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Planck
The Discovery of the Cosmic Microwave Background
Since we'll be talking a great deal about microwaves in this presentation, we should define them before we get much further:

The electromagnetic spectrum
The electromagnetic spectrum.
Microwaves are similar to visible light, except they have wavelengths that are about 10,000 times longer than those of light we can see. Due to the stretching of space, the oldest light we can now detect has been stretched so that we detect it now as microwaves.

When viewed in the light of Edwin Hubble's discovery of the red shifts of galaxies some 30 years earlier, this microwave background could be nothing other than the highly red-shifted and cooled relic radiation from a very hot infant universe...

The Horn reflector antenna at Bell Telephone Laboratories in Holmdel, New Jersey
Data from the FIRAS instrument on the COBE satellite plotted over a temperature spectrum of a blackbody with a temperature of 2.725 Kelvin.
...now seen at a wavelength of approximately 2 millimeters, corresponding to a black body temperature of approximately 3 Kelvin.

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