# This is one of the corner-stones of the Hot Big Bang model.

N2 - Big bang nucleosynthesis (BBN) describes the production of the lightest nuclides via a dynamic interplay among the four fundamental forces during the first seconds of cosmic time. A brief overview of the essentials of this physics is given, and new calculations presented of light-element abundances through Li6 and Li7, with updated nuclear reactions and uncertainties including those in the neutron lifetime. Fits are provided for these results as a function of baryon density and of the number of neutrino flavors Nν. Recent developments are reviewed in BBN, particularly new, precision Planck cosmic microwave background (CMB) measurements that now probe the baryon density, helium content, and the effective number of degrees of freedom Neff. These measurements allow for a tight test of BBN and cosmology using CMB data alone. Our likelihood analysis convolves the 2015 Planck data chains with our BBN output and observational data. Adding astronomical measurements of light elements strengthens the power of BBN. A new determination of the primordial helium abundance is included in our likelihood analysis. New D/H observations are now more precise than the corresponding theoretical predictions and are consistent with the standard model and the Planck baryon density. Moreover, D/H now provides a tight measurement of Nν when combined with the CMB baryon density and provides a 2σ upper limit Nν

## The neutron lifetime can be well approximated by

### Big Bang nucleosynthesis - Wikipedia

But, as matter condenses out of the cooling Universe, a morestable epoch is entered, one where the slow, gentle force of gravitydominates over the nuclear forces of earlier times.Much of the hydrogen that was created at recombination was used up inthe formation of galaxies, and converted into stars.

### Big Bang Nucleosynthesis - UC Berkeley Astronomy w

gravity), so this time, and until today, is called thematter era.Today, radiation in the form of photons have a very passiverole in the evolution of the Universe.