Assessing cosmic acceleration with the Alcock-Paczyński effect in the SDSS-IV quasar catalogue

The geometry of the Universe may be probed using the Alcock-Paczyński (AP) effect, in which the observed redshift size of a spherical distribution of sources relative to its angular size varies according to the assumed cosmological model. Past applications of this effect have been limited, however, by a paucity of suitable sources and mitigating astrophysical factors, such as internal redshift-space distortions and poorly known source evolution. In this Letter, we introduce a new test based on the AP effect that avoids the use of spatially bound systems, relying instead on sub-samples of quasars at redshifts z 1.5 in the Sloan Digital Sky Survey IV, with a possible extension to higher redshifts and improved precision when this catalogue is expanded by upcoming surveys. We here use this method to probe the redshift-dependent expansion rate in three pertinent Friedmann-Lemaître-Robertson-Walker cosmologies: Λ cold dark matter (ΛCDM), which predicts a transition from deceleration to acceleration at z ∼0.7; Einstein-de Sitter, in which the Universe is always decelerating; and the Rh = ct universe, which expands at a constant rate. ΛCDM is consistent with these data, but Rh = ct is favoured overall.