Cosmic Microwave Background Radiation
COSMIC MICROWAVE BACKGROUND RADIATION
The cosmic microwave background radiation (CMBR) comprises the remnant photons from an early period after the Big Bang in which the electrons, protons, and photons constituted a hot plasma filling the universe. The CMBR has the spectral form of blackbody radiation. The expansion of the universe stretches the wavelengths of the CMBR photons, reducing their energy and thus cooling off the radiation. The present temperature of the CMBR is 2.728 K, and so it is sometimes called the 3K background. The intensity of this blackbody radiation peaks at a wavelength of about 3 mm; in the micro-wave range, the CMBR is the dominant source of signal observed by telescopes looking up through the disk of our galaxy and away from known point sources (like supernova remnants). Other notable features of the CMBR are its isotropy (meaning that in all directions on the sky, its temperature is measured to be the same to within a fraction of a percent) and its lack of any sizable polarization. The existence of such relic radiation is one of the foundations of modern cosmology. Any theory attempting to address the large-scale disposition and history of the universe must satisfactorily explain such radiation, including its relative isotropy and resemblance to a perfect blackbody.
The CMBR was discovered serendipitously in 1965 by Robert W. Wilson and Arno A. Penzias of Bell Laboratories in Holmdel, New Jersey. In 1978 Penzias and Wilson were awarded the Nobel Prize in Physics for their discovery. The work that lead to the discovery stands as an excellent example of careful scientific method. Penzias and Wilson made their discovery in the course of characterizing an antenna-receiver system designed to calibrate the absolute intensity of several astronomical sources known to emit radio waves. Absolute calibration of an antenna-receiver system requires understanding every source of thermal noise to which it responds. Noise sources include the room temperature (300 K) radiation from the Earth and from the metal comprising the antenna itself, thermal emission from the atmosphere, and noise from the amplifier. Through a laborious series of tests, Penzias and Wilson characterized their system to better than 1 K, so that the "excess antenna temperature" of 3 K was undeniably external to their apparatus. As they mulled over the import of their result, Penzias and Wilson became aware of ongoing theoretical work at nearby Princeton University. There, P. James E. Peebles and Robert H. Dicke had just deduced that in a universe evolving from a big bang, relic thermal radiation should be detectable. In fact, David Wilkinson and Peter G. Roll had already begun building an experiment to try to measure it. The two groups collaborated and published back-to-back papers announcing the discovery and offering a cosmological interpretation for it.
The universe is presently observed to be expanding; therefore, at earlier times it was smaller than it is today and thus much denser. At some point, its contents included electrons, protons, and photons, among other things. (Other constituents included neutral particles like neutrons and neutrinos, as well as dark matter, which does not interact with the plasma.) Initially, the universe was so hot that the electrons and protons were completely ionized. This plasma was opaque; photons did not travel in straight lines for long, as they were continuously interacting with (or scattering off) the charged particles. However, as the universe expanded and cooled, it was eventually no longer hot enough to sustain the plasma. Photons no longer had enough energy to ionize hydrogen when it chanced to form, and eventually all the electrons and protons were bound up in hydrogen. Suddenly, the universe became transparent to the photons, since they quit scattering off particles. This epoch in the history of the universe is sometimes referred to as the time of last scattering for that reason. Another name is decoupling, since the neutralization of the plasma decoupled the photons from the matter. This epoch occurred a few hundred thousand years after the Big Bang. The universe is 10 to 20 billion years old, so the CMBR photons reaching the Earth today have been traveling straight toward the planet for the last 10 to 20 billion years.
The most noticeable aspect of the CMBR is its lack of features. The absolute temperature of the CMBR has been measured from wavelengths of 50 cm to 0.5 mm; across this broad range, no significant deviations from a blackbody shape have been observed. The Far Infrared Absolute Spectrometer (FIRAS) on board the COBE satellite limited any distortions at wavelengths between 0.5 and 5 mm to be smaller than 0.1 percent. The CMBR is isotropic to one part in a thousand. At that level, it exhibits a dipole anisotropy. (Anisotropy means lack of isotropy; a related term is inhomogeneity.) The dipole arises because the solar system is moving with respect to the CMBR. In the direction the solar system is moving, the CMBR appears blue-shifted (to hotter temperatures) by the Doppler effect. In the opposite direction, the CMBR is red-shifted. Primordial anisotropy in the CMBR (meaning anisotropies observed today that are interpreted as reflecting conditions at the time of last scattering) was first measured by the Differential Microwave Receiver (DMR), also on board COBE. Detection of the anisotropy was announced in 1992. These measurements were very hard to make because (aside from the dipole) the anisotropy is tiny: ten parts in a million.
The extreme isotropy of the CMBR (prior to the DMR result) puzzled scientists for two reasons. The first is that the rest of the universe today is not at all isotropic. How could the present lumpy matter distribution (planets, stars, galaxies, and so on) have evolved from a completely isotropic beginning? The answer, of course, is that the beginning was not completely isotropic. Study of the small anisotropies of the CMBR, and how the amplitudes of such anisotropies vary with spatial scale, reveals features of the early universe. In the decade after the COBE result, numerous ground-based and balloon-borne experiments made measurements of CMBR anisotropy at many spatial scales. In the summer of 2002, the MAP satellite was launched with the goal of measuring the CMBR anisotropy on many scales with unprecedented accuracy.
The second problem involves causality. At the time of last scattering, the universe was only a few hundred thousand years old, so light could only have traveled a distance R of a few hundred thousand light-years at most. Since information cannot be transmitted faster than light can travel, causality arguments dictate that regions in the universe farther apart than R cannot be in thermal equilibrium. Today, R subtends an angle a bit smaller than a degree, and yet the CMBR is isotropic over much larger regions than a degree. The solution to this conundrum was inflation, an epoch in which regions that were originally in causal contact moved apart much more rapidly than the speed of light because of an extremely rapid period of expansion of the universe.
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Suzanne T. Staggs