Future radio surveys of intergalactic hydrogen clouds could offer the first direct measurement of the Universe’s acceleration. By far, the primary support for cosmic acceleration comes from supernova data, which is indirect, as it assumes the validity of Einstein’s general relativity and that the Universe is homogeneous, in order to derive equations that relate distance to velocity and luminosity.
In a paper published in Physical Review Letters (Phys. Rev. Lett. 113, 041303, 2014), CITA-KIAA Joint Postdoctoral Fellow Hao-Ran Yu and his colleagues investigated the potential of using dense hydrogen clouds for a direct acceleration measurement. These clouds, which occupy the suburbs in between galaxies, are detected through their absorption of radio emission from a background quasar. Astronomers can measure the velocity of one of these clouds by observing the deviation, or redshift, of the hydrogen absorption line at 21 centimeters wavelength. Because the 21-centimeter line is narrow (compared to galaxy emission lines), it’s possible to observe very small velocity changes. Yu et al. argue that upcoming wide-sky radio surveys will measure the velocity of hundreds of thousands of hydrogen clouds. If these surveys make a few adaptations, like increasing their frequency resolution, they could measure—over the course of a decade—cosmically relevant accelerations of around a few millimeter/second/year. (ref - Michael Schirber, APS editor)
Paper link: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.041303
(Physical Review Letters news and commentary in physics, highlights, editor’s suggestion, and featured in physics)
APS synopsis link: http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.113.041303
(Cover page of APS, on 24 July)
Image Caption: The Canadian Hydrogen Intensity Mapping Experiment (CHIME), located in a radio-quiet valley near Penticton, BC, Canada.
Image credit: Prof. Keith Vanderlinde; Dunlap Institute for Astronomy & Astrophysics, University of Toronto
