Cosmological Constant or Intergalactic Dust? Constraints from the Cosmic Far-Infrared Background

Recent observations of Type Ia supernovae (SNe) at redshifts 0 < z < 1 reveal a progressive dimming that has been interpreted as evidence for a cosmological constant of Ω_Λ ~ 0.7. An alternative explanation of the SN results is an open universe with Ω_Λ = 0 and the presence of ≳0.1 μm dust grains with a mass density of Ω_dust ~ a few × 10^-5 in the intergalactic (IG) medium. The same dust that dims the SNe absorbs the cosmic UV/optical background radiation around ~1 μm, and reemits it at far-infrared (FIR) wavelengths. Here we compare the FIR emission from IG dust with observations of the cosmic microwave (CMB) and cosmic far-infrared backgrounds (FIRB) by the DIRBE/FIRAS instruments. We find that the emission would not lead to measurable distortion of the CMB, but would represent a substantial fraction (≳75%) of the measured value of the FIRB in the 300-1000 μm range. This contribution would be marginally consistent with the present unresolved fraction of the observed FIRB in an open universe. However, we find that IG dust probably could not reconcile the standard Ω = 1 CDM model with the SN observations, even if the necessary quantity of dust existed. Future observations, capable of reliably resolving the FIRB to a flux limit of ~0.5 mJy, along with a more precise measure of the coarse-grained FIRB, will provide a definitive test of the IG dust hypothesis in all cosmologies.