Primordial light element abundances

After few minutes the Universe evolved through conditions of temperature and density which permitted the first synthesis of astrophysically interesting abundances of D, He, He and Li. The relic abundances are sensitive probes of the nucleon density and so are the CMB acoustic oscillations, somewhat 400000 years later, which allow a stringent cross check. The CMB high precision estimate of the baryon density by WMAP is currently used as input parameter for standard big bang nucleosynthesis (SBBN) to interpret primordial abundances rather then being directly derived from the observations of light elements as was common use before. New atomic physics and identification of systematics lead to an upwards revision of the He primordial abundance at Yp=0.2477±0.0029 (Peimbert et al 2007) removing a major source of tension between SBBN and WMAP. The D/H as measured in QSO high redshift absorbing clouds shows an excess of scatter but the mean value is found in spectacular agreement with the WMAPΩb prediction. The Li/H recently redetermined in halo dwarfs is more than a factor 4 lower than expected. We argue that the difference reduces to a factor 2 when the IRFM Teff scale is adopted. Diffusion has been suggested to have depleted Li in halo dwarfs by the required amount to remove the gap, however this would imply an implausible high abundance of the more fragile Li detected in some halo dwarfs, thus leaving the puzzle open. 1. John Beckman and the light elements The organizers asked me to review the status of the light elements, which is one of J.E. Beckman’s interests although a bit aside the main theme of this unique and enjoyable conference. John’s interest on light elements dates back to the 80’s at the epoch of his definitive move to IAC. The start was a search for BeII lines in HD76932 by means of IUE spectra, a star belonging to the first sample of halo stars were Francois and Monique Spite observed the primordial Li. The upper limit provided for Be showed that spallation processes of high energy cosmic rays were not an alternative to primordial nucleosynthesis to make up for all or a fraction of the Li observed in the star, thus supporting its primordial origin (Molaro & Beckman 1984). This work was followed by the first Be detections in halo stars by means of the Image Photon Counting System at the Isaac Newton Telescope on La Palma and reported in Rebolo et al (1988a). In the same years a Li observational campaign was conducted from La Palma with INT and first results presented at the second IAP workshop in Paris (Beckman et al 1986), and then in a more definitive shape in Rebolo et al (1988b). This was the first confirmation of the primordial Spite and Spite halo plateau made by and independent group, significantly enlarging the sample of halo stars in number