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The Question

(Submitted May 01, 2009)

What causes the CMB to vibrate exclusively in the microwave frequency range? Why not some other frequency range, or at a variety of frequencies? What does this tell us about the nature of space, or the conditions at the time of the big bang, or both?

The Answer

The CMB follows a blackbody, or Planck spectrum, which is uniquely determined by the temperature of the emitting object - in this case the Universe . The temperature we see today is 2.7 kelvin, and this reflects how much the Universe has expanded since the time when the Universe became transparent to radiation. The universe became transparent when atoms could form, about half a million years after the Big Bang, at a temperature of about 3,000 degrees. The ratio of the temperatures then and now (3,000 degrees to ~ 3 degrees) is the ratio of the size of the universe then to the size now - a factor of 1,000.

This is also the ratio of the wavelength of light scattered then and now.

There is a huge body of literature that explains the CMB in great detail, including websites for COBE and WMAP, two satellites that made many measurements of the CMB and the uniformity of the CMB.

Here is one place to get started:
http://imagine.gsfc.nasa.gov/docs/features/exhibit/map_foggy.html

Jay and Jeff
for Ask an Astrophysicist

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