The Cosmic Microwave Background as a Cosmological Probe

Observations of the Cosmic Microwave Background (CMB) have already proven to be a powerful probe of physical conditions in the early universe (z ∼ 10). These observations have made precision tests of the standard cosmological model and appear to confirm that we live in a bizarre universe mostly composed of dark matter and dark energy. Over the next several years, new experiments will make high resolution measurements of the microwave background. These measurements use the microwave background as a “backlight” for studying the universe at low redshift. The scientific value of these high resolution CMB observations will be significantly enhanced by cross-correlating these observations with optical, radio and X-ray observations. 1. Standard Cosmology Model: Answers and Questions Recent observations have established a new standard model of cosmology. With only five basic parameters (the age of the universe, the density of matter, the density of atoms, the amplitude of primordial fluctuations and their scale dependence), this model fits both microwave background observations of the physical conditions in the early universe and observations of the large-scale distribution of galaxies (Spergel et al. 2003). While remarkably simple, the new standard cosmological model is also rather bizarre. It implies that protons, neutrons and electrons compose only 5% of the energy density of the universe. Cosmologists believe that most of the mass in the universe is composed of weakly interacting subatomic particles, the so called “dark matter”, which has never been directly detected. We also believe that all of the matter comprises only 25% of the total energy density of the universe, with the balance comprised of some kind of “dark energy” associated with empty space. We do not even understand the distribution of the ordinary matter. In the local universe, observed stars, gas and dust appear to account for only a small fraction of the total density in baryons (Fukugita et al. 1998; Fukugita & Peebles 2004). As is often true in science, answering old questions such as: “What is the shape of the universe?” “What is the age of the universe?” and “What seeds galaxy formation?” has led to new questions: “What is the dark energy?” “What is the dark matter?” and “How do galaxies emerge from fluctuations in the early universe?” This talk will focus on the role of microwave background experiments to address these questions. My talk will both describe recent results from WMAP and look forward to the next generation of small scale experiments that will probe the CMB sky with higher angular resolution.