and Cosmology

8. Cosmology III: The Cosmological Parameters
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Galactic Foreground. The measured temperature distribution
of the microwave radiation is a superposition
of the CMB and of emission from Galactic (and extragalactic)
sources. In the vicinity of the Galactic disk,
this foreground emission dominates, which is clearly
visible in Fig. 1.17, whereas it seems to be considerably
weaker at higher Galactic latitudes. However,
due to its different spectral behavior, the foreground
emission can be identified and subtracted. We note
that the Galactic foreground basically consists of three
components: synchrotron radiation from relativistic
electrons in the Galaxy, thermal radiation by dust, and
bremsstrahlung from hot gas. The synchrotron component
defines a spectrum of about I ν ∝ ν −0.8 , whereas the
dust is much warmer than 3 K and thus shows a spectral
distribution of about I ν ∝ ν 3.5 in the spectral range
of interest for CMB measurements. Bremsstrahlung
has a flat spectrum in the relevant spectral region,
I ν ≈ const. This can be compared to the spectrum of the
CMB, which has a form I ν ∝ ν 2 in the Rayleigh–Jeans
region.
There are two ways to extract the foreground emission
from the measured intensity distribution. First, by
observing at several frequencies the spectrum of the microwave
radiation can be examined at any position, and
the three aforementioned foreground components can
be identified by their spectral signature and subtracted.
As a second option, external datasets may be taken into
account. At larger wavelengths, the synchrotron radiation
is significantly more intense and dominates. From
a sky map at radio frequencies, the distribution of synchrotron
radiation can be obtained and its intensity at
the frequencies used in the CMB measurements can be
extrapolated. In a similar way, the infrared emission
from dust, as measured, e.g., by the IRAS satellite (see
Fig. 2.11), can be used to estimate the dust emission
of the Galaxy in the microwave domain. Finally, one
expects that gas that is emitting bremsstrahlung also
shows strong Balmer emission of hydrogen, so that the
bremsstrahlung pattern can be predicted from an Hα
map of the sky. Both options, the determination of the
foregrounds from multifrequency data in the CMB experiment
and the inclusion of external data, are utilized
in order to obtain a map of the CMB which is as free
from foreground emission as possible – which indeed
seems to have been accomplished in the bottom panel
of Fig. 1.17.
Fig. 8.26. The antenna temperature (∝ I ν ν −2 )oftheCMBand
of the three foreground components discussed in the text, as
a function of frequency. The five frequency bands of WMAP
are marked. The dashed curves specify the average antenna
temperature of the foreground radiation in the 77% and 85%
of the sky, respectively, in which the CMB analysis was conducted.
We see that the three high-frequency channels are not
dominated by foreground emission
The optimal frequency for measuring the CMB
anisotropies is where the foreground emission has
a minimum; this is the case at about 70 GHz (see
Fig. 8.26). Unfortunately, this frequency lies in a spectral
region that is difficult to access from the ground.
From COBE to WMAP. In the years after the COBE
mission, different experiments performed measurements
of the anisotropy from the ground, focusing
mainly on smaller angular scales. In around 1997,
evidence was accumulating for the presence of the
first Doppler peak, but the error bars of individual
experimental results were too large at that time to
clearly localize this peak. The breakthrough was then
achieved in March 2000, when two groups published
their CMB anisotropy results: BOOMERANG and
MAXIMA. Both are balloon-based experiments, each
observing a large region of the sky at different frequencies.
In Fig. 8.27, the maps from the BOOMERANG
experiment are presented. Both experiments have unambiguously
measured the first Doppler peak, localizing
it at l ≈ 200. From this, it was concluded that we live
in a nearly flat Universe – the quantitative analysis of
the data yielded Ω m +Ω Λ ≈ 1±0.1. Furthermore, clear