The phenomenology of the acoustic peaks in the temperature and polarization
is essentially described by 4 observables and the
initial conditions [Hu et al, 1997]. These are the angular
extents of the sound horizon , the particle horizon at matter radiation
equality
and the damping scale
as well as the value of the baryon-photon momentum density
ratio
.
sets the spacing between of the peaks;
and
compete to determine their amplitude through radiation
driving and diffusion damping.
sets the baryon loading and, along with the potential well depths
set by
, fixes the modulation of the even and odd peak heights.
The initial conditions set the phase, or equivalently the location of the first
peak in units of
, and an overall tilt
in the power spectrum.
In the model of Plate 1, these numbers are
(
),
,
,
and
and in this family of models the parameter sensitivity is approximately
[Hu et al, 2001]
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and . Current
observations indicate that
,
,
,
, and
([Knox et al, 2001]; see also [Wang et al, 2001,Pryke et al, 2001,de Bernardis et al, 2001]), if gravitational
waves contributions are subdominant and the reionization redshift is low as assumed
in the working cosmological model (see §2.1).
The acoustic peaks therefore contain three rulers for the angular diameter
distance test for curvature, i.e. deviations from . However contrary to popular belief, any one of
these alone is not a standard ruler whose
absolute scale is known even in the working cosmological model. This is reflected
in the sensitivity of these scales to other cosmological parameters. For example,
the dependence of
on
and hence the Hubble constant is quite strong. But in
combination with a measurement of the matter-radiation ratio from
, this degeneracy is broken.
The weaker degeneracy of on the baryons can likewise be broken from a measurement
of the baryon-photon ratio
. The damping scale
provides an additional consistency check on the implicit
assumptions in the working model, e.g. recombination and the energy contents
of the Universe during this epoch. What makes the peaks so valuable for this
test is that the rulers are standardizeable
and contain a built-in consistency check.
There remains a weak but perfect degeneracy between and
because they both appear only in
. This is called the angular diameter distance
degeneracy in the literature and can readily be generalized to dark energy
components beyond the cosmological constant assumed here. Since the effect of
is intrinsically so small, it only creates a correspondingly
small ambiguity in
for reasonable values of
. The down side is that dark energy can never be isolated
through the peaks alone since it only takes a small amount of curvature to mimic
its effects. The evidence for dark energy through the CMB comes about by allowing
for external information. The most important is the nearly overwhelming direct
evidence for
from local structures in the Universe. The second is
the measurements of a relatively high Hubble constant
; combined with a relatively low
that is preferred in the CMB data, it implies
but at low significance currently.
The upshot is that precise measurements of the acoustic peaks yield precise
determinations of four fundamental parameters of
the working cosmological model: ,
,
, and
. More generally, the first three can be replaced by
,
,
and
to extend these results to models where the underlying assumptions
of the working model are violated.