—8— The Cosmic Microwave Background Jean-Michel LamarreI Abstract The Cosmic Microwave Background (CMB) is one of the pillars on which the Big Bang theory relies. High-quality maps of this radiation strongly constrain the history of the Universe and its content. Such maps require accurate and sensitive measurements of tiny random features on a strong uniform background. Stray ra- diation must be rejected extremely efficiently and the response and noise of the instrument should be known to better than a percent, while full sky maps are needed for optimal data reduction. This is better achieved outside the atmosphere in the conditions of space. In a field where fundamental physics and astrophysics are closely related, the major advances came from the COBE and WMAP space- craft, and further progress is now expected from the Planck mission and also from recently developed ground-based experiments with dedicated goals achievable on fractions of the sky. From ground-based discovery to space exploration The Cosmic Microwave Background was generated a few hundred thousand years after the Big Bang, when ionized hydrogen was cooling down enough to recombine and become transparent. Photons were scattered a last time by the “last scattering surface” and then travelled for about 14 109 years until the present. The expansion of the Universe has redshifted the CMB× radiation from the infrared to the millimetre wave range, in which we can detect them in our epoch. Mapping the CMB provides a picture of the past Universe, or most exactly of the part that is visible to us now. One can expect from its observation to gain an impressive wealth of information and a large number of ways to question our understanding of physics and astronomy. It should unveil features that may verify or falsify the most currently accepted principles. Its simple existence is additional evidence in favour of the Big Bang theory. The light emitted by the last scatter- ing surface informs us about the physical conditions (density and temperature) prevailing only 3 105 ato4 105 a after the Big Bang, but also about what hap- pened earlier. This× can be compared× with the fact that we can study accurately the internal structure of the Sun by observing the movements of its surface. The ILERMA, Observatoire de Paris and CNRS, Paris, France 149 150 8. The Cosmic Microwave Background difference is that this concerns the Universe as a whole, from its earliest moments up to now. Promises of major discoveries were more than kept by two space mis- sions, COBE and WMAP, which have produced high-quality maps of the sky at millimetre wavelengths. A third one, the Planck mission, has a strong potential for new discoveries. Although the CMB is the most important source of photons in the sky outside the solar system, its first detection came rather late, probably because the signal from ground-based radio receivers results from the co-addition of receiver noise, signal picked-up from the antenna environment, thermal emission from the antenna and the atmosphere, and radiation from the sky. Understand- ing and separating signal components with a high-quality antenna was a full-time job at which Penzias and Wilson (1965) were occupied when they discovered an “excess antenna temperature” that was not identified with any of the first three components. They published this result, for which they won the 1978 Nobel Prize for physics. After the discovery of the CMB, it became clear that observing from above the atmosphere was an efficient way to get rid of the atmosphere itself and of the thermal emission from the immediate environment of the antenna. Dicke dif- ferential radiometers aboard a high altitude U2 airplane measured a small dipole in the CMB radiation (Smoot et al 1977). But even at the altitude of stratospheric balloons, i.e., 30 m to 40 km, the residual atmospheric emission proved to remain a strong source of uncertainty in large parts of the spectrum (Woody and Richards 1981). These experiments have confirmed the richness of the domain and proved the efficiency of new instrumental concepts that found their full potential only when brought to space on the COBE spacecraft. The Cosmic Background Explorer (COBE) COBE, launched in November 1989 into a Sun-synchronous orbit, included two microwave experiments (Boggess et al 1992). Both instruments have unveiled, with an angular resolution of 7◦, an image of the microwave sky nearly uncontaminated by the radiation from Earth or from other sources in the sky. The Far Infrared Absolute Spectrophotometer (FIRAS) has measured the sky radiation between 30 GHz and 600 GHz with a spectral resolution of 30 GHz (Mather et al 1990). The measured CMB spectrum (Figure 8.1, right) is near to that of a black body at (2.725 0.001) K (Fixsen and Mather 2002). Deviations from a perfect black body