Professor, Department of Astronomy and Astrophysics
University of Chicago

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Properties of Cosmic Light

Key Concepts

Because of the expansion, the microwave background is very cold now - 3 degrees above absolute zero. It's wavelength has been stretched out of the visible and into the microwave regime of millimeters to centimeters. This is the same type of radiation that heats your food but a few percent of its wavelength - and of course not especially tuned to cook things!

In terms of photons, or packets of light, there are quite a few of them in the microwave background -- about 400 per cubic centimeter. Since they travel at the speed of light that means quite a large number of them are whizzing through each patch of space each second. Unfortunately, just as your eyes aren't tuned to "see" radio or TV waves, they aren't tuned to see microwaves. TV waves at a fraction of a meter are reasonably close to microwaves in wavelength. In fact a few percent of the TV "snow" you see between channels comes from the microwave background.

Unfortunately since the advent of cable people like you and me have to turn to experimental physicists to observe them. They use specially tuned antennas and receivers to detect microwave photons. When they look out on the sky, what they see is remarkably uniform across the sky. To 1 part in 100,000 the temperature in one direction is the same as in the other. However, there are small variations (or ripples) in the temperature that we observe. These wrinkles are useful in determining how objects like galaxies formed in the universe.