CMBFIT: Rapid WMAP likelihood calculations with normal parameters

We present a method for ultrafast confrontation of the Wilkinson Microwave Anisotropy Probe (WMAP) cosmic microwave background observations with theoretical models, implemented as a publicly available software package called CMBFIT, useful for anyone wishing to measure cosmological parameters by combining WMAP with other observations. The method takes advantage of the underlying physics by transforming into a set of parameters where the WMAP likelihood surface is accurately fit by the exponential of a quartic or sextic polynomial. Building on previous physics based approximations by Hu et al., Kosowsky et al., and Chu et al., it combines their speed with precision cosmology grade accuracy. A FORTRAN code for computing the WMAP likelihood for a given set of parameters is provided, precalibrated against CMBFAST, accurate to $\Delta \ln \mathcal{L}\sim 0.05$ over the entire $2\sigma$ region of the parameter space for 6 parameter “vanilla” $\Lambda \mathrm{CDM}$ models. We also provide 7-parameter fits including spatial curvature, gravitational waves and a running spectral index.