Present critical density of Universe rho_crit = 3 H^2 / 8 Pi G H = Hubble Constant = 2.2E-18 sec^-1 rho_crit = 8.6E-27 kg / m^3 What particle mass does this represent? The length scale associated with a quantum particle of mass m is the Compton wavelength lambda where lambda = h / m c This translates to the following density: rho_particle = mass / volume rho_particle = mass / lambda^3 rho_particle = m / (h / m c)^3 rho_particle = m^4 c^3 / h^3 Let us equate the two densities: rho_particle = rho_crit m^4 = h^3 * 8.6E-27 / c^3 m = 1.7E-38 kg m = 1 meV / c^2 The Cosmic Microwave Background has a photon energy of 1 meV. Surprising coincidence or not? I suppose this is not surprising if the mass and radiation energy at the recombination time were similar. Or maybe it is surprising as mass density is supposed to scale as R^-3 whereas radiation density scales as R^-4.