Abstract: Our Universe is big, old, and full of structures. All of them are big mysteries in the classical Big Bang Cosmology. Inflationary cosmology, which postulates accelerated expansion in the early Universe, explain all of them. Inflation is driven by a quantum field, whose quantum fluctuations are stretched to fill the entire Universe. Their trace is observed in the Cosmic Microwave Background (CMB), and they also yield density fluctuations to form galaxies etc. Furthermore, where the fluctuations were large, they may have formed primordial black holes and may have been the origin of the black holes found by LIGO and Virgo. We have calculated not only the lowest-order perturbations, but also the loop effects, and found that the distribution of fluctuations must be highly Gaussian, and that the formation of primordial black holes would be inconsistent with the CMB.