From Gamow to COBE

Today the Universe is characterized by a richness of complexity. Structure exists on scales from stars to superclusters. Ordinary ``baryonic'' matter, in the form of protons, nuclei and their accompanying electrons, is found in stars, diffuse hot gas, cold gas, and other forms; the admixture varies greatly with environment.

Most of the matter in the universe is simply dark, known to exist only because of its gravitational effects. Its composition is unknown, and most of it is probably not baryonic. It is hard to imagine that one could, from observations of the present Universe alone, sort out how it all happened. During its earliest moments, however, the Universe was much simpler - a smooth gas of photons, baryons and dark-matter particles.

The Cosmic Microwave Background (CMB) radiation is a snapshot of the Universe 300,000 years after the beginning when these photons last scattered. At that time the opaque universal plasma had finally cooled down enough to become a transparent gas of neutral atoms. The CMB serves us as a ``cosmic Rosetta stone.''

FIG.1: COBE DMR 4-year full sky temperature map of the CMB in galactic coordinates. The dipole variation due to the Solar System motion has been removed. The equatorial band is foreground Milky Way radiation. Elsewhere the colors indicate fluctuations of tens of microKelvin. The DMRs resolution is 7 degrees. The blowups of a 7 degree circle indicate the finer detail that might be revealed by the next generation MAP and Planck satellites. The left blowup is a simulation of an open Universe ( ) and the right is a simulation of a flat Universe ( ).

Like the Rosetta Stone, the CMB was found by accident. The story begins with theorist George Gamow and his colleagues Ralph Alpher (now of Union College) and the late Robert Hermann, who saw the early Universe as a nuclear oven in which the light elements in the periodic table were cooked. They realized that the nuclear yields depend upon the temperature of the radiation left today. During the late 1940s and early 1950s they made predictions ranging from 5K to 50K for this putative relic radiation.

Not until 1964 did anyone actually go out and look for this radiation. Unaware of the earlier work by Gamow and others and motivated by a more precise calculation of the temperature by their colleague P.J.E. Peebles, the late Robert Dicke, David T. Wilkinson, and Peter Roll were setting up an experiment on the roof of the physics building to detect the microwave echo of the big bang when Arno Penzias and Robert Wilson of Bell Labs discovered an unexplained celestial microwave hiss. Even before the Internet, physics gossip traveled near the speed of light. The Princeton quartet soon heard about the Penzias-Wilson hiss and quickly provided the big-bang interpretation.

Almost overnight cosmology was transformed from the provence of a handful of astronomers to a major field in its own right. Measurements made at electromagnetic wavelengths from tens of cm to less than a millimeter established the black body character of the CMB. The hot big-bang model was on its way to becoming the standard cosmology (see Box 1).

As it turns out, only the lightest nuclei - D, He, He and Li - were made in the big bang, the rest came much later, made by nuclear reactions in stars and elsewhere. The agreement between measured and predicted abundances of the light elements is today one of the key tests of the standard cosmology.

In 1989, after more than a decade of preparation (including a major redesign after the Challenger disaster), NASA launched the Cosmic Background Explorer (COBE), a satellite devoted to the study the microwave and infra-red backgrounds. The results from COBE exceeded the hopes of even the most optimistic. The Far InfraRed Absolute Spectrometer (FIRAS) determined the CMB temperature T to four significant figures ( K) and showed that any spectral deviations from a Planck spectrum were less than 0.005%. The CMB is the most precise black body known in Nature and could only have arisen from the very hot, dense conditions that existed in the early Universe.

The search for variations (anisotropy) in the intensity of the CMB across the sky began with Penzias and Wilson. They estimated the temperature to be isotropic to within about 10%. In 1976, flying an instrument on a U2 spy plane, a group led by Berkeley physicists Richard Mueller and George Smoot established a 3mK dipolar temperature variation across the sky, arising from the motion of the Solar System with with respect to the rest frame defined by the CMB. COBE greatly refined this measurement to yield a Solar System velocity of km/sec in that frame, and it even detected the annual variation due to Earth's motion around the sun - the ultimate proof of Copernicus' hypothesis.

On smaller angular scales, the anisotropy maps the distribution of matter in the early Universe because variations in the early matter density lead to temperature fluctuations of similar size. It is generally assumed that the abundance of structure seen in the Universe today - galaxies, clusters of galaxies, superclusters, voids and great walls - evolved by gravitational amplificiation from small primeval density inhomogeneities. Theoretical expectations for the magnitude of the CMB fluctuations have decreased from the early 1 0.000000e+00stimates to more precise estimates of around 0.001alculated in recent years. For two decades, the instrument builders had to watch the goalposts recede faster than they could build more sensitive experiments.

By 1992 small-scale anisotropy had still not been detected. Upper limits were already as stringent as K on angular scales ranging from tens of degrees to fractions of a degree. It was not certain how much further the observers could push before foreground emission from the Milky Way and extragalactic objects became insurmountable. Some even questioned the general idea that structure evolved primarily by the action of gravity. But then in April 1992 at the American Physics Society meeting in Washington DC, the COBE Differential Microwave Radiometer (DMR) team announced evidence for temperature fluctuations of K on an angular scale of ten degrees (see Fig.1).

The theorists had escaped disgrace, and cosmology was again transformed overnight. With the COBE detection the final piece of the standard cosmology was in place, and the testing of models for the formation of structure in the Universe, most of them motivated by the physics of the early Universe, could begin. The race to map the early Universe by means of CMB anisotropy was on.