Q:

Weak interactions in nucleosynthesis

During the Bing Bang nucleosynthesis the particles present in the universe are,  photons ($\gamma$), neutrinos ($\nu$), antineutrinos ($\overline{\nu}$), electrons($e^{+}$), positrons ($e^{+}$), neutrons($n$) and protons ($p$). And hey interact by the reactions
$$n \leftrightarrow p+e^{-}+\overline{\nu}    $$
$$\nu+n \leftrightarrow  p +e^{-}$$
$$e^{+}+n \leftrightarrow   p + \overline{\nu}$$
And the book that I am reading (Cosmology of Weinberg) says that the reactions between the particles are mediated by weak interactions. I don't understand why this happen? What happen with the other forces?
Maybe the temperature has something to do $T\approx 10 $ MeV

A:

Only the weak interaction can change down quarks into up quarks, and thus neutrons into protons. Under the electromagnetic, strong, and gravitational interactions, up quarks stay up quarks and down quarks stay down quarks.
This Feynman diagram, borrowed from Wikipedia's article "Free neutron decay", shows how the first process works. A down quark in the neutron turns into an up quark by emitting a $W^-$ boson, which then decays into an electron and an antineutrino.

