and Cosmology

8. Cosmology III: The Cosmological Parameters
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Fig. 8.28. Power spectrum of the CMB angular fluctuations,
measured with the BOOMERANG experiment. These results
were published in 2002, based on the same data as
the previously released results, but using an improved analysis.
Plotted are the coefficients l(l + 1)C l /(2π) as a function
of wave number or the multipole order l ∼ 180 ◦ /θ, respectively.
The first three peaks can clearly be distinguished; they
originate from oscillations in the photon–baryon fluid at the
time of recombination. In the panel on the right, the fluctuation
spectra of several cosmological models which provide
good fits to the CMB data are plotted. The model denoted
“weak” (solid curve) uses the constraints 0.45 ≤ h ≤ 0.90,
t 0 > 10 Gyr, and it has Ω Λ = 0.51, Ω m = 0.51, Ω b h 2 = 0.022,
h = 0.56, and accordingly t 0 = 15.2 Gyr. The short-dashed
curve (“strong H 0 ”) uses a stronger constraint h = 0.71 ±
0.08, and yields Ω Λ = 0.62, Ω m = 0.40, Ω b h 2 = 0.022,
h = 0.65, and accordingly t 0 = 13.7Gyr
ments, given their error bars, are statistically compatible
with the power spectrum that results from the weighted
mean.
With the optimally averaged power spectrum, we
can now determine the cosmological model which best
describes these data. Under the assumption of a flat
model, we obtain Ω Λ = 0.71 ± 0.11 and a baryon density
of Ω b h 2 = 0.023 ± 0.003, in excellent agreement
with the value obtained from primordial nucleosynthesis
(see Eq. 4.62). Furthermore, the spectral index
of the primordial density fluctuations is constrained to
n s = 0.99 ± 0.06, which is very close to the Harrison–
Zeldovich value of 1. In addition, the Hubble constant
is estimated to be h = 0.71 ± 0.13, again in extraordinarily
good agreement with the value obtained from
local investigations using the distance ladder, which is
a completely independent measurement. These agreements
are truly impressive if one recalls the assumptions
our cosmological model is based upon.
Baryonic Oscillations in the Galaxy Distribution. As
an aside, though a very interesting one, it should be
mentioned here that the baryonic oscillations which
are responsible for generating the acoustic peaks in the
CMB anisotropy spectrum have now also been observed
in the large-scale distribution of galaxies. To understand
how this can be the case, we consider what happens after
recombination. Imagine that recombination happened
instantaneously; then right at that moment there are density
fluctuations in the dark matter component as well as
the acoustic oscillations in the baryons. The photons can
stream freely, due to the absence of free electrons, and
the sudden drop of pressure in the baryon component
reduces the sound speed from c/ √ 3 essentially to zero.
We said before that the baryons can then fall into the
potential wells of the dark matter. However, since the
cosmic baryon density is only about six times smaller
than that of the dark matter, the baryonic density fluctuations
at recombination are not completely negligible