International Team Measures Early-Universe Helium With Record Precision

An international team of astronomers, including researchers from the University of Minnesota Twin Cities, has measured the amount of helium created in the universe's first five minutes with unprecedented precision — a milestone in what scientists call "precision cosmology." The findings appear in five papers in The Astrophysical Journal.

Using 130 hours of observation on the Large Binocular Telescope, the team cut the uncertainty in this cosmic value to 0.5%, three times better than before. The result bears on the Big Bang theory — that the universe expanded from an extremely dense, hot state about 13.8 billion years ago — which rests on three pillars: the universe's expansion, the cosmic microwave background, and the abundance of light elements like helium and deuterium. Helium had not been measured as precisely as the other two.

Instead of measuring many galaxies and extrapolating an initial helium level, the team focused on 15 of the most chemically "pristine," small, remote galaxies found — time capsules preserved almost as they were shortly after the Big Bang. Using spectrographs built at Ohio State University, researchers analyzed more than 10 helium and 15 hydrogen lines, accounting for small effects that matter only at sub-percent precision.

"This is a physics experiment on a grand scale and one of the biggest findings in my entire 40-year career," said Evan Skillman, a physics professor at the University of Minnesota. "It's a fundamental number that tells us specifically about the conditions of our universe in its first five minutes." Richard Pogge, an astronomy professor at Ohio State, said the MODS spectrographs took 12 years to build. The team also used the helium value to calculate the number of neutrino families — the lightest subatomic particles — in the early universe.

The team also included researchers from eight other universities and institutes in the U.S., Mexico and Canada. "We promised a half-percent uncertainty in our proposal, and we got there," Skillman said.