Pinning Down the Universe’s Rate of Expansion

Captured From: http://physics.aps.org

db456444-fef0-4582-9ae6-d8555d72c411Paul Hooper at Spirit Design, with Mat Pieri and Gongbo Zhao, ICG
Visualization of the experiment carried out by the Baryon Oscillation Spectroscopic Survey (BOSS). Redshift data from 140,000 quasars collected by a 2.5-meter telescope at Apache Point, New Mexico, provide the most accurate measurement to date of the expansion rate of the Universe.

In 1998, observations of supernovae led to the remarkable conclusion that the Universe’s expansion is accelerating—a finding most often explained by a yet-to-be-deciphered form of dark energy. In a talk at a session on cosmology, Andreu Font-Ribera from Lawrence Berkeley National Lab reported on the most precise measurement of that expansion to date, carried out by the Baryon Oscillation Spectroscopic Survey (BOSS). And the new result pushes our knowledge further back in time: we now know that 10.8 billion years ago, the Universe was expanding by 1% every 44 million years.

The BOSS analysis relies on data from 140,000 quasars collected by a 2.5-meter telescope at Apache Point, New Mexico. BOSS researchers have measured the expansion of the Universe by mapping the redshifts of light passing through intergalactic hydrogen clouds. These clouds have been imprinted with so-called baryon acoustic oscillations—sound waves created in the exploding plasma of the early Universe.

Light from extremely distant but very bright quasars passes through the hydrogen clouds, which have high- and low-density regions caused by the baryon oscillations. The clouds are moving away from the quasars with the expansion of the Universe, so there are redshifts in the spectral absorption lines that yield information on the rate of expansion. And because the baryon oscillations are peaks and troughs of density, researchers can tie a given redshift to a particular position.

The team used two complementary techniques to get the uncertainty in the measured expansion rate down to 2%. One method, autocorrelation, compared the absorption in nearby quasar spectra. The other, cross-correlation, analyzes the amount of absorption as a function of separation from a quasar. As the analysis of the data set continues, researchers hope to be able to understand better the nature of the dark energy that is causing the accelerating expansion.

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