Distance Duality Test: The Evolution of Radio Sources Mimics a Nonexpanding Universe

Abstract: Distance duality relation (DDR) marks a fundamental difference between expanding and nonexpanding Universes, as an expanding metric causes angular diameter distance smaller than luminosity distance by an extra factor of $(1+z)$. Here we report a test of this relation using two independent samples of ultracompact radio sources observed at 2.29 GHz and 5.0 GHz. The test with radio sources involves only geometry, so it is independent of cosmological models. Since the observed radio luminosities systematically increase with redshift, we do not assume a constant source size. Instead, we start with assuming the intensive property, luminosity density, does not evolve with redshift and then infer its evolution from the resultant DDR. We make the same assumption for both samples, and find it results in the same angular size-redshift relation. Interestingly, the resultant DDR is fully consistent with a nonexpanding Universe. Imposing the DDR predicted by the expanding Universe, we infer the radio luminosity density evolves as $\rho_L\propto(1+z)^3$. However, the perfect agreement with a nonexpanding Universe under the assumption of constant luminosity densities poses a conspiracy and fine-tuning problem: the size and luminosity density of ultracompact radio sources evolve in the way that precisely mimics a nonexpanding Universe.