GBT26A-544

Direct Measurement of the Cosmic Acceleration: A Quarter-Century GBT Baseline

Abstract

The expansion of the universe is accelerating today due to dark energy (whatever that is) and decelerated in the past due to dark matter (again, whatever that is). The acceleration was first inferred from observations of supernovae by comparing luminosity distances and redshifts. Studies of dark energy generally rely on distance indicators and the cosmological distance ladder, but an exception is the secular redshift drift: in an accelerating universe, redshifts are dynamic, and observations of the rate of change of cosmological redshifts provide a model-independent and distance ladder-independent measurement of the acceleration. In 2012, we attempted a first measurement of the acceleration of 21 cm HI absorption lines using multi-epoch GBT observations and obtained a non-significant acceleration of 5.5 +/- 2.2 m/s/yr. We propose to add a 2025 or 2026 epoch, providing a nearly quarter-century time baseline since our first observations in 2003. Simulations suggest that we can break the m/s/yr threshold, reaching roughly +/- 70 cm/s/yr, by observing 10 systems over 25 hours. Improvement by nearly two orders of magnitude is therefore possible. Observations will also provide new limits on the cosmic evolution of physical constants and enable study of these absorbers themselves.

Investigators

Name	Institution
GBT Operator	Green Bank Observatory
Jeremy Darling *	Colorado at Boulder, University of

* indicates the PI