Cosmology is undergoing an explosive burst of activity, fueled both by new, accurate astrophysical data and by innovative theoretical developments. Cosmological parameters such as the total density of the universe and the rate of cosmological expansion are being precisely measured for the first time, and a consistent standard picture of the universe is beginning to emerge. This is exciting, but why talk about astrophysics at a school for particle physicists? The answer is that over the past twenty years or so, it has become evident that the the story of the universe is really a story of fundamental physics. I will argue that not only should particle physicists care about cosmology, but you should care a lot. Recent developments in cosmology indicate that it will be possible to use astrophysics to perform tests of fundamental theory inaccessible to particle accelerators, namely the physics of the vacuum itself. This has proven to be a surprise to cosmologists: the old picture of a universe filled only with matter and light have given way to a picture of a universe whose history is largely written in terms of the quantum-mechanical properties of empty space. It is currently believed that the universe today is dominated by the energy of vacuum, about 70% by weight. In addition, the idea of inflation postulates that the universe at the earliest times in its history was also dominated by vacuum energy, which introduces the intriguing possibility that all structure in the universe, from superclusters to planets, had a quantum-mechanical origin in the earliest moments of the universe. Furthermore, these ideas are not idle theorizing, but are predictive and subject to meaningful experimental test. Cosmological observations are providing several surprising challenges to fundamental theory.
These lectures are organized as follows. Section 2 provides an introduction to basic cosmology and a description of the surprising recent discovery of the accelerating universe. Section 3 discusses the physics of the cosmic microwave background (CMB), one of the most useful observational tools in modern cosmology. Section 4 discusses some unresolved problems in standard Big-Bang cosmology, and introduces the idea of inflation as a solution to those problems. Section 5 discusses the intriguing (and somewhat speculative) idea of using inflation as a "microscope" to illuminate physics at the very highest energy scales, where effects from quantum gravity are likely to be important. These lectures are geared toward graduate students who are familiar with special relativity and quantum mechanics, and who have at least been introduced to general relativity and quantum field theory. There are many things I will not talk about, such as dark matter and structure formation, which are interesting but do not touch directly on the main theme of the "physics of nothing." I omit many details, but I provide references to texts and review articles where possible.