The cosmic microwave background
The cosmic microwave background-
Anisotropies in the CMB -
he Cosmic Microwave Background (CMB) is the most disT tant, and therefore the most ancient source of electromagnetic radiation that can be directly observed from Earth in any frequency range. The Cosmic Background Explorer satellite (COBE) has measured its sub-millimeter emission, which is that of a nearly perfect blackbody at 2.73 K. The relative deviation from a pure Planck spectrum is very small (typically less than 10-5). This emission is attributed to the primordial Universe when it was about 300 000 years old and warm enough (3000K) to ionise the hydrogen gas that constitutes most of its mass. Owing to the expansion of the Universe, this radiation was red-shifted by the Doppler effect by a factor of about 1000, and thanks to the cooling due to the expansion, it couId travel and reach us through the very transparent neutral hydrogen. The discovery of the CMB and even more the measurement of its Planck spectrum by the COBE-FIRASl experiment is the most compelling evidence for a hot big bang model. The existence of the CMB was predicted by Alpher, Bethe and Gamow as a very unique feature of a hot big bang in which the nudeosynthesis of most of the He, D and 7Li seen in the Universe (but which cannot be produced in stars) is produced in the early phases of a hot big bang.
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Furthermore, the existence of very small anisotropies was pre
dicted at a level of 10-5 in the simplest model to explain
large-scale structure formation in the Universe. In this model
anisotropies result from the gravitational instability acting on in
homogeneities generated in the very early Universe. These
anisotropies were indeed found at the right level by the COBE
DMR2 experiment.
In this model, these fluctuations carry information on the
physics of the very early Universe because the spectrum of fluctu
ations is conserved at least on the largest scale in the expansion.
The fluctuations on smaller scales are not gravitationally unsta
ble during the whole history of the Universe preceding
recombination. The fluctuations behave as acoustic oscillations
leaving characteristic peaks of the anisotropies in the power
spectrum. The lowest frequency acoustic peak corresponds to an
angular scale, which depends mainly on thegeometry of the spa
tial part of space-time. In fact, in the simplest model of structure
formation, this power spectrum, if measures down to small
enough scales and with enough accuracy, contains information
allowing cosmologists to get very precisely all the cosmological
parameters (space-time geometry, relative contribution of the
various terms contributing to the dynamics of the Universe, etc.).
The measurements of the small-scale anisotropies of the CMB
are thus becoming one of the main tools of observational cos
mology.
. New results from the balloon-borne experiments
BOOMERanG3 and MAXlMA4 and from ground-based experi
ments using radio detectors and interferometryS·6, were recently
published. They gave a first view of the small-scale anisotropyof
the CMB, unveiling the predicted peaks in the power spectrum of
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its angular distribution. The BOOMERanG
CMB map shown in figure 1 is a high-signal
to-noise rendition of the anisotropies of the
background radiation as they emerged at
recombination when the Universe was a billion ,
times denser than at present. These
anisotropies reflect the inhomogeneities of the 1Ii'
early Universe. Figure 2 shows the combined
power spectrum of these recent experiments.
The predicted acoustic peaks are very clearly
seen. The most striking result is that the posi
tion of the first one is within a few percent of
the position predicted for a Universe with a
Euclidean spatial part of the metric: "the Uni
verse is flat"! 10
Following COBE, NASA launched in the sum
mer of 2001 the MAP satellite7, which will be a
second-generation CMB space experiment. The
detectors are passively, i.e., radiatively cooled radio-type receivers
with sensitivity comparable to the balloon-borne bolometer experi
ments but with significantly better capabilities for large-scale
measurements and control ofsystematics.
10
......-.-The ultimate mission: ESA's 'Planck' satellite
The 'Planck' project of the European Space Agency, to be
launched in 2007, is intended to be the third generation of CMB
space experiments, pushing to its limits the knowledge that will
be retrieved from the CMB observation with unprecedented
angular resolution and sensitivity.
The six spectral bands of the High Frequency Instrument
(HFI) on 'Planck' cover the frequency range between 100 and
1000 GHz with an angular resolution of about 5 arcmin. Its sensi
tivity will be limited, in the CMB channels, by the statistical
fluctuations of the CMB itself, which makes 'Planck' a kind of
ultimate experiment. It will al (...truncated)