Friedmann equations: Difference between revisions

This term originally was used as a means to determine the [[shape of the universe|spatial geometry]] of the universe, where {{math|''ρ''_c}} is the critical density for which the spatial geometry is flat (or Euclidean). Assuming a zero vacuum energy density, if {{mvar|Ω}} is larger than unity, the space sections of the universe are closed; the universe will eventually stop expanding, then collapse. If {{mvar|Ω}} is less than unity, they are open; and the universe expands forever. However, one can also subsume the spatial curvature and vacuum energy terms into a more general expression for {{mvar|Ω}} in which case this density parameter equals exactly unity. Then it is a matter of measuring the different components, usually designated by subscripts. According to the [[Lambda-CDM model|ΛCDM model]], there are important components of {{mvar|Ω}} due to [[baryon]]s, [[cold dark matter]] and [[dark energy]]. The spatial geometry of the [[universe]] has been measured by the [[Wilkinson Microwave Anisotropy Probe|WMAP]] spacecraft to be nearly flat. This means that the universe can be well approximated by a model where the spatial curvature parameter {{mvar|k}} is zero; however, this does not necessarily imply that the universe is infinite: it might merely be that the universe is much larger than the part we see. (Similarly, the fact that [[Earth]] is approximately flat at the scale of the [[Netherlands]] does not imply that the Earth is flat: it only implies that it is much larger than the Netherlands.)