Skillman and Olive Measure the Early-Universe Helium

Evan Skillman, of the Minnesota Institute for Astrophysics, and Keith Olive of the Fine Theoretical Physics Institute, are part of The Large Binocular Telescope Primordial Helium Project which used precise helium measurements of the first five minutes after the Big Bang to provide useful constraints for cosmologists on possible physics beyond the standard model. John H. Miller, a graduate student in the School was also a co-author.

The series of five papers published in The Astrophysical Journal details how the team was able to leverage 130 hours of observation time on the Large Binocular Telescope to reduce the uncertainty of this fundamental cosmic value to just 0.5% — three times better than previous standards.

“This is a physics experiment on a grand scale and one of the biggest findings in my entire 40-year career,” said Evan Skillman, University of Minnesota College of Science and Engineering Distinguished Professor in the School of Physics and Astronomy. “It’s a fundamental number that tells us specifically about the conditions of our universe in its first five minutes. It has diagnostic power that speaks directly to the Standard Model of Physics.”

The Big Bang Theory shows how the universe expanded from an extremely high-density and high-temperature state approximately 13.8 billion years ago. The theory rests on three pillars: expansion of the universe, cosmic microwave background and the abundance of light elements like helium and deuterium. The last pillar — helium — hasn’t been studied and measured as extensively as the previous two pillars.

To reach this level of precision, the team moved away from traditional extrapolation methods — estimating an unknown value by extending a trend line of known data — which requires measuring many galaxies and estimating the initial helium level. Instead, they focused on 15 of the most "pristine" small, remote galaxies ever discovered. These galaxies are so chemically unevolved that they act as “time capsules,” preserved almost exactly as they were shortly after the Big Bang.

Using Multi Object Double Spectrographs built at Ohio State University, the researchers analyzed more than 10 helium lines and 15 hydrogen lines simultaneously. This allowed them to account for small systematic effects that were previously deemed negligible but become critical when aiming for sub-percent accuracy.

"The MODS spectrographs took 12 years to build from conception to first light on sky,” said Richard Pogge, College of Arts and Sciences Distinguished Professor of Astronomy at Ohio State University. “This is the kind of project we designed them to do, and to see them deliver is enormously satisfying. It’s not everyday you can help build instruments that measure something fundamental about our universe."

The precise measurement not only confirms history, but provides a precise look at the Standard Model of Physics. By determining the exact amount of helium, the team was able to calculate the number of neutrino families — the lightest subatomic particles — present in the early universe. 

“We promised a half-percent uncertainty in our proposal, and we got there,” added Skillman. “In the unpredictable world of science, that doesn’t happen very often.” 

In addition to the University of Minnesota Twin Cities and The Ohio State University, the team included researchers from Gonzaga University, Northwestern University, The University of Texas at Austin, Indiana University, University of California Santa Cruz, University of Illinois, Universidad Nacional Autónoma de México and TRIUMF.