So today I was solving a problem from my book. I even did not understand it. I saw the solution but even did not understand the solution, The question is,
"In 1959 Lyttleton and Bondi suggested that the expansion of the Universe could be explained if matter carried a net charge. Suppose that the Universe is made up of hydrogen atoms with a number density $N$, which is maintained a constant. Let the charge on the proton be: $ep = – (1 + y)e$ where $e$ is the electronic charge. (a) Find the critical value of $y$ such that expansion may start."
It was given in the solution that expansion starts if coulomb repulsion on a hydrogen atom, at $R$, is larger than gravitational attraction. Why does this gravitational attraction come into play? Also could someone explain me the question and solution thoroughly?