Hi All

SM... lateral thinking as always but you're wrong.

Firstly, a radiation dominated Universe has a different equation of state to dark energy and different dynamics.

Secondly, nuclear fusion - chiefly hydrogen -> helium - converts a mere 0.007 of the hydrogen to energy. And the brightest stars fuse about 10% of their total mass of hydrogen - the rest doesn't reach the core.

Even converting hydrogen -> iron liberates a mere 0.009 of total mass. Any higher fusion reaction stages are endothermic and use energy - gravitational collapse [and rebound off a neutron star core] is needed to power core-collapse supernova NOT fusion. There might be energy pulled from the quantum vacuum by the collapse too, but not enough to do the job.

Adam