Toward a Direct Measurement of the Cosmic Acceleration

Understand

We present precise HI 21 cm absorption line redshifts observed in multiple epochs to directly constrain the secular redshift drift dz/dt_o or the cosmic acceleration, dv/dt_o.

• 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 dz/dt_o = (-2.3 +/- 0.8) x 10^-8 yr^-1 or dv/dt_o = -5.5 +/- 2.2 m/s/yr.