Assessing Cosmic Acceleration with the Alcock-Paczynski Effect in the
  SDSS-IV Quasar Catalog
  The geometry of the Universe may be probed using the Alcock-Paczynski (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 catalog is expanded by
upcoming surveys. We here use this method to probe the redshift-dependent
expansion rate in three pertinent Friedmann-Lemaitre-Robertson-Walker (FLRW)
cosmologies: LCDM, which predicts a transition from deceleration to
acceleration at z ~ 0.7; Einstein-de Sitter, in which the Universe is always
decelerating; and the R_h=ct universe, which expands at a constant rate. LCDM
is consistent with these data, but R_h=ct is favoured overall.
