Introduction

In 1964, two scientists at AT&T Bell Laboratories were using a radio telescope to detect radio waves from our galaxy. Arno Penzias and Robert Wilson detected a background "noise" in the microwave range of the electromagnetic spectrum. The wavelength of a microwave is longer than that of visible light and is between the wavelengths of radio and infrared waves. The microwaves they detected were isotropic (uniform in all directions) so they couldn't be attributed to one particular source. Penzias and Wilson suspected that the microwave disturbance came from the telescope itself, but after cleaning and checking it many times the noise persisted.

Penzias and Wilson continued to search for an explanation for the background microwave radiation they were detecting. Around the same time, Robert Dicke was expanding on a theory that there should be a residue of background radiation filling the universe that originated in the big bang. Penzias and Wilson soon joined Dicke in reporting their observations.

According to Dicke's interpretation of the big bang theory, the universe was filled with hot, dense matter shortly after the Big Bang. For the first few hundred thousand years, photons scattered off electrons and went nowhere. Eventually, radiation gained independence from matter during the "Era of Decoupling." The radiation, later called the cosmic microwave background (CMB), became less dense and cooled down as the universe expanded. The current temperature of the CMB is only three degrees Kelvin. Another effect the universe's expansion had on the CMB was the stretching out of the radiation's wavelength. The Doppler shift caused the radiations, which were once gamma rays, to stretch into microwaves. The current wavelength of remnant radiation is 21 centimeters.

For those new to the idea, it is difficult to understand why radiation emitted billions of years ago still surrounds us. Some people think of the big bang in terms of the explosion of dynamite in which radiation travels faster than the matter (the stick of dynamite) and escapes it; however, radiation created from the big bang is quite different. The universe has no center and lacks edges. Matter is evenly distributed throughout the universe and radiation would have to leave the universe in order to escape matter. For this reason, the background radiation fills, and always will fill, the universe.

Until the observation of the cosmic microwave background by Penzias and Wilson, the only observational evidence of a big bang was the red shift (See The Doppler Effect) observed by Edwin Hubble. The red shift, which is caused by the Doppler effect, is the shift of light to lower frequencies like that of the color red. This shift shows that galaxies rush away from each other, and by tracing the galaxies back in time we find that matter originated from a point. The observation of the CMB also supports the big bang theory and is far less ambiguous than the red shift. In addition, the CMB can be used as a tool for learning about the early stages of the universe.

Studying the Cosmic Microwave Background

After the big bang, some regions of the universe were slightly denser than others. Due to the gravitational force, the dense regions gradually became even denser. These dense regions later became galaxies and then clusters of galaxies and superclusters (See Galaxies, Galaxy Groups, and Galaxy Clusters). The areas that were less dense after the big bang became large voids between the superclusters.

Before the creation of galaxies and superclusters, photons separated from matter and created the cosmic background radiation. Since the decoupling of radiation and matter had occurred before the formation of galaxies and superclusters, the CMB provides information about the earliest stages of the universe. The CMB contains slight variations in temperature due to contrasts in density during the early decoupling era. By observing the background radiation's temperature variation, also called its anisotropy, we can determine the primordial universe's density variation. From this, we can understand the initial conditions of the universe and the subsequent evolution of its structure.

COBE images courtesy of NASA

In 1989, NASA launched a satellite called the Cosmic Background Explorer (COBE) to explore the background radiation's anisotropy throughout the universe. The COBE satellite found that the CMB's temperature variation is about one part in 100,000. The image to the left represents the data found. In the top figure, the blue represents zero degrees Kelvin and the red represents four degrees Kelvin. In this figure, the temperature of the CMB seems uniform. In the middle figure, in which blue represents the temperature of 2.724^o Kelvin and red represents 2.732^o Kelvin, the variations are visible. The middle figure contains extraneous anisotropy caused by the motion of the instruments relative to the CMB. The motion of the instruments is accounted for in the bottom figure.

In addition to helping to determine the general evolution of the universe's structure, the CMB can help us find many important values. The Hubble Constant, which is the universe's rate of expansion, can be determined with more precision. Omega, which is the density of the universe, may also be found by observing the background radiation. Although the universe did not leave behind many clues to its creation, the CMB provides scientists with more than just a peek into its early stages.