  The Probe of Inflation and Cosmic Origins (PICO) is an imaging polarimeter
that will scan the sky for 5 years in 21 frequency bands spread between 21 and
799 GHz. It will produce full-sky surveys of intensity and polarization with a
final combined-map noise level of 0.87 $\mu$K arcmin for the required
specifications, equivalent to 3300 Planck missions, and with our current
best-estimate would have a noise level of 0.61 $\mu$K arcmin (6400 Planck
missions). PICO will either determine the energy scale of inflation by
detecting the tensor to scalar ratio at a level $r=5\times 10^{-4}~(5\sigma)$,
or will rule out with more than $5\sigma$ all inflation models for which the
characteristic scale in the potential is the Planck scale. With LSST's data it
could rule out all models of slow-roll inflation. PICO will detect the sum of
neutrino masses at $>4\sigma$, constrain the effective number of light particle
species with $\Delta N_{\rm eff}<0.06~(2\sigma)$, and elucidate processes
affecting the evolution of cosmic structures by measuring the optical depth to
reionization with errors limited by cosmic variance and by constraining the
evolution of the amplitude of linear fluctuations $\sigma_{8}(z)$ with
sub-percent accuracy. Cross-correlating PICO's map of the thermal
Sunyaev-Zeldovich effect with LSST's gold sample of galaxies will precisely
trace the evolution of thermal pressure with $z$. PICO's maps of the Milky Way
will be used to determine the make up of galactic dust and the role of magnetic
fields in star formation efficiency. With 21 full sky legacy maps in intensity
and polarization, which cannot be obtained in any other way, the mission will
enrich many areas of astrophysics. PICO is the only single-platform instrument
with the combination of sensitivity, angular resolution, frequency bands, and
control of systematic effects that can deliver this compelling, timely, and
broad science.
