Hi guys, I have a question about the mass of the universe, and inflation/dark energy. As I am sure you know, inflation is supposed to stretch the universe so that the visible universe is only a small part of the true size of the universe. This guarantees that the visible universe is flat. However, and I quote from "Relativity, Gravitation, and Cosmology" by Ta-Pei Cheng, "The Friedmann equation requires a flat universe to have a mass/energy density exactly equal to the critical density. Yet observationally, including both the baryonic and dark matter, we can only find less than a third of this value. Thus it appears that to have a flat universe we would have to solve a missing energy problem." As a result, it is suggested that the value of dark energy must be 75% of all the mass of the universe. But surely inflation guarantees a flat universe, even if the visible universe only includes a grain of sand? How can cosmologists accept the inflation hypothesis, and then say we need more dark energy to make a flat universe? Thanks a lot.