After completing his Ph.D., Weinberg worked as a postdoctoral researcher at Columbia University (1957–59) and University of California, Berkeley (1959) and then was promoted to faculty at Berkeley (1960–66). He did research in a variety of topics of particle physics, such as the high energy behavior of quantum field theory, symmetry breaking, pion scattering, infrared photons and quantum gravity (soft graviton theorem). It was also during this time that he developed the approach to quantum field theory described in the first chapters of his book The Quantum Theory of Fields and started to write his textbook Gravitation and Cosmology, having taken up an interest in general relativity after the discovery of cosmic microwave background radiation. He was also appointed the senior scientist at the Smithsonian Astrophysical Observatory. The Quantum Theory of Fields spanned three volumes and over 1,500 pages, and is often regarded as the leading book in the field.