# Method for Direct Measurement of Cosmic Acceleration by 21-cm Absorption Systems

 Title Method for Direct Measurement of Cosmic Acceleration by 21-cm Absorption Systems Publication Type Journal Article Year of Publication 2014 Authors Yu, H-R, Zhang, T-J, Pen, U-L Journal Physical Review Letters Volume 113 Pagination 041303 Keywords Astronomical catalogs atlases sky surveys databases retrieval systems archives etc., Dark energy, Distances redshifts radial velocities, Observational cosmology, spatial distribution of galaxies 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 {$Łambda$}CDM universe with 5{$\sigma$} confidence. This acceleration test requires modest data analysis and storage changes from the normal processing and cannot be recovered retroactively. } DOI 10.1103/PhysRevLett.113.041303