Other Questions

Why is it that the cosmological constant and the matter density today are of the same order \Lambda \sim\#\rho ? This is a strange coincidence, given that \Lambda \sim \# \rho , \rho changes with time as the universe expands, but \Lambda , being a constant of nature, should not change. That means that the rough equivalence between \Lambda and \rho is only true for the time we live in now–a confounding situation known as the “coincidence problem.” It would suggest that we are now occupying a special time in cosmic history–a proposition that many physicists find unsettling.