2012ApJ...761L..26D

We present precise H I 21 cm absorption line redshifts observed in multiple epochs to directly constrain the secular redshift drift {dot}z or the cosmic acceleration, Δv/Δt_{cir}. A comparison of literature analog spectra to contemporary digital spectra shows significant acceleration likely attributable to systematic instrumental errors. However, we obtain robust constraints using primarily Green Bank Telescope digital data. Ten objects spanning z = 0.09-0.69 observed over 13.5 years show {dot}z = -2.3± 0.8^–8/yr or Δv/Δt _{cir}= -5.5±2.2 m/s/yr. The best constraint from a single object, 3C 286 at < z > = 0.692153275(85), is {dot}z = (1.6 ± 4.7^–8/yr or Δv/Δt _{cir}= 2.8±8.4 m/s/yr. These measurements are three orders of magnitude larger than the theoretically expected acceleration at z = 0.5, {dot}z = 2x 10^–11/yr or Δv/Δt_{cir}= 0.3 cm/s/yr, but they demonstrate the lack of peculiar acceleration in absorption line systems and the long-term frequency stability of modern radio telescopes. A comparison of UV metal absorption lines to the 21 cm line improves constraints on the cosmic variation of physical constants: Δ(α² g_pµ)/α² g_pµ = (- 1.2±1.4)x10^–6 in the redshift range z = 0.24-2.04. The linear evolution over the last 10.4 Gyr is (- 0.2±2.7)x10^–16/yr, consistent with no variation. The cosmic acceleration could be directly measured in ∼125 years using current telescopes or in ∼5 years using a Square Kilometer Array, but systematic effects will arise at the 1 cm/s/yr level.