Astro2020 White Paper: A Direct Measure of Cosmic Acceleration

TL;DR

This paper discusses the potential for directly measuring cosmic acceleration through redshift drift, emphasizing the importance, challenges, and future prospects of detecting this subtle effect with upcoming astronomical facilities.

Contribution

It highlights the feasibility of detecting redshift drift within a five-year observational baseline using dedicated experiments, accelerating progress in understanding cosmic acceleration.

Findings

Redshift drift directly measures the Universe's expansion history.

Upcoming facilities like ESO-ELT and SKA could detect redshift drift within 20-30 years.

Dedicated experiments could achieve detection with only a five-year baseline.

Abstract

Nearly a century after the discovery that we live in an expanding Universe, and two decades after the discovery of accelerating cosmic expansion, there remains no direct detection of this acceleration via redshift drift - a change in the cosmological expansion velocity versus time. Because cosmological redshift drift directly determines the Hubble parameter H(z), it is arguably the cleanest possible measurement of the expansion history, and has the potential to constrain dark energy models (e.g. Kim et al. 2015). The challenge is that the signal is small - the best observational constraint presently has an uncertainty several orders of magnitude larger than the expected signal (Darling 2012). Nonetheless, direct detection of redshift drift is becoming feasible, with upcoming facilities such as the ESO-ELT and SKA projecting possible detection within two to three decades. This timescale is uncomfortably long given the potential of this cosmological test. With dedicated experiments it should be possible to rapidly accelerate progress and detect redshift drift with only a five-year observational baseline. Such a facility would also be ideal for precision radial velocity measurements of exoplanets, which could be obtained as a byproduct of the ongoing calibration measurements for the experiment.