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Footprints of Creation

Discovery of the Cosmic Background Radiation

Before 1964, there were two competing theories of cosmology: the Steady State theory, championed by Fred Hoyle, and George Gamow's Big Bang theory. Hoyle and his colleagues maintained that the universe has no beginning or end and that, as the universe expands, matter is spontaneously created to maintain the constant density of the universe.

Then, in a serendipitous discovery, Arno Penzias and Robert Wilson detected cosmic background radiation. They were looking for signals from the early communications satellites, but they were plagued by background radio noise, corresponding to a temperature of about about 3 degrees Kelvin, that would not go away. The signal was equally strong in all directions, regardless of the position of the Sun or the Milky Way relative to the observer on Earth. Later measurements showed that the radiation was extremely uniform--they detected variations of less than one part in 10,000. Clearly this radiation was coming from the distant parts of the universe.

This cosmic background radiation was right in line with Big Bang theory. Immediately following the Big Bang, the universe was tremendously hot. When matter and photons parted company 300,000 years after the Big Bang, the radiation was free to travel through space. Big Bang theorists knew that if their scenario was correct, this radiation must still be in the universe--after all, where could it go? They also knew that, because of the Doppler Shift associated with the expansion of the universe, these photons must have been "stretched" into radio waves of much longer wavelength--in the microwave region of the electromagnetic spectrum.

Moreover, measurements taken in years to come would show that the cosmic background radiation is consistent with that emitted by a blackbody. A blackbody is a hypothetical object that absorbs all radiation falling on it. When heated, it would emit radiation at characteristic energies or wavelengths that correspond to it s temperature alone. In practice, such a body would have to be totally isolated. These conditions describe the hot, dense, nearly perfectly opaque universe in its first 300,000 years.

The Steady State theory, which had no way of accounting for the cosmic background radiation whose characteristics so much resembled a blackbody's, was put to rest.

COBE: Another Boost for the Big Bang

COBE Satellite

In 1989, an instrument aboard the Cosmic Background Explorer (COBE) satellite measured the cosmic background radiation with unprecedented accuracy.
JPEG Image (30.6 KB); Caption; Credits and Copyrights

COBE Sky Map

COBE measured a cosmic background radiation spectrum that fits perfectly with the predicted blackbody spectrum at 2.735 degrees Kelvin.
JPEG Image (10.2 KB); Caption; Credits and Copyrights

A Cosmological Conundrum

However, the smoothness, or isotropy, of the cosmic background radiation posed several problems for the original Big Bang model.

First, there's the origin of structure. How can we reconcile the large-scale structures we see today with the smoothness of the early universe? How could galaxies, galaxy clusters, and superclusters have arisen from such uniformity? This is known as the Smoothness Problem.

Until 1992, the cosmic background radiation was thought to vary by no more than one part in 10,000. But then, a team of scientists using NASA's Cosmic Background Explorer satellite (COBE) detected minute ripples--variations of one-hundred thousandth of a degree, or one part in 100,000--in the microwave background. Cosmologists now believe that these tiny density variations could have "seeded" the large-scale structures that we see today. They're using high performance computing technology to demonstrate the viability of this hypothesis in advanced simulations.

Next, there's the so-called Horizon Problem. There is a maximum distance, called the horizon distance, that a light signal could have traveled since the beginning of the universe. The cosmic radiation that reaches our radio antennas from opposite directions, say east and west, originated in regions 30 billion light years apart. Information can't be transmitted faster than the speed of light, so how could two disconnected regions be so uniform?

Then, there's the Flatness Problem. The ratio of the average density of the universe to the density that's just sufficient to halt expansion--the critical density--is one, or very nearly so. This ratio, called Omega, must have been very close or equal to one (1.0) an instant after the Big Bang. Greater, and the universe would have collapsed under its own weight in an instant; smaller, and it would have quickly expanded into infinity. How could the average density of the universe be identical to the critical density so soon after the Big Bang? Since a value of one corresponds to a flat universe, this is known as the Flatness Problem.

Inflation Saves the Day

In 1979, Alan Guth, then a postdoctoral student, added a burst to the Big Bang theory that he named inflation. Inflation neatly explained both the horizon and flatness problems. During inflation, the universe grew exponentially. Once-intimate particles of matter were flung into the farthest reaches of the universe at speeds far greater than the speed of light, and at the same time flattening out any pre-existing curvature of the universe.

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NCSA. Last modified 11/1/95.