[Gugel]

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What are your credentials?

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I'm not claiming to know everything and please correct me if I'm mistaken anywhere. I took some high level astronomy/cosmology courses in college and read a bit cosmology in my spare time. I'll be able to answer most questions in layman's terms.

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Please explain why a specific cosmic microwave background radiation was definitive proof that the big bang theory was correct. Just curious how Georges Lemaître could have predicted that.

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When we look at Cosmic Background Radiation, we are actually seeing a snapshot of the Universe relatively soon after the Big Bang took place (~400,000 years after). It was the moment the Universe had cooled enough so that it went from being opaque to becoming transparent. It was also the moment energy and matter became two separate things. Anyway, this Cosmic Background Radiation is really far out and receding from us fast so it's grossly distorted (redshifted) into the microwave part of the light spectrum. But if we account for the redshift, we can determine the wavelength of the light that originally left the CBR and from that, we can determine the temperature (~2800 C). 400,000 years ago, every single part of the Universe was uniformly ~2800 C (there was no empty, cold space).