Deuterium

I also work with Signe Riemer-Sorensen on a primeval deuterium-to-hydrogen abundance measurement using absorption features in spectra from a high-redshift quasar.

Why is the deuterium interesting in cosmology? In the aftermath of the big bang, in the short time after energy became matter, heavier particles than hydrogen ions (protons) were formed. This period in the cosmological timeline is called the big bang nucleosynthesis.

So what is it that actually happens when heavier (than hydrogen) atoms are formed? The process is called fusion. The second lightest element in the periodic table is helium, but hydrogen atoms cannot fuse directly and become helium, they need the intermediate hydrogen isotope deuterium, where the nucleus comprises a proton and a neutron.

But in the early universe, the temperature was rather high (it was just after the big bang, after all), and it was high enough to break up the protons and neutrons in deuterium nuclei (more precisely were the photons' energy higher than the binding energy of deuterium).

So, to get helium in the universe, we need the universe to have cooled enough to allow deuterium to exist. How much it must have cooled, depends on the number of photons there were back then. Measurements of the deuterium abundance between us and far-away light sources (QSOs or quasars) can reveal the ratio between the number of photons to the number of baryons (which protons and neutrons are) in the early universe.

By comparing the amount of hydrogen and deuterium in gas clouds (who cause absorption lines in spectra from quasars), I get an indication of the primeval deuterium abundance, and hence also an indication of the cosmological baryon abundance.

This web page was last updated in 2015.