  The observed angular power spectrum of the cosmic microwave background
temperature anisotropy, $C_l$, is a convolution of a cosmological radiative
transport kernel with an assumed primordial power spectrum of inhomogeneities.
Exquisite measurements of $C_l$ over a wide range of multipoles from the
Wilkinson Microwave Anisotropy Probe (WMAP) has opened up the possibility to
deconvolve the primordial power spectrum for a given set of cosmological
parameters (base model). We implement an improved (error sensitive)
Richardson-Lucy deconvolution algorithm on the measured angular power spectrum
from WMAP assuming a concordance cosmological model. The most prominent feature
of the recovered $P(k)$ is a sharp, infra-red cut off on the horizon scale. The
resultant $C_l$ spectrum using the recovered spectrum has a likelihood far
better than a scale invariant, or, `best fit' scale free spectra
($\Delta\ln{\cal L}=25$ {\it w.r.t.} Harrison Zeldovich, and, $\Delta\ln{\cal
L}=11$ {\it w.r.t.} power law with $n_s=0.95$). The recovered $P(k)$ has a
localized excess just below the cut-off which leads to great improvement of
likelihood over the simple monotonic forms of model infra-red cut-off spectra
considered in the post WMAP literature. The recovered $P(k)$, in particular,
the form of infra-red cut-off is robust to small changes in the cosmological
parameters. We show that remarkably similar form of infra-red cutoff is known
to arise in very reasonable extensions and refinements of the predictions from
simple inflationary scenarios. Our method can be extended to other cosmological
observations such as the measured matter power spectrum and, in particular, the
much awaited polarization spectrum from WMAP.
