- The
cosmic microwave background (CMB)
reaches us from a starting point that is
near the particle horizon: i.e.,
the edge of the observable universe
which is currently at a
proper distance
∼ 14.3 Gpc ≅ 46.6 Gly
(see Wikipedia: Observable universe).
This starting point is, in fact, a
sphere centered us
called the
last scattering surface (LSS).
The diagram represents
the LSS
as a big circle.
- Note
      r_proper = a(t)*r_comoving ,
where r_proper is proper distance,
r_comoving is (cosmological) comoving distance,
a(t) is the cosmic scale factor,
and t is cosmic time
measured from time zero at the
big bang singularity
of the
Friedmann-equation Λ models---which
is overwhelmlingly before they apply by a fraction of
second.
Comoving distances
are constant with respect to cosmic time.
For the usual convention that
cosmic scale factor a(t=present) =1 ,
the proper distances
are equal to the
comoving distances at present.
So the radius of the
(current) observable universe
is always ∼ 14.3 Gpc ≅ 46.6 Gly
in comoving distance.
- When the CMB
started out coming toward us our (current)
observable universe
was much smaller.
The expansion of the universe
caused it to grow to it's present size.
However, its comoving volume was the same as now.
The diagram
shows the unchanging comoving volume, in fact: i.e., the area encompassed by the
LSS.
- The CMB
started at the
recombination epoch
∼ 378,000 years after the
Big Bang
at cosmological redshift of z ≅ 1100
(see Wikipedia: Concordance model: Parameters).
- The comoving radius of
the particle horizon
at the recombination epoch
(i.e., the
recombination epoch
"observable universe")
was much smaller
than comoving radius of
our (current)
observable universe.
The small circles
represent particle horizons
for the points they are centered on.
- No signal could have reached from
one small circle to the other
since time zero in the
Friedmann-equation Λ models.
They are NOT
causally connected in those
models and CANNOT have influence each other.
- However, the uniformity of the
CMB
shows that all points on the
LSS
to have had the same
temperature
to about 1 part in 10**5
(see Wikipedia:
Cosmic Background Explorer: Intrinsic anisotropy of CMB).
How could this be if those points are NOT
causally connected?
There is NO way that the
points could be
in near thermodynamic equilibrium
(i.e., at nearly the
temperature) by
influencing each other in the
Friedmann-equation Λ models.
- One solution is just to say the
observable universe started
out as extremely homogeneous as
initial condition---a
"just so story".
- However,
cosmologists and
physicists in general
don't like the idea of a beginning of time out of nothing---especially with
fine-tuned initial conditions.
Because why?
So the extremely homogeneity of the
early universe is
a problem which is called
horizon problem.
The most popular possible solution (i.e., explanation)
since circa
1979 is
inflation cosmology which
posits a background universe
out of which our observable universe
formed and solves the horizon problem---but
at the expense of creating more
problems---but physicists like
new problems to solve.
-
One could solve the problem by just saying the universe
was created ex nihilo with
uniform conditions at the
Big-Bang singularity
or at the Planck time.
But why then was it NOT created exactly uniform?
And anyway physicists do NOT like to accept at blank wall at the
Planck time.
They want to know what happened before.
Inflation
solves the horizon problem by saying all the
observable universe
and more
started from a minute region of space that was
CAUSALLY-CONNECTED and in thermodynamic equilibrium
and inflationary expansion blew it up
to sizes that the expansion of
Friedmann-equation Λ models
could NOT achieve.
This solves the horizon problem and also indicates that
beyond the region of inflation there are other
pocket universes.
- By the by,
Steven Weinberg (1933--)
considers the
horizon problem
the one initial condition problem of the
observable universe
for which there are no plausible alternatives to
inflation
(see
Weinberg, Steven, 2008, p. 208).
Some disagree.
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