Big bang nucleosynthesis as a probe of new physics

The remarkable agreement between the observed light-element 
abundances and the predictions of big bang nucleosynthesis (BBN) makes 
BBN a powerful probe of physics beyond the Standard Model (BSM). In this 
talk, I will present the BBN constraints on a specific BSM particle, the 
Majoron, and discuss its cosmological implications. I will also 
highlight the caveats of the standard BBN scenario in light of the 
long-standing cosmological lithium problem and briefly outline a 
possible solution.