In the Universe’s early history, its temperature was high enough to prohibit the formation of atoms, and
the Universe was therefore ionized. Approximately 3 105 yr after the Big Bang, corresponding to
a redshift zrec
1000, the temperature dropped enough to allow the formation of atoms,
a point known as “recombination”. For photons, the consequence of recombination was that
photons no longer scattered from ionized particles but were free to stream. After recombination,
these primordial photons reddened with the expansion of the Universe, forming the cosmic
microwave background (CMB) which we observe today as a black-body radiation background at
2.73 K.
In the early Universe, structure existed in the form of small density fluctuations ( 0.01) in the
photon-baryon fluid. The resulting pressure gradients, together with gravitational restoring forces, drove
oscillations, very similar to the acoustic oscillations commonly known as sound waves. At the same time, the
Universe expanded until recombination. At this point, the structure was dominated by those oscillation
frequencies which had completed a half-integral number of oscillations within the characteristic size of the
Universe at recombination; this pattern became frozen into the photon field which formed the
CMB once the photons and baryons decoupled. The process is reviewed in much more detail
in [60].
The resulting “acoustic peaks” dominate the fluctuation spectrum (see Figure 4). Their angular scale is
a function of the size of the Universe at the time of recombination, and the angular diameter distance
between us and zrec. Since the angular diameter distance is a function of cosmological parameters,
measurement of the positions of the acoustic peaks provides a constraint on cosmological parameters.
Specifically, the more closed the spatial geometry of the Universe, the smaller the angular diameter distance
for a given redshift, and the larger the characteristic scale of the acoustic peaks. The measurement of the
peak position has become a strong constraint in successive observations (in particular Boomerang, reported
in [30] and WMAP, reported in [146
] and [145
]) and corresponds to an approximately spatially flat
Universe in which
1.
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But the global geometry of the Universe is not the only property which can be deduced from the fluctuation
spectrum10.
The peaks are also sensitive to the density of baryons, of total (baryonic plus dark) matter, and of dark
energy (energy associated with the cosmological constant or more generally with w components).
These densities scale with the square of the Hubble parameter times the corresponding dimensionless
densities (see Equation (5
)) and measurement of the acoustic peaks therefore provides information on the
Hubble constant, degenerate with other parameters, principally the curvature energy
and the index w in the dark energy equation of state. The second peak strongly constrains the
baryon density,
, and the third peak is sensitive to the total matter density in the form
.
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