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

Abraham Loeb

Published 1998-02-11Version 1

The redshift of all cosmological sources drifts by a systematic velocity of order a few m/s over a century due to 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, Omega_M and Omega_Lambda, and the Hubble constant H_0. 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 a hundred quasars set a decade apart, with the HIRES instrument on the Keck 10 meter telescope. The signal would appear as a global redshift change in the Lyman-alpha 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.