Scientists have measured the amount of helium produced during the Universe's first five minutes with unprecedented precision, giving a clearer picture of the conditions that existed shortly after the Big Bang.
The findings, presented in five papers published in The Astrophysical Journal, rest on 130 hours of observations using the Large Binocular Telescope. By analysing distant galaxies, the researchers reduced the uncertainty in this fundamental measurement to just 0.5 per cent, making it three times more precise than previous estimates. The work was described in materials provided by the University of Minnesota and published by ScienceDaily.
Helium matters because of where it came from. According to the Big Bang theory, the Universe began expanding from an extraordinarily hot, dense state approximately 13.8 billion years ago. Three major lines of evidence support that account: the continuing expansion of the Universe, the cosmic microwave background radiation, and the amounts of light elements such as helium and deuterium that formed in its earliest moments.
Of the three, the light elements have had the least attention. Expansion and the microwave background have been studied extensively, while measurements of primordial helium have historically received less. Determining its original abundance more accurately allows researchers to investigate the physical conditions that prevailed when the Universe was only minutes old.
To reach the new precision the team abandoned the conventional approach, which involves observing large numbers of galaxies and extrapolating the results to establish how much helium existed before later generations of stars altered their chemical composition. Instead the researchers selected 15 exceptionally pristine, small, distant galaxies that have undergone relatively little chemical evolution, and which act as cosmic time capsules.
The instruments did the rest. Using spectrographs developed at Ohio State University, the team examined more than 10 helium spectral lines and 15 hydrogen lines simultaneously, which made it possible to identify and correct subtle sources of measurement error that earlier studies could largely overlook. Such small effects become critical once uncertainty is being pushed below one per cent.
"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 Distinguished Professor in the School of Physics and Astronomy at the University of Minnesota's College of Science and Engineering. "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 hardware was a long time coming. "The MODS spectrographs took 12 years to build from conception to first light on sky," said Richard Pogge, a 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."
Beyond refining the early chemical inventory, the measurement is a test of fundamental physics. The amount of helium created shortly after the Big Bang depends on the physical conditions at the time, including the types of elementary particles present, and from their figure the researchers were able to calculate the number of neutrino families in the early Universe.
"We promised a half-percent uncertainty in our proposal, and we got there," Skillman added. "In the unpredictable world of science, that doesn't happen very often." The collaboration brought together scientists from the University of Minnesota Twin Cities, Ohio State University, Gonzaga University, Northwestern University, the University of Texas at Austin, Indiana University, the University of California Santa Cruz, the University of Illinois, the Universidad Nacional Autónoma de México and TRIUMF.
It is worth comparing the durations involved. The subject of the research lasted five minutes. The observing campaign took 130 hours, and the instruments that made it possible took 12 years to build.
The first five minutes of the Universe have therefore now been looked at for roughly 1,560 times their own length, through apparatus that took rather longer than that again to assemble.
The precision figure can be put to work as well. Half a per cent of five minutes is one and a half seconds. The Department of Unnecessary Research is pleased to report that 298.5 seconds of the early Universe are now settled, and that one and a half seconds remain outstanding.
The margin was also agreed in advance, which this newspaper regards as the most impressive detail in the whole collaboration. The team promised half a per cent in the proposal and delivered half a per cent, making these the only five minutes in the history of the Universe to have had their error bars negotiated before they were examined.
The remaining second and a half is not expected to be resolved by the original observers, one of whom has already given a 40-year career to the other 298.5.