Why Is the Temperature of the Universe 2.726k?

NASA’s Cosmic Background Explorer (COBE) Satellite has recently made the most accurate measurement of the temperature of the Universe determining it to be 2.726± 0.01K. In trying to understand why the temperature has this value, one is led to discover the most fundamental features of the Universe—an early, radiation-dominated epoch, enormous entropy per nucleon, synthesis of the light elements around three minutes after the bang, and a small excess of matter over antimatter—as well as some of the most pressing issues in cosmology today, the development of structure in the Universe and the identification of the nature of the ubiquitous dark matter. 1 The Cosmic Background Radiation The existence of the cosmic background radiation (CBR) is one of the cornerstones of the standard cosmology, or hot big-bang model [1]. Indeed, its very existence provides the evidence that the Universe began from a hot state [2]. The temperature of the cosmic background radiation has recently been measured to unprecedented precision by the Far InfraRed Absolute Spectrophotometer (FIRAS) instrument on NASA’s Cosmic Background Explorer (COBE) Satellite [3]: T0 = 2.726K± 0.01K; (1) the FIRAS results are shown in Fig. 1. Measurements of the CBR temperature, made over the period of almost thirty years since its discovery by Penzias and Wilson [4], now span almost three and half decades in wavelength, from about 0.04 cm to 70 cm, and are all consistent with the COBE temperature. Deviations from a perfect blackbody spectrum are less than 0.03% over the wavelengths probed by COBE, 0.05 cm − 0.5 cm [3]. The CBR is probably the most well studied and best black body known; indeed, the COBE collaboration plans to use their data to test the form of the Planck Law itself [5]. With a number density of 411 cm the photons in the CBR by a wide margin account for most of the (known) particles in the Universe, outnumbering atoms by a factor of around a billion. The surface of last scattering for the CBR is the Universe itself at an age of a few 100,000 years (see Fig. 2), and thus the CBR provides a fossil record of the infant Universe. As such its every property has been studied—spectrum, polarization, and spatial isotropy—revealing important information about the evolution of the