“It is the most fundamental number in cosmology,” said Adam Riess, 38, an astronomer at the Space Telescope Science Institute and Johns Hopkins University, who discovered 10 years ago that “dark energy” is speeding up the expansion of the universe. This spring, Riess announced that he and his comrade, Lucas Macri of the University of Texas, had used the Hubble Space Telescope to make the most precise measurement yet of this parameter.
Hubble constant, Riess reported, is 74 km per second per megaparsec. It means that for every additional million parsecs (about 3.26 million light-years) a galaxy is from us, it is going 74 km per second faster.
“I’m not saying we’re going to get to 1 per cent,” Riess said, “but we might.” “I think Adam’s work is nice,” said Freedman, who has led a large space telescope effort to measure the constant. But she and others added that some parts of Riess’ scheme could be vulnerable to systematic errors.
The stakes are bigger than just dark energy. Cosmologists would like to know if the universe is in fact 13.7 billion years old, full of dark matter and dark energy, and speckled with galaxies that grew by gravity from random microscopic fluctuations in the Big Bang. That universe is described mathematically by half a dozen fundamental parameters, from which the Hubble constant can be calculated. But to test the model “at a physically interesting level”—in the words of John Huchra of the Harvard-Smithsonian Center for Astrophysics—the Hubble constant needs to be actually measured to high accuracy.
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