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fireball should have a low temperature of no more than a few tens of kelvins peaking in the spectrum’s microwave. Incidentally, too, the Princeton researchers were constructing a microwave antenna to search for this radiation when Penzias and Wilson went public with their discovery.
It was not until 1989 that COBE (the Cosmic Background Explorer) was able to confirm that the temperature was 2.725 K.
Source: Bennet, Donahue, Schneider, Voit. The Cosmic Perspective. Volume II. Stars and Galaxies. 6th Edition. 678.
By looking at the CMB, we observe almost all the way to the universe’s beginning, since these waves have not interacted with matter since the universe was 400,000 years old, less than a thousandth of today’s size.
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