The Big Bang Theory
If the universe is expanding, where were the galaxies in the distant past? Hubble observed that in every direction you look, galaxies at a given distance are moving away at the same rate. This is what you would expect if the universe were expanding uniformly.
Astronomers theorize that the universe came into being at a single moment, in an event called the big bang. According to this theory, all the matter and energy of the universe were at one time concentrated in an incredibly hot region smaller than the period at the end of this sentence. The big bang theory states that the universe began in an instant, billions of years ago, in an enormous explosion.
After the Big Bang
The universe expanded quickly and cooled down after the big bang. After a few hundred thousand years of expansion, the universe was still much smaller and hotter than it is now, but cool enough for atoms to form. Gravity pulled atoms together into gas clouds that eventually evolved into stars in young galaxies. As shown in Figure 26, the sun and solar system formed about 4.6 billion years ago, when the universe was about two thirds of its present size.
Figure 26 The universe began with the big bang 13.7 billion years ago. The first stars and galaxies formed 200 million years later. The solar system, and Earth, formed about 9 billion years after the big bang.
dEvidence for the Theory
In 1965, two American physicists, Arno Penzias and Robert Wilson, noticed a signal on their radio telescope that they couldn't explain. They eventually realized that they were detecting a faint distant glow in every direction. Today this glow is called the cosmic microwave background radiation. This glow is energy produced during the big bang, still traveling throughout the universe. The existence of cosmic microwave background radiation and the red shift in the spectra of distant galaxies strongly support the big bang theory.
The big bang theory describes how the expansion and cooling of the universe over time could have led to the present universe of stars and galaxies. It offers the best current scientific explanation of the expansion of the observable universe. Variations of the theory continue to be proposed and are being tested with new observations.
Age of the Universe
Since astronomers know how fast the universe is expanding now, they can infer how long it has been expanding. If you traveled backward in time, all of the matter in the universe would be at its starting point 13 to 14 billion years ago. Recent measurements of the microwave background radiation have led to a more precise age. Astronomers now estimate that the universe is 13.7 billion years old.
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