The Cosmic Microwave Background

Home , Archeops, Astron (spacecraft), Bolometer, Cosmic Background Imager

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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 radiation 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 spacecraft, 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 scattering 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 happened 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 missions, 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 differential 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. The sensitive data is the C(l), i.e., the power spectrum of the anisotropy map, that must be measured at moments l of the spherical harmonics ranging from zero (mean brightness of the sky) to about 3000 (features smaller than 7′) and a sensitivity of a few microkelvins. A first view of the C(l) was obtained through a number of remarkable ground-based and balloon-borne experiments. This requires a huge improvement with respect to COBE. The Boomerang (de Bernardis 153 et al 2000), Maxima (Hanany et al 2000) and Archeops (Benoˆıt et al 2003) balloon- borne experiments have shown images of the CMB on small fractions of the sky with good signal-to-noise