Taurus: A Balloon-Borne Polarimeter for Cosmic Reionization and Galactic Dust

The optical depth of the universe looking back to the epoch of last scattering expresses the probability that a photon from the cosmic microwave background (CMB) will scatter on its way to our instruments. Its value is sensitive to the timing and details of cosmic reionization by the first stars. Knowledge of this cosmological parameter is also crucial to precision measurements of others, notably future attempts to constrain the sum of neutrino masses via gravitational lensing of the CMB. Measuring this optical depth requires a high-fidelity map of CMB polarization on the very largest scales, a measurement that has proven challenging thus far due to Galactic foregrounds, Earth's atmosphere, and instrumental effects. We propose to develop and deploy Taurus, a mid-latitude super-pressure balloon mission to map the polarization of the microwave sky with fidelity at large angular scales. Large scale fidelity will be achieved by combining the minimal atmosphere of the balloon platform, an observing strategy designed to mitigate instrumental systematic errors, and sensitivity surpassing that of the Planck satellite. Taurus will measure the CMB and Galactic dust emission over about 70% of the sky in several bands, including high frequencies (>220 GHz) that have proven challenging from the ground. In combination with synchrotron maps from existing space- and ground-based instruments, Taurus will measure the universe's reionization depth to within a factor of 2 of the sample variance limit. Combined with the CMB lensing measurements and the large-scale structure data expected to be available in the next five years, Taurus will "measure the neutrino mass to a level of 0.05 eV, the lower limit implied by current neutrino mixing experiments", a goal stated in the last Decadal Survey of Astronomy and Astrophysics, and yet to be achieved. Taurus will also produce measurements of polarized Galactic dust emission of lasting value, both for modeling the interstellar medium and for foreground removal from future CMB data. In the final data release from the Planck collaboration, the value of the optical depth to reionization is insensitive to the addition of Planck data at the angular scales theoretically most sensitive to this parameter. This direct result of leftover instrumental systematics in the Planck data illustrates the lack of any sensitive measurement of the polarization of the CMB at the largest angular scales. Taurus will provide this measurement, enabling sensitive new tests of the standard cosmological model, and a new window into physics beyond the standard model.