How would nucleosynthesis be different if the neutron were stable? If the strong nuclear force were just 2% stronger, the neutron would be a stable particle instead of having a half life of about 13 minutes. What difference would that have made to Big Bang nucleosynthesis, to the growth of structure, to the formation of stars, nucleosynthesis in stars?
 A: Assuming that the proton is heavier than the neutron, by more than the mass of the electron (plus the mass of a neutrino, plus the ionization energy of hydrogen), this is easy to answer, it would just make hydrogen unstable to decay to a neutron an an electron positron pair, so that a mostly hydrogen universe will decay into neutrons and electron-positron pairs, which wil annihilate into photons. So I will assume that the difference between neutron mass and proton mass is less than the mass of the electron, so that both the proton and the neutron are stable.
The most drastic effect of this is on big-bang nucleosynthesis, where two new stable species can be created, neutrons and tritium, and He-3 would be unstable to inverse beta-decay into tritium. So you would produce hydrogen, deuterium, tritium, helium, lithium, and neutrons. The initial conditions are mostly neutrons, not mostly protons, because the mass is inverted, and so you would get a lot of He-4, very little H-1, and most of the universe's mass would consist of stable neutrons and alpha-particles. These neutrons might collide to form neutron clusters, which would then beta-decay to protons once the binding energy was greater than the neutron proton mass difference.
There wouldn't be stars, but there might be gravitationally bound neutron clusters. Neutrons are neutral, and find it hard to dissipate energy, but the time scales are long, so they might be able to eventually settle down into neutron-star-like objects.
