 We define the critical density of the universe as the density that would give us flat space. Any more than this, and we have a closed universe. Any less than this, and we have an open universe. The flat matter-dominated version is called the Einstein Decider Universe after the scientists that developed the model. This critical mass density comes to around five protons per cubic meter. The actual density of interstellar space is on the average of about one proton per cubic centimeter. That's a million times denser than the critical density. But much of the universe is made up of vast voids with far less than this. So five protons per cubic meter could be the number we actually have. Cosmologists like to work mostly with ratios. In this case, we have the ratio of the energy density over the critical density called the density parameter omega. Current measurements have it at very close to one. It is the sum of all forms of energy that fill the universe. At this point in our analysis, we have three components that add up to one. Radiation, mass, and curvature.