Synopsis: Direct Test of Cosmic Acceleration

Future radio surveys of intergalactic hydrogen clouds could offer the first direct measurement of the Universe’s acceleration.

The consensus among cosmologists is that the Universe is accelerating, but this is inferred from a model of the expansion history and an unproven assumption about the uniformity of the Universe. Upcoming radio telescope surveys could offer a more direct observation of acceleration by tracking velocity changes in intergalactic hydrogen clouds, as described in Physical Review Letters. The authors outline a few modifications in data collection that are needed to achieve a sufficiently precise acceleration measurement.

The primary support for cosmic acceleration comes from supernova data. In 1998, astronomers detected that certain supernova are fainter—and therefore farther away from us—than expected. The implication is that the Universe is accelerating, rather than decelerating as normal gravitational interactions would predict. However, this conclusion 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.

Hao-Ran Yu of the Beijing Normal University in China 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 made a few adaptions, like increasing their frequency resolution, they could measure—over the course of a decade—cosmically relevant accelerations of around 1 millimeter/second/year. – Michael Schirber


Announcements

More Announcements »

Subject Areas

AstrophysicsCosmology

Previous Synopsis

Next Synopsis

Fluid Dynamics

Turbulent Times

Read More »

Related Articles

Synopsis: Lightweight Particles Might Explain Missing Lithium
Cosmology

Synopsis: Lightweight Particles Might Explain Missing Lithium

The apparent lack of lithium in the Universe, relative to theoretical expectations, could be explained by hypothetical lightweight and electrically neutral particles. Read More »

Synopsis: Gravitational Waves May Hold Dark Matter Secret
Astrophysics

Synopsis: Gravitational Waves May Hold Dark Matter Secret

A theoretical analysis examines the possibility that the black holes detected by LIGO serve as dark matter. Read More »

Synopsis: Cosmic Magnetism Revisited
Cosmology

Synopsis: Cosmic Magnetism Revisited

An analysis of the polarized emission from some 3000 distant radio sources places a stringent upper limit on the strength of the cosmological magnetic field. Read More »

More Articles