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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:
http://isaac.exploratorium.edu/~pauld/activities/astronomy/expandinguniverselecture.html
http://map.gsfc.nasa.gov/
Hope this helps,
Michael Arida for Ask an Astrophysicist
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