are less± than 50 parts per million of its peak emission. This very pure spectrum ruled out a number of alternative theories developed to explain the CMB without a Big Bang. The differential maps (Figure 8.1, left) produced by the Differential Microwave Radiometer (DMR) show an incredibly featureless sky (Smoot et al 1992). COBE has revealed that the observable Universe is uniform at very large scales, which has strong theoretical implications. To produce such a homogeneous image, a link must exist between regions of the last scattering surface that are too far from each other to have interacted since the Big Bang. This could have happened only in a very early period, before a phase of extremely rapid inflation that separated these regions. It is only by increasing the contrast thousands of times (Figure 8.1, middle left) that one can see a dipole component. The spectrum of the dipole has been measured by FIRAS and its shape is consistent with the 151 Figure 8.1: Results from the COBE spacecraft show that microwave sky is occupied by a nearly featureless source with a spectrum near to perfect black body at 2.7 K, the Cosmic Microwave Background radiation. The FIRAS results (right, Fixsen et al 1996) show the measured spectrum and the theoretical one from a black body. Maps reconstructed from the FIRAS and the DMR data (left, from http: //lambda.gsfc.nasa.gov/product/cobe/) show a very uniform sky (upper left). It is only by increasing the contrast thousands of times that one can see the 3 mK dipole component (middle left). Further contrast enhancement reveals that there are, in addition to galactic emission, very small but significant deviations from uniformity (DMR data, lower left). assumption that it is produced by a Doppler shift of the monopole. Its value of (3.372 0.007) mK implies that the Sun’s peculiar speed with respect to a co- moving± frame is (371 1) kms−1. Further contrast enhancement reveals a very small but significant random± “anisotropy”, i.e., deviations from uniformity, with RMS amplitude of (29 1) K at the angular resolution of 10◦ (Bennett 1996). Such anisotropies were± keenly expected as a major sign that we understood the nature and the origin of the Cosmic Microwave Background radiation (CMBR). They were expected to be the remnants of quantum fluctuations during the first moments of the Big Bang. And without them, it would have been very difficult to understand how such an extremely uniform source could have produced the highly structured Universe that we can observe now in our neighbourhood. J. Mather and G. Smoot received the 2006 Nobel Prize for physics for the results from the COBE spacecraft. Although its angular resolution was only 7◦ and the signal to noise only 2 per beam on anisotropies, COBE has produced a wealth of major discoveries. 152 8. The Cosmic Microwave Background Figure 8.2: Left (Bond et al 1994): Power spectra as a function of “l” for scale-invariant models, with various values given to the cosmological constant (ΩΛ = 0;0.4; 0.5), Hubble constant (H = 0.5 and 0.65), and baryonic content (ΩB =0.5 and 0.3). Right (Hinshaw et al 2009): C(l), measured by WMAP based on the five year data including the best fit model (see Table 8.3). Entering the era of precision cosmology with WMAP and Planck Imprints of the early Universe history and seeds of future large structures were expected to be observable in the CMB anisotropy (Silk 1968; Sunyaev and Zeldovich 1970). On angular scales 1◦, the CMB probes fluctuations in the gravitational potential (Sachs–Wolfe effect,≥ Sachs 1967) while smaller scales probe the sound waves prior to recombination. Even before the discovery of the CMB anisotropies by COBE, it was understood that high-sensitivity measurements could be and had to be made by new types of instruments. It was shown by Bond et al (1994) and Jungman et al (1996) that accurate, high resolution measurements of the CMB could be used to determine many of the cosmological parameters of our Universe not fixed by the various models of cosmology, for example its content (mass, energy), its geometry (curvature), and its dynamics (past and future of the expansion), as shown in Figure 8.2. These constraints can be used alone or in conjunction with other tracers of the large scale properties of the Universe. They provide strong indications on what happened during the first moments of the Big Bang, well before CMB photons are emitted. This is possible in particular because the physics that takes place at and behind the last scattering surface is amazingly simple and well understood (Sunyaev and Chluba 2007). In a way similar to solar physicists who study the internal structure of the Sun by observing its visible surface, cosmologists analyse the imprint left by physical phenomena that happened long before and are not directly observable.