The cosmic microwave background — Astronomy, 11–13 years
A faint microwave glow comes from every direction in space. It is ancient light released when the young universe became transparent, giving astronomers a snapshot of its early state.
An afterglow from the young universe
The cosmic microwave background is a very faint glow spread across the whole sky. It began as light in the early universe, when matter was a hot, glowing fog. About 380,000 years after the universe began expanding, the fog cooled enough for atoms to form and light to travel freely. That light has stretched into microwaves as space expanded.
Why astronomers look for it
A story about the early universe needs evidence, not only a clever idea. If the universe began hot and dense and then expanded, old light should still fill space, cooled and stretched by that expansion. In 1965, Arno Penzias and Robert Wilson detected an unwanted microwave hiss from every direction. It turned out to be the predicted afterglow, not a fault in their equipment.
From 3000 K to 2.7 K
Use absolute temperature, measured in kelvins. First, the ancient glowing universe was about 3000 K when this light escaped. Today the background is measured at about 2.7 K. Divide 3000 by 2.7: the light’s source was roughly 1,100 times hotter. The expansion did not erase the light; it stretched its waves until detectors receive them as microwaves.
The reasonable mistake
It is easy to imagine the background as a bright shell with a definite edge around the universe. The name and the all-sky image can suggest that. In fact, it is light arriving from every direction, released when the universe became transparent; there is no special wall at its edge. We see different parts of that ancient light because light travels to us from different distances.
What its tiny patterns tell us
Space missions map tiny temperature differences in the background, much smaller than one part in a thousand. These patterns are not pictures of stars; they are slightly denser and less dense patches in the young universe. Gravity later grew the denser patches into galaxies and clusters. Comparing the map with models helps estimate the universe’s age, contents and early structure.
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