Speed of early universe’s expansion determined

Rate known as Hubble constant now known with great precision for ancient universe

SAVANNAH, Ga. — Ask an astronomer how fast the universe is currently expanding and you’ll get a fuzzy answer (SN: 4/5/14, p. 18). But thanks to a measurement with unprecedented precision, astronomers can now confidently cite the speed of cosmic expansion that was occurring nearly 11 billion years ago.

Physicists with the Baryon Oscillation Spectroscopic Survey reported April 7 at a meeting of the American Physical Society that they had improved on their 2012 measurement (SN: 12/29/12, p. 9). They did so by tracking the distances to, and recession speeds of, gas clouds backlit by more than 140,000 distant ultrabright galaxies called quasars. BOSS pinned down a figure — the universe was stretching by 1 percent every 44 million years — with more precision than estimates of the universe’s current expansion rate have.

Due to quasars’ extreme brightness, BOSS is the only expansion-rate survey able to probe the early universe, when gravity was slowing down the stretching of space-time triggered by the Big Bang. Scientists want to explore how, over time, gravity’s influence waned and dark energy took over, causing cosmic expansion to accelerate. Adding another layer of intrigue, BOSS’s measurement is slightly at odds with theoretical predictions, team physicist Andreu Font-Ribera of Lawrence Berkeley National Laboratory says.

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