Observational Cosmology - Unit 3 Solutions
7. Heavier elements require higher temperatures to synthesise them (since there is a larger Coulomb barrier to be overcome), but the temperature of the Universe is rapidly dropping as it expands. By the time He has been synthesised, the temperature has simply dropped too low for reactions to proceed much further. In addition, the lack of any stable nuclide with A=8 causes a delay in the reaction path beyond Li.