Chapter 14
They looked for dung but found gold, which is just the opposite of the experience of most of us.
Ivan Kaminov, on Penzias and Wilson's discovery


After a brief discussion of one of the classic questions of historical cosmology, Olbers' paradox, this chapter describes the cosmic background radiation, or CBR. The cosmic background radiation was discovered in 1964 by two Bell Laboratories scientists, Arno Penzias and Robert Wilson, who had originally intended to use a radio telescope to study Galactic emissions and were puzzled by a persistent noise in the instrument that they could not explain. Once the CBR was understood, scientists struggled for over two decades to measure its spectrum. Much of the most interesting portion of the spectrum is absorbed by the Earth's atmosphere. Rocket and balloon flights gave important information, but were prone to considerable experimental error. The COBE satellite has given us our best data to date on the CBR. It has determined that the CBR corresponds to blackbody radiation with a temperature of a little more than 2.7 K. COBE also found the Doppler shift due to the motion of the Earth with respect to the CBR. After subtracting away this so-called dipole shift, COBE found tiny residual temperature fluctuations, which represent the imprints of the small inhomogeneities in the early universe that grew into the large scale structure (galaxy clusters, superclusters, voids) that we see today. The redshift formula for the CBR temperature (eqn. 12.2) tells us how the blackbody temperature varies with redshift. For example, the blackbody temperature was 2700 Kelvins at z = 1000. This redshift corresponds approximately to the time when electrons combined with protons to form hydrogen atoms; as a result of this event the universe became transparent, allowing the CBR photons to stream into space.

COBE Images

This is a COBE map showing the dipole temperature differences on the microwave background. This temperature difference is created by the Doppler effect as the Earth, Solar System, and Milky Way move with respect to the microwave background radiation. The faint linear structure across the center of the image is due to the plane of the Milky way galaxy.

This COBE map shows fluctuations in the temperature of the microwave background after the effects due to the dipole and the Milky Way have been subtracted out. The level of the fluctuations is less than 20 millionths of a degree. Unlike the dipole temperature variations, these fluctuations are believed to be intrinsic to the CBR itself, resulting from slight gravitational redshifts (blueshifts) due to slight over (under) densities in the early universe at the time of recombination.

The discovery of these fluctuations was very significant, for they are due to the small variations in matter density that were present when the CBR decoupled from matter only about 300,000 years after the big bang. These small density variations are the "seeds" that will grow into the galaxies and galaxy clusters in the present universe. The spatial resolution of the COBE satellite was not good enough to observe the fluctuations in enough detail to draw detailed conclusions. A new satellite, the Wilkinson Microwave Anisotropy Probe (named in honor of David Wilkinson) was launched in 2001 to study the CBR in greater detail.

The above picture represents the temperature fluctuations observed by the WMAP satellite. Compare this image with the one above from COBE. The greater detail visible in the WMAP image is due to its superior resolution. This data allowed astronomers to determine the apparent size of the largest amplitude temperature fluctuations in the CBR. The size of this fundamental peak can be predicted from theory for different types of cosmological models. By comparing the observed scale with the predictions astronomers were able to determine that the universe is best described by a model with flat geometry.

A great deal of information can be obtained through a detailed study of the structure in the CBR. The WMAP mission continues to take data, and the European Science Agency will launch the Planck satellite in 2007 to gather addition information from this crucial time in the history of the universe.

For more information see Questions and Answers related to Chapter 14.

Wayne Hu of the University of Chicago has written a web-based Introduction to the CBR.

University of Virginia Professor Mark Whittle has prepared a presentation about the CBR fluctuations which uses sound to illustrate how these fluctuations evolve. See The Sounds of the Universe.

To learn about the latest studies of the CBR, try the Microwave Anisotropy Probe cosmology page.

Visit the home page of Planck Satellite to learn about its planned mission.

Original content © 2005 John F. Hawley