Foundations of Modern Cosmology

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Chapter 12: A Message From the Big Bang

Chapter Summary

After a brief discussion of one of the classic questions of historical cosmology, Olber's 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. 12.1) 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

Figure 12.7 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.

Figure 12.5 COBE map of the sky showing 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.

Another important clue to the early history of the universe is the abundance of the lightest elements. George Gamow and his collaborators developed the theory of nucleosynthesis in the big bang in the late 1940's and early 1950's. Indeed, their work led to the first prediction of the CBR and its temperature. Unfortunately, their prediction of microwave photons from the big bang was not taken seriously until it was rediscovered by others in the mid 1960's. The discovery of the CBR led scientists to realize that they had overlooked clues to the existence of the cosmic background for over 15 years.

The existence of the CBR is a stunning confirmation that the standard models are the best candidates to describe the universe. Thus we would like to know the values of q, Ho, Omega, the age of the universe, and the geometry of the universe k. If we could measure these parameters accurately, we would be able to determine which of the many choices best describes the universe in which we live. The second part of Chapter 12 describes various tests that can, at least in principle, measure the parameters of the standard model of cosmology.

The Hubble Space Telescope is now providing exciting new data that may help answer some cosmological questions. Since the telescope orbits the Earth above the obscuring atmosphere it has greater resolving power than ground based telescopes. Fig. 12.9 is an image showing the components of the HST.

Here is a very beautiful picture of the HST in orbit taken during the last servicing mission.

Points to Ponder How do the various cosmological parameters relate to one other? Under what circumstances might we require a cosmological (lambda) constant?
Potential Pitfalls
Questions & Answers

Questions and Answers related to Chapter 12.

Web World

Visit the COBE Home Page for more detailed information and pictures of the COBE satellite.

The latest discoveries about the structure of the microwave background can be found at the home page of the BOOMERanG Project. The fluctuations observed are consistent with the flat universe.

Professor Joseph Silk has written some internet essays on cosmological principles, Olber's paradox and the steady state model. Essays by Joe Silk.

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