The cosmic microwave background

Europhysics News, Jul 2018

Jean Michel Lamarre, Jean-Loup Puget

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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 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 " ~. _ .• r U :5 bUUU r-';2 4000 ­ .3 I': "2 ~ , ~ ;;. 2000 I JU UU I:J ~u .t<A lLJe~1 _ _ _~~ 1J'U:)IJUI1t',(,.tI. I I I I I I I I ' I , I I I I I I I I I 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 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 scales. 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 10' For a detailed discussion of all these questions we refer the reader to a more extensive review by Bouchet, Puget and Lamarre.8 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 this experiment. 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 .

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Jean Michel Lamarre, Jean-Loup Puget. The cosmic microwave background, Europhysics News, 212-213, DOI: 10.1051/epn:2001603