Telescope Survey of 54 Galaxies Pins Big Bang Helium at 24.58% of the Universe's Mass
The Large Binocular Telescope team's result lands within a hair of Planck's cosmic microwave background figure. About 90% of the helium around today was made in the first minutes, and about 10% came from stars.
Astronomers have made a new measurement of how much helium the big bang produced, and it agrees closely with what the afterglow of the early universe predicts. A team using the Large Binocular Telescope found that primordial helium makes up 24.58% of the cosmos's initial matter by mass. The Planck satellite's observations of the cosmic microwave background gave 24.67%.
In the first moments after the big bang, two elements were forged: hydrogen and helium. Tiny traces of light elements such as lithium were produced too, but nothing of consequence, as Brian Koberlein wrote in Universe Today. The early universe was about three parts hydrogen to one part helium. Every heavier element, from the oxygen in your lungs to the iron in your blood, came later, made through astrophysical processes such as fusion in stars, supernovae, or collisions of neutron stars. Even now these heavier elements are so rare that astronomers lump them together as metals.
Astronomers study the big bang partly through how much hydrogen and helium it left behind. That sounds simple, but there is a catch: all the hydrogen we see was made in the big bang, while not all the helium was. Helium is also the most common product of stellar fusion, so over billions of years stars have been adding to the supply. That steadily raises the helium fraction and skews any measurement of the original amount.
The Large Binocular Telescope Primordial Helium Abundance project, known as LBT Yp, set out to get around the problem. It has published a series of results in The Astrophysical Journal, with papers led by Evan Skillman, Erik Aver and Tsung-Han Yeh. The team focused on measuring hydrogen and helium inside metal-poor galaxies. Those distant galaxies are made mostly of early stars and contain few heavy elements. The logic is that if a galaxy is low in metals, stars have not yet polluted its gas with much helium.
Using interstellar gas regions in a sample of 54 galaxies, the team arrived at 24.58% primordial helium by mass. That is within about 0.1 percentage points of the 24.67% Planck extracted from the cosmic microwave background. The agreement strengthens one of the three pillars of the big bang model.
Comparing the new value with local measurements of helium abundance, the team concluded that about 90% of cosmic helium is primordial and the other 10% was formed by stellar fusion. That split offers clues to the pace of star formation in galaxies, and it could help scientists better understand how different elements are made.
The result is a test passed rather than a surprise. Two completely different methods, one reading the light of gas in dwarf galaxies today and the other reading radiation released when the universe was young, land on nearly the same answer. When independent measurements of one number agree this well, theorists have less room to adjust the early universe's recipe, and any new physics has to leave the helium count intact.
