Cosmic distance-duality as a probe of exotic physics and acceleration

In cosmology, distances based on standard candles (e.g., supernovae) and standard rulers (e.g., baryon oscillations) agree as long as three conditions are met: (1) photon number is conserved, (2) gravity is described by a metric theory with (3) photons traveling on unique null geodesics. This is the content of distance duality (the reciprocity relation) which can be violated by exotic physics. Here we analyze the implications of the latest cosmological data sets for distance duality. While broadly in agreement and confirming acceleration we find a 2-sigma violation caused by excess brightening of SNIa at $z>\mathrm{}0.5,$ perhaps due to lensing magnification bias. This brightening has been interpreted as evidence for a late-time transition in the dark energy but because it is not seen in the $d_A$ data we argue against such an interpretation. Our results do, however, rule out significant SNIa evolution and extinction: the “replenishing” gray-dust model with no cosmic acceleration is excluded at more than 4-sigma despite this being the best fit to SNIa data alone, thereby illustrating the power of distance duality even with current data sets.