Moments after our Universe was born in a hot Big Bang, space-time ‘inflated’ in a rapid, exponential expansion. Inflation is a cornerstone of modern cosmology, explaining the extreme smoothness of the cosmic microwave background (CMB) and the geometric flatness of the Universe. While the basic predictions of inflation have been verified in increasingly precise measurements, the physical process driving inflation remains elusive. The answer surely lies in the unknown and fundamental physics of space, time and matter. Astrophysical observations provide the best constraints on the nature of inflation, such as whether one field or multiple fields were responsible for inflation. SPHEREx, a mission in NASA's Medium Explorer (MIDEX) program that was selected for Phase A in August 2016, is an all-sky survey satellite designed to probe the origin and destiny of our Universe; but also to explore whether planets around other stars could harbor life; and explore the origin and evolution of galaxies. These themes are addressed by a single survey, with a single instrument. After detailing the underlying physical motivations, I will explain how SPHEREx will measure the large-scale distribution of galaxies and determine the degree of statistical ‘non-Gaussianity’ of the distribution of inflationary ripples which depends on the nature of the field(s) producing inflation. At the same time, this catalog will enable strong scientific synergies with Euclid and WFIRST.