title: Method for direct measurement of cosmic acceleration by 21-cm absorption systems.
abstract: So far there is only indirect evidence that the Universe is undergoing an accelerated expansion. The evidence for cosmic acceleration is based on the observation of different objects at different distances and requires invoking the Copernican cosmological principle and Einstein's equations of motion. We examine the direct observability using recession velocity drifts (Sandage-Loeb effect) of 21-cm hydrogen absorption systems in upcoming radio surveys. This measures the change in velocity of the same objects separated by a time interval and is a model-independent measure of acceleration. We forecast that for a CHIME-like survey with a decade time span, we can detect the acceleration of a ΛCDM universe with 5σ confidence. This acceleration test requires modest data analysis and storage changes from the normal processing and cannot be recovered retroactively.
authors.affiliation: Department of Astronomy, Beijing Normal University, Beijing 100875, China and Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China.
authors.forename: Hao-Ran
authors.initials: HR
authors.lastname: Yu
authors.affiliation: Department of Astronomy, Beijing Normal University, Beijing 100875, China.
authors.forename: Tong-Jie
authors.initials: TJ
authors.lastname: Zhang
authors.affiliation: Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, M5S 3H8, Ontario, Canada.
authors.forename: Ue-Li
authors.initials: UL
authors.lastname: Pen