Most of the helium in the Universe formed in the first few minutes after the Big Bang, while stars added the rest over billions of years. Astronomers have now determined the primordial helium fraction more precisely than ever before. To do so, they studied gas in dwarf galaxies where matter has largely retained its original composition. The result agrees with theoretical predictions and the three known types of neutrinos.
A Quarter of Primordial Matter
Primordial helium accounted for 24.58% of the mass of ordinary matter. The theory of light-element nucleosynthesis, incorporating data from the Planck space observatory, predicts 24.67%. The difference is smaller than the measurement uncertainty. A series of papers from the project has been published in the peer-reviewed journal The Astrophysical Journal.
This comparison is meaningful because the amount of primordial helium depends on only three quantities: the density of ordinary matter, the lifetime of a free neutron, and the number of types of particles that traveled at nearly the speed of light during that era. The first two are known very precisely, so helium provides a test of the third.
The Contribution of Stellar Nucleosynthesis
Helium is measured in gas clouds ionized by young, massive stars. However, stars also produce helium themselves through nuclear fusion. According to The Ohio State University, approximately one-tenth of the helium in today’s Universe originated in stars.
To minimize the contribution of stellar helium, researchers select galaxies with very low metallicity. Astronomers call all elements heavier than helium “metals,” and their proportion in matter is known as metallicity. These elements form inside stars, so where there are few metals, there is also little stellar helium. The final value was determined using 15 gas clouds in 13 such galaxies, according to Universe Today.
Helium’s Infrared Line
The brightness of helium emission lines depends on both the temperature and density of the gas. Different combinations of these quantities produced similar spectra, keeping the uncertainty at around 1.3%. Infrared emission from helium at a wavelength of 1,083 nanometers helped resolve this problem. Its brightness depends most strongly on gas density, allowing the effects of temperature and density to be separated.
The observations were conducted with the Large Binocular Telescope in Arizona, which has two mirrors measuring 8.4 meters in diameter. Using the new spectral line and the same instruments for all galaxies reduced the uncertainty from 1.3% to approximately 0.5%.
Three Types of Neutrinos
Each additional type of such particles would have increased the energy density of the early Universe and accelerated its expansion. This would have left neutrons less time to turn into protons, resulting in more helium. The abundance of these particles is expressed in terms of the number of neutrino species.
Combined with data on the cosmic microwave background and primordial deuterium, the new measurement gives 2.925 ± 0.082 neutrino species. The Standard Model of particle physics predicts exactly three, and the result is consistent with this. The previous estimate had an uncertainty of 0.141, making the new one approximately 1.7 times more precise.
This level of precision is already sufficient to rule out one hypothesis involving new physics. If another unknown particle had existed in the early Universe and interacted strongly with the hot matter, it would have added approximately 0.57 to the number of neutrino species. An exception would be possible only if such a particle stopped interacting very early, before helium formed.
Next, the project’s authors will search for additional metal-poor galaxies in data from the DESI spectroscopic sky survey. The more galaxies they find with nearly primordial compositions, the less they will have to rely on assumptions about how much helium stars have added.
