  These four lectures cover four topics in modern cosmology: the cosmological
constant, the cosmic microwave background, inflation, and cosmology as a probe
of physics at the Planck scale. The underlying theme is that cosmology gives us
a unique window on the ``physics of nothing,'' or the quantum-mechanical
properties of the vacuum. The theory of inflation postulates that vacuum
energy, or something very much like it, was the dominant force shaping the
evolution of the very early universe. Recent astrophysical observations
indicate that vacuum energy, or something very much like it, is also the
dominant component of the universe today. Therefore cosmology gives us a way to
study an important piece of particle physics inaccessible to accelerators. The
lectures are oriented toward graduate students with only a passing familiarity
with general relativity and knowledge of basic quantum field theory.
