in the universe, is the scale factor (essentially the 'size' of the universe), and is the curvature parameter — that is, a measure of how curved spacetime is. A positive, zero or negative value of corresponds to a respectively closed, flat or open universe. The constants and are Newton's gravitational constant and the speed of light, respectively. Cosmologists often simplify this equation by defining a critical density, . For a given value of , this is defined as the density required for a flat universe, i.e. . Thus the above equation implies . Since the constant is known and the expansion rate can be measured by observing the speed at which distant galaxies are receding from us, can be determined. Its value is currently around . The ratio of the actual density to this critical value is called Ω, and its difference from 1 determines the geometry of the universe: corresponds to a greater than critical density, , and hence a closed universe. gives a low density open universe, and Ω equal to exactly 1 gives a flat universe. The Friedmann equation, can be re-arranged into which after factoring , and using , leads to The right hand side of the last expression above contains constants only and therefore the left hand side must remain constant throughout the evolution of the universe. As the universe expands the scale factor increases, but the density decreases as matter (or energy) becomes spread out. For the standard model of the universe which contains mainly matter and radiation for most of its history, decreases more quickly than increases, and so the factor will decrease. Since the time of the Planck era, shortly after the Big Bang, this term has decreased by a factor of around and so must have increased by a similar amount to retain the constant value of their product. Current value of Ω Measurement The value of Ω at the present time is denoted Ω0. This value can be deduced by measuring the curvature of spacetime (since , or , is defined as the density for which the curvature ). The curvature can be inferred from a number of observations. One such observation is that of anisotropies (that is, variations with direction - see below) in the Cosmic Microwave Background (CMB) radiation. The CMB is electromagnetic radiation which fills the universe, left over from an early stage in its history when