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The Question

(Submitted October 01, 2003)

I have read and heard in many discussions about the CMB that it firsts started out as a Gamma Ray radiation and eventually should pass all the electromagnetic spectrum down to the Radio Wave portion as the Universe continue it's accelerated expansion. My question is: Was there a time in the Universe where it was bathed in Visible Light? Is this an era of the Olbers' Paradox? Can we measure the precise age of the Universe using the "rate of change" of this background radiations as it evolves with the expansion of the Universe? And lastly, The presence of CMB at the temperature of about 3.0 Kelvin, doesn't it imply a "bounded" Universe. The Universe is not infinite?

The Answer

First, the presence of the CMB does not mean that universe is necessarily bounded, in fact current measurements in its anisotropy show that the universe is unbounded now.

You are correct, at a certain period in the past the CMB was at visible wavelengths, and before that was at gamma-ray temperatures, though at that point it wasn't background radiation as the mater and energy were strongly interacting.

The CMB takes its origin around 380,000 years after the big bang when matter and energy decoupled. (energy became free to travel through matter and not get constantly absorbed and reemitted) When it first became free to travel, the CMB was at ~4000K or in the orange band of the spectrum. If anyone with sensory organs sensitive in the visible band was around to see, the night sky would have glowed faintly in orange.

More details can be found at:

Hope this helps,
Michael Arida for Ask an Astrophysicist

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