Dark Energy and Its Implications for Gravity

1 It is convenient to measure the energy densities of the different species, which drive the expansion of the universe, in terms of this critical density using the dimensionless parameters Ω i = ρ i /ρc (with i denoting the different components like baryons, dark matter, radiation, etc.)The critical energy density is defined as ρc = 3H 2 0 /8πG where H 0 = ȧ/a is the expansion rate of the universe.