Ten Things Everyone Should Know About Inflation

These lecture notes are organized into ten lessons that summarize the status of inflationary cosmology. 1 Inflation is a Bold and Expansive Paradigm The hot big-bang cosmology is very successful. It provides a physical description of the Universe from about 10 sec onward [1]. However, it raises fundamental questions about initial conditions: The origin of the smoothness and flatness of our Hubble volume, the small (one part in 10) density inhomogeneities needed to seed all the structure seen in the Universe today, and the tiny baryon asymmetry that results in the existence of matter today. Inflation explains how a region of size much, much greater than our Hubble volume could have become smooth and flat [2] as well as the origin of the density inhomogeneities needed to seed structure [3]. With regard to the smoothness and flatness, inflation is a temporary fix: It does not guarantee that the observable Universe in the exponentially distant future will be isotropic and homogeneous [4]. Models of inflation are based upon well defined, albeit speculative physics – usually the semi-classical evolution of a weakly coupled scalar field. The physics is speculative because a) there is no evidence for the existence of even a single fundamental scalar field and b) the energy scale associated with inflation is typically much greater than 1TeV and in most models around 10GeV. I believe that it is fair to say that inflation has revolutionized the way cosmologists view the Universe. It leads to the current working hypothesis for an extension of the standard cosmology: The Inflation/Cold Dark Matter Paradigm. This paradigm has the potential to extend the standard cosmology back to times as early as 10 sec and address almost all the pressing questions in cosmology. The key elements of this paradigm are: flat Universe, nonbaryonic dark matter in the form of slowly moving elementary particles (cold dark matter), and nearly scale-invariant, adiabatic density perturbations. As I will emphasize, the inflation/cold dark matter paradigm is highly testable and a flood of observations are doing so. At the outside, within the next decade this paradigm will have been falsified or more firmly established. There are even grander implications of inflation, albeit very difficult to test [5]. Cosmologists have long used the Copernican principle to argue that the entire Universe must be smooth because of the smoothness of our Hubble volume. In the post-inflation view, our Hubble volume is smooth because it is a small part of a region that underwent inflation, and thus it need not reflect the large-scale features of the Universe as a whole. On the largest scales the structure of the Universe is likely to be very rich: Different regions may have undergone different amounts of inflation, beginning at different times; some regions may not have undergo inflation and may have collapsed to black holes; other regions may be governed by different realizations of the laws of physics because they evolved into different vacuum states of equivalent energy. It is likely that most of the volume of the Universe is still undergoing inflation and that inflationary patches are being constantly produced (eternal inflation). In this case, “the age of the Universe” is a meaningless concept: Our expansion age merely measures the time back to the end of our inflationary event. If inflation is correct, it will be a major advance in our understanding of the origin and evolution of our Hubble volume and it will open a new window on physics beyond