A Geometrical Test of the Cosmological Energy Contents Using the Lyman-alpha Forest

Abstract

In this Letter we explore a version of the test of cosmological geometry proposed by Alcock and Paczynski (1979), using observations of the Lyman-alpha forest in the spectra of close quasar pairs. By comparing the correlations in absorption in one quasar spectrum with correlations between the spectra of neighboring quasars one can determine the relation of the redshift distance scale to the angle distance scale at the redshift of the absorbers, $z \sim 2 - 4$. Since this relationship depends on the parameters of the cosmological model, these parameters may be determined using the Lyman-alpha forest. While this test is relatively insensitive to the density parameter $Ω_m$ in a dust-dominated universe, it is more sensitive to the presence of a matter component with large negative pressure (such as a cosmological constant $Λ$) and its equation of state. With only 25 pairs of quasar spectra at angular separations $0.5' - 2'$, one can discriminate between an $Ω_m = 0.3$ open universe ($Λ=0$) and an $Ω_m = 0.3$ flat ($Λ$-dominated) universe at the $4-σ$ level. The S/N can be enhanced by considering quasar pairs at smaller angular separations, but requires proper modeling of nonlinear redshift space distortions. Here the correlations and redshift space distortions are modeled using linear theory.