compute. In order to differentiate between these scenarios, precise measurements of the Hubble parameter as a function of redshift are needed. Measured observables of dark energy The density parameter, , of various components, , of the universe can be expressed as ratios of the density of to the critical density, : The Friedman equation can be rewritten in terms of the density parameter. For the current prevailing model of the universe, ΛCDM, this equation is as follows: where m is matter, r is radiation, k is curvature, Λ is dark energy, and w is the equation of state. Measurements of the CMB from WMAP put tight constraints on many of these parameters; however it is important to confirm and further constrain them using an independent method with different systematics. The BAO signal is a standard ruler such that the length of the sound horizon can be measured as a function of cosmic time. This measures two cosmological distances: the Hubble parameter, , and the angular diameter distance, , as a function of redshift . By measuring the subtended angle, , of the ruler of length , these parameters are determined as follows: the redshift interval, , can be measured from the data and thus determining the Hubble parameter as a function of redshift: Therefore, the BAO technique helps constrain cosmological parameters and provide further insight into the nature of dark energy. See also Baryon Oscillation Spectroscopic Survey BINGO (telescope) Euclid (spacecraft) References External links Martin White's Baryon Acoustic Oscillations and Dark Energy Web Page Review of Baryon Acoustic Oscillations SDSS BAO Press Release Physical cosmology Baryons