Spectrum of the Cosmic Microwave Background: Results and Prospects

Abstract

The precise measurement of the long-wavelength spectrum is the last unexplored frontier for the cosmic microwave background (CMB). The Cosmic Background Explorer (COBE) showed that the CMB spectrum follows a blackbody form at millimeter and sub-mm wavelengths. Balloon-borne and ground-based measurements at longer wavelengths lack sufficient precision to measure likely spectral distortions and provide only a general confirmation of the blackbody shape: distortions as large as 5% could exist at wavelengths of several centimeters or longer without violating existing observations. Measurements at centimeter wavelengths probe different physical processes than the COBE results at shorter wavelengths. Planned measurements offer the prospect of measuring the long-wavelength free-free emission from the primeval plasma to establish the epoch and nature of reionization in the early universe.