held constant (roughly equivalent to holding the CMB temperature constant) and the curvature density parameter is fixed by the value of the other three. Apart from the Planck satellite, the Wilkinson Microwave Anisotropy Probe (WMAP) was instrumental in establishing an accurate age of the universe, though other measurements must be folded in to gain an accurate number. CMB measurements are very good at constraining the matter content and curvature parameter It is not as sensitive to directly, partly because the cosmological constant becomes important only at low redshift. The most accurate determinations of the Hubble parameter are currently believed to come from measured brightnesses and redshifts of distant Type Ia supernovae. Combining these measurements leads to the generally accepted value for the age of the universe quoted above. The cosmological constant makes the universe "older" for fixed values of the other parameters. This is significant, since before the cosmological constant became generally accepted, the Big Bang model had difficulty explaining why globular clusters in the Milky Way appeared to be far older than the age of the universe as calculated from the Hubble parameter and a matter-only universe. Introducing the cosmological constant allows the universe to be older than these clusters, as well as explaining other features that the matter-only cosmological model could not. WMAP NASA's Wilkinson Microwave Anisotropy Probe (WMAP) project's nine-year data release in 2012 estimated the age of the universe to be years (13.772 billion years, with an uncertainty of plus or minus 59 million years). However, this age is based on the assumption that the project's underlying model is correct; other methods of estimating the age of the universe could give different ages. Assuming an extra background of relativistic particles, for example, can enlarge the error bars of the WMAP constraint by one order of magnitude. This measurement is made by using the location of the first acoustic peak in the microwave background power spectrum to determine the size of the decoupling surface (size of the universe at the time of recombination). The light travel time to this surface (depending on the geometry used) yields a reliable age for the universe. Assuming the validity of the models used to determine this age, the residual accuracy yields a margin of error near one percent. Planck In 2015, the Planck Collaboration estimated the age of the universe to be billion years, slightly higher but within the uncertainties of