Roger Penrose has a theory about a cyclical conformal universe, which requires that the universe eventually only contains photons. As photons travel at the speed of light, they do not experience the passage of time, so they also do not experience distance (space). So in the deep remote future, when only photons are left, the difference between the very large and miniscule disappears, and you get all photons in the universe existing at the same point at the same time, which causes a new big bang. At least, that is Penrose's theory. I believe they found some evidence in the CMB supporting it.