Why do we believe in the Big Bang model? There are four main pieces of evidence:

• microwave background
• recession of the galaxies
• abundance of chemical elements
• irregularity of microwave background

Let's look at each one:

Microwave Background: The Big Bang theory predicts that space is bathed in weak microwave radiation. Such radiation was discovered in 1965 by Penzias and Wilson. Careful measurements of the intensity of this radiation over a large range of wavelengths matched the predictions to extraordinarily high accuracy.

M83, a spiral galaxy about 15 million light years away and receding at about 330 km/sec (© Anglo-Australian Observatory)

This image shows two years of accumulated data from the Cosmic Background Explorer (COBE) satellite. The different colors show tiny differences, about one part in a hundred thousand, between the temperature of the microwave background at different places in the sky. (image courtesy of NASA)

Recession of the Galaxies: Well before the Big Bang theory was proposed, the astronomer Edwin Hubble discovered that distant galaxies are MOVING AWAY from Earth with a recession velocity proportional to their distance. This observation established the expansion of the universe. The illustration shows a receding galaxy located about a thousandth of the way to the edge of the visible universe.

Abundance of Chemical Elements: Observations of spectral lines (see illustration) from distant galaxies show that the universe is about three-quarters hydrogen and one-quarter helium. This is just the ratio predicted from a Big Bang explosion that produced intense gamma radiation that inhibited the formation of elements beyond helium. As the universe expanded and cooled, this gamma radiation became the microwave background.

Irregularity of the Microwave Background: The way matter clumps together in stars and galaxies leads to the prediction that the intensity of the microwave background has tiny irregularities. The theory predicts that the background radiation has a blotchy pattern, with a characteristic size of half a degree (about the angular size of the moon) or less. The irregularities are only about one part out of one hundred thousand, but they have in fact been observed, as shown in the illustration of results from the Cosmic Background Explorer (COBE) probe.

Light given off by excited hydrogen atoms, with wavelength increasing from left to right; note the separate lines, unlike the continuous pattern of colors in a rainbow; these lines are characteristic of hydrogen (image courtesy of HyperPhysics, © Rod Nave)