The findings, reported in The Astrophysical Journal, are based on 130 hours of observations made with the Large Binocular Telescope. By analyzing light from distant, primitive galaxies, the researchers gained a clearer view of conditions that existed shortly after the Big Bang.

This greater precision gives scientists a powerful new way to investigate the origins of the universe and test the Standard Model of physics. Such fundamental tests have a long history of advancing scientific understanding and contributing to technological progress.

“This is a massive physics experiment and one of the biggest discoveries of my entire 40-year career,” said Evan Skillman, Distinguished Professor in the Department of Physics and Astronomy in the University of Minnesota’s College of Science and Engineering. “This is a fundamental number that tells us specifically what the state of the universe was like during the first five minutes. It has diagnostic power that directly impacts the Standard Model of physics.”

Why primordial helium is a clue to the Big Bang

According to the Big Bang theory, the universe began expanding from an extremely hot, dense state about 13.8 billion years ago. Three major lines of evidence support this explanation: the continued expansion of the universe, cosmic microwave background radiation, and the quantities of light elements such as helium and deuterium formed during the universe’s earliest moments.

Cosmic expansion and the cosmic microwave background have been studied extensively, but measurements of primordial helium have historically received less attention. Determining the universe’s original helium abundance more precisely can reveal the physical conditions that existed when the cosmos was only minutes old.

To achieve this new level of precision, the researchers moved away from traditional methods of estimating primordial helium. Those methods typically involve observing large numbers of galaxies and extrapolating the results to determine how much helium existed before later generations of stars altered the universe’s chemical composition.

Instead, the team selected 15 very primitive, small, and distant galaxies. Because these galaxies have undergone relatively little chemical evolution, they serve as cosmic “time capsules,” preserving clues about the early composition of the universe.

Advanced spectrometer helps achieve record precision

The research team used the MODS spectrometer, developed at Ohio State University, to examine more than 10 helium spectral lines and 15 hydrogen spectral lines simultaneously. Studying these signals together allowed the researchers to identify and correct subtle sources of measurement error that had largely been overlooked in previous studies.

These small effects become especially important when scientists are trying to reduce uncertainty below 1%. Accounting for them was essential to achieving the new 0.5% precision.

“The MODS spectrometer took 12 years to build from concept to first light in the sky,” said Richard Poggi, Distinguished Professor of Astronomy in the College of Arts and Sciences at The Ohio State University. “This is the kind of project we designed them to do, and it’s very satisfying to see them come to fruition. It’s not every day you get to help build an instrument that measures something fundamental in our universe.”

Primordial helium offers a test of basic physics

The new measurement not only refines estimates of the universe’s oldest chemical composition; it also provides an important test of the Standard Model of physics. The amount of helium created immediately after the Big Bang depended on the physical conditions at the time, including the types of elementary particles present.

Using the precise helium measurement, the researchers were able to calculate the number of neutrino families in the early universe. Neutrinos are extremely light elementary particles whose behavior is important to scientists’ understanding of fundamental physics.

The result strengthens existing evidence for the Big Bang while providing a more accurate way to test whether established physical theories correctly describe the universe’s first minutes.

“We promised to bring the uncertainty of the proposal down to half a percent, and we delivered,” Skillman added. “In the unpredictable world of science, that rarely happens.”

The international collaboration brought together scientists from the University of Minnesota Twin Cities, The Ohio State University, Gonzaga University, Northwestern University, the University of Texas at Austin, Indiana University, the University of California at Santa Cruz, the University of Illinois, the National Autonomous University of Mexico, and TRIUMF.