Observations of primordial abundances

The primordial abundances of the light elements are not measured easily and simultaneously. The main difficulties come from systematic uncertainties in inferring abundances from observations and in modeling their chemical evolution since the Big Bang. 

Helium-4 can be observed in galactic and extragalactic HII regions - regions of hot and ionized gas - using either optical or radio recombination lines. However the best determinations come from observations of Hell — Hel recombination lines in extragalactic HII regions. 

Deriving an abundance from the observed lines should be straightforward. However, corrections must be applied to compensate for excitation effects. 

In stars, most of the hydrogen is converted into helium-4 which is converted into heavier elements. Then, an excess of helium-4 can return into the interstellar medium. Therefore, one must account for a possible helium enrichment. 

There is a lot of recent and independent observations and analyses leading to some values of Yv. In particular, the value inferred by Izotov and Thuan (1998) is  

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The paper is organized as follows in section 2, we briefly review the geometry and dynamics of the Universe and then give the Einstein-Friedman-Lemaitre (hereafter EEL) equations section 3 introduces some important quantities needed for observations in section 4, we rapidly present some solutions of the EEL equations, i.e. some cosmological models in section 5, the Standard Big Bang Nucleosynthesis Model is described while section 6 shows a statement of observations of primordial abundances in section 7, we confront the predictions of the Standard Big Bang Nucleosynthesis (hereafter SBBN) model to the observations of the primordial abundances a brief conclusion is... 

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