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Old December 15th 17, 05:38 AM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default A quasar, too heavy to be true

In article ,
Martin Brown writes:
The universe is opaque until the universe cools to allow neutral
hydrogen when it becomes transparent.


I'm afraid that's backwards. The universe starts out with opacity
dominated by scattering. After recombination (a slight misnomer --
that epoch is the first appearance of atoms), the universe was opaque
to ultraviolet light. Only after reionization did the universe
become transparent to UV. That corresponds to redshifts of something
like 6 to 9 or so, but the details are not yet known. Reionization
was almost certainly patchy so occurred at different times in
different locations.

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[[Mod. note -- Maybe there's a confusion here about wavelengths.
Is this chart correct?

visible/IR hard UV
before recombination opaque opaque
(roughly z 1100)

between recombination & reionization transparent opaque
(roughly 8 z 1100)

after reionization transparent transparent
(roughly z 8)

Here "hard UV" means UV with a photon energy high enough that H atoms
can be photoionized.
-- jt]]