In the search for signals from the early universe, the Heidelberg scientist Prof. Dr Georg Wolschin deals with the question of whether and how residual spectral lines from the recombination phase with the formation of the first elements can be detected in the cosmic background radiation – which can be measured very precisely today. It is not yet possible to detect these recombination lines of hydrogen and helium atoms, which were emitted some 380,000 years after the Big Bang, but it could be possible in the future as measuring equipment improves. Prof. Wolschin, who researches at the Institute for Theoretical Physics of Heidelberg University, bases his proposition on a model of the Lyman-Alpha line, the strongest spectral line of hydrogen released during recombination. 

After the Big Bang, the early universe initially contains elementary particles and radiation. It isn’t until around 380,000 years later that a new phase of development follows with the emergence of the elements. Once the cosmos cools to a temperature of around 3,000 Kelvin, protons, deuterons and helium nuclei can combine with electrons to form hydrogen and helium. The spectral lines released during this process are superimposed on the cosmic background radiation released immediately after the Big Bang, which has a continuous spectrum. As the cosmos expands, the background radiation continues to cool and shifts to larger wavelengths, i.e. lower frequencies. Today, it can be detected very accurately in the microwave range as a cosmic microwave background (CMB) with a temperature of 2.725 Kelvin. Its first measurement happened in 1964 at a fixed frequency, and later it became possible to measure the mean value of the CMB spectrum very precisely, particularly during space missions. It agrees with that of a black body. Spatial fluctuations in the temperature distribution were also discovered. According to Prof. Wolschin, this makes it possible to draw far-reaching conclusions about structure formation in the early universe.

Science is also interested in exploring frequency fluctuations, which can be traced back to the radiation emitted during recombination. Georg Wolschin’s research deals with the Lyman-Alpha line, which can also be used to derive the long-range distribution of hydrogen in the universe. To find out whether this spectral line is detectable in today’s CMB spectrum, the scientist is modelling its temporal broadening and “attenuation” as well as how it shifts to lower frequencies as the cosmos expands. Based on his calculations in a non-linear model, the signal from the hydrogen Lyman-Alpha line is likely to be about seven orders of magnitude lower than the CMB measurement – too weak to be measured with today’s technology. 

“Detecting frequency modulations of the cosmic microwave background as remnants of the spectral lines of hydrogen released during recombination requires considerable experimental effort, i.e. new space missions with even more sensitive tools,” explains the Heidelberg scientist. “However, finding signals of this kind from the recombination epoch in the cosmic background radiation would mark a great success in cosmological research,” says Prof. Wolschin. The results of his study were published in the journal “Scientific Reports”.