, only a fraction F_halo <10% of the SXB can be in the form of diffuse
X-ray halos around AGN. We therefore limit the intergalactic opacity to
optical/infrared photons from large dust grains (with radii in the range
a=0.2-2.0 mum) to a level tau_GD<0.15(F_halo/10%) to a redshift z~1. Our
results are only weakly dependent on the grain size distribution or the
redshift evolution of the intergalactic dust. Stacking X-ray images of AGN can
be used to improve our constraints and diminish the importance of dust as a
source of systematic uncertainty for future supernova surveys which aim to
improve the precision on measuring the redshift evolution of the dark energy
equation-of-state.
