New study gives dark energy a boost

Measurements provide further evidence for accelerating expansion of universe

COSMIC YARDSTICK Precisely gauging the distance to the galaxy NGC 5584 and other celestial objects helped astronomers obtain the most precise value ever measured for the Hubble constant, an indicator of the present-day expansion rate of the universe. NASA, ESA, A. Riess, L. Macri (Texas A&M Univ.), Hubble Heritage Team (STScI/AURA)

New evidence bolsters the case that a bizarre form of energy is uniformly accelerating the expansion of the universe, refuting one of the alternative models developed by researchers who refuse to accept the idea.

The new study, which measures the present-day expansion of the universe to unprecedented accuracy, also suggests that the cosmos may be slightly older than previously calculated.
A team of astronomers used the Hubble Space Telescope’s new infrared camera to refine the Hubble constant — a number that indicates the current rate at which galaxies are receding from one another due to cosmic expansion. By precisely measuring the distance to various celestial objects and then gauging the speed at which they are receding from each other, the team measured a Hubble constant of 73.8 kilometers per second per megaparsec. That means that for every million parsecs (3.26 million light-years) separating two distant galaxies, they move apart 73.8 kilometers per second faster.
The value has an uncertainty of only 3.3 percent, Adam Riess of Johns Hopkins University and the Space Telescope Science Institute in Baltimore, Md., and his colleagues report in the April 1 Astrophysical Journal. That margin of error is about 30 percent better than the previous value, reported in 2008 (SN Online: 5/5/08). 
“The Hubble constant is the first and most important number in cosmology,” says Michael Turner of the University of Chicago. “It is hard to measure accurately.  Any improvement — and this is a real improvement — has broad implications.”
Although the Hubble constant measures only the current rate of cosmic expansion, given certain assumptions the new value implies that the universe is about 75 million years older than the previous estimate of 13.75 billion years, Riess says. 
Obtaining a precise value of the Hubble constant also places new restrictions on one alternative to “dark energy” as the driving force behind accelerated cosmic expansion, says Riess. In the alternative scenario, Earth and its environs would sit at the center of a vast void a few billion light-years across (SN: 6/7/08, p. 12). That configuration would produce an optical illusion making it appear as if the universe’s expansion is accelerating.
But such a setup would require a significantly lower value of the Hubble constant than the one Riess and his colleagues have now measured to high precision. Previous measurements of the constant were already at odds with the void model, but the added precision of the new study refutes the model conclusively, Riess says. Having Earth live at the center of a void “is a really weird model, but so is dark energy,” he notes. “It might be a matter of taste which model someone thinks is weirder, but now we have data to show that dark energy is favored.”   
The new measurement “makes life very difficult” for the void scenario, acknowledges one of its developers, Robert Caldwell of Dartmouth College. Turner notes that “while this does not completely rule out the ‘we are at the center of the universe’ alternative to dark energy, it does put a stake in its heart.”
In combination with data from NASA’s Wilkinson Microwave Anisotropy Probe, which surveys radiation left over from the Big Bang, the new results also suggest that dark energy is what physicists call the “cosmological constant.” It refers to a constant density of energy residing in the vacuum of space, suggested by Einstein as an addition to his equations of general relativity but later abandoned.

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