Optical depth
τ_rec
since reionization: Most modern value 0.0561(71) which means I_modern=I_recombination*exp(-τ)
≅ I_recombination*(1-τ) ≅ 0.94*I_recombination.
So most CMB
has NOT been scattered by
free electrons
by Thomson scattering
which randomize its direction.
Randomization would erase our ability to know about
know of the primordial density fluctuations
at the
recombination era t = 377,770(3200) Jyr
= 1.192*10**13 s (z = 1089.80(21)).
About the same amount of scattering has occurred to
diffuse extragalactic background radiation (DEBRA).
Note,
if the
τ_rec >> 1,
there would still probably be fluctuations
in the
CMB
and DEBRA,
but they would NOT be primordial, but would be due to
fluctuations
free electron density
in the intergalactic medium (IGM)
due to a hypothetical higher density
of the intergalactic medium (IGM)
and/or some hypothetical feature of
structure formation (AKA large-scale structure formation).
Actually, in the late 1980s and
early 1990s, people were worrying
if the primordial density fluctuations
were too small to be seen even if
τ_rec << 1
as it is.
The CMB
temperature
standard deviation (i.e., σ)
is ∼ 100 microkelvins around the
cosmic present CMB mean temperature
T = 2.72548(57) K (Fixsen 2009).
Put another way, the relative
temperature variations are only 1 in 25000
(Wikipedia:
Cosmic microwave background radiation: Features).
