Modern cosmology is about the theoretical physics discribing the evolution of the entire universe all the way back to the beginning of time. This book starts with the basics of the Standard Big Bang Model, which contained a few minor problems with enormous consequences. The viewpoint of space and time was again revised to allow the universe to begin with a tiny fraction of time in which the universe went through a phase of enormous expansion; called the "Inflationary Phase". During this rapid expansion the very small sizes of quantum fluctuations were streched out to considerable density perturbations in the initial cosmic plasma. These regions of increased density have grown in time due to their own gravity, eventually leading to the formation of galaxies and stars. These seeds of todays universe can still be observed as fluctuations in the Cosmic Microwave Background Radiation; which is light of this initial cosmic plasma after a journey of 13.7 billion years arriving at our telescopes. This book gives a clear and structured way of predicting these fluctuations from inflationary quantum field theory, and their String Theory origin.