Direct Measurement of Cosmological Parameters from the Cosmic Deceleration of Extragalactic Objects

The redshift of all cosmological sources drifts by a systematic velocity of order a few meters per second over a century as a result of the deceleration of the universe. The specific functional dependence of the predicted velocity shift on the source redshift can be used to verify its cosmic origin and to measure directly the values of cosmological parameters, such as the density parameters of matter and vacuum, Ω_M and Ω_Λ, and the Hubble constant H₀. For example, an existing spectroscopic technique, which was recently employed in planet searches, is capable of uncovering velocity shifts of this magnitude. The cosmic deceleration signal might be marginally detectable through two observations of ~10² quasars set a decade apart, with the HIRES instrument on the Keck 10 m telescope. The signal would appear as a global redshift change in the Lyα forest templates imprinted on the quasar spectra by the intergalactic medium. The deceleration amplitude should be isotropic across the sky. Contamination of the cosmic signal by peculiar accelerations or local effects is likely to be negligible.