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.
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
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
. 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.
......-.-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 also measure the polarization of the
CMB in three channels, which will give independent and unique
information on the CMB anisotropy.
This kind of accuracy on the CMB can be achieved only by
removing the various astrophysical foregrounds emitting at these
frequencies. Among these, one expects the emission of dust and
gas in our own galaxy and from other galaxies. Clusters of galax
ies, that contain high-temperature gas detected in the X-rays,
distort the CMB spectrum by inverse Compton scattering. This is
the Sunyaev-Zeldovich Effect (SZE), which makes clusters of
galaxies good tracers of the dynamics of the Universe at large
The six bands in HFI and four more in LFI (Low Frequency
Instrument, also on 'Planck') are needed to characterise and
eliminate these various components by use of their spectral and
spatial signature. Planck must be considered not only as the third
generation of CMB satellites, but also as the first sub-Terahertz
all-sky survey of modern astronomy. Several thousands of galax
ies, young stellar objects, clusters of galaxies will be detected,
many of them for the first time. Nearly every field of astronomy
will benefit from 'Planck's' results. The 'Planck' project is commit
ted to deliver a set of well-defined products to the scientific
community at large.
The extremely high sensitivity (dTIT < 2 10-6) as well as the
high angular resolution of 'Plancl<' made possible by the use of
"Spider web" type bolometers developed at Caltech and flown on
the BOOMERanG, MAXIMA and ARCHEOPS experiments.
'Planck' will also be the first experiment that measures the polar
ization signal of the CMB on large scales.
europhysics news NOVEMBER/DECEMBER 2001
For a detailed discussion of all these questions we refer the
reader to a more extensive review by Bouchet, Puget and
2 Differential Microwave Radiometers
About the authors
Jean-Michel Lamarre, has 20 years of experience in IR and
submm space astronomy. He contributed to the development of
this field in France by leading or contributing to the conception
of a number of experiments: EMILIE at the South Pole, the bal
loon-borne AROME experiment, and the space projects:
CRYOSPIR, AELITA, SAMBA, FIRE and FIRST. He was the PI of
the imaging channel of IKS on the VEGA sounder to the comet
Halley, and ofSPM-PRONAOS that measured the positive part of
the SZ effect. He played a major role in the birth of the Planck
HFI concept and design, and is now the instrument scientist of
Jean-Loup Puget is an astrophyicist and Research Director at
CNRS. He is Mission Scientist for the ESXs Infrared Space
Observatory (ISO), and also PI for the High Frequency Instru
ment (HFI) on ESXs Planck mission. In 1984, together with Alain
Leger, he discovered the presence of large quantities of aromatic
hydrocarbons in interstellar matter.
References and Explanations of Abbreviations 1 Far Infrared Absolute Spectrophotometer 3 P. de Bernardis et al. 2000 , A flat universe from high resolution maps ofthe CMBR , Nature 404 , 955 .
4 A. T. Lee et aI., A high spatial resolution analysis ofthe MAXIMA-l cosmic microwave background anisotropy data, astro-ph/Ol04459.
5 A.D. Miller et al., A Measurement ofthe Angular Power Spectrum ofthe CMB from l=100 to 400, Accepled by Astrophys .J.Lett., Astro-ph/990642l.
6 N.W. Haverson et al., DASIfirst results: A Measurement ofthe Cosmic Microwave background Angular Power Spectrum , Astro-ph/Ol04489.
8 ER. Bouchet , J.1. Puget , J.M. Lamarre 2000 , The cosmic microwave background: from detector signals to constraints on the early universe physics , in 'The Primordial Universe', Binetruy et al., eds. (Les Ulis, Paris: EDP Sciences) pp. 103 - 220 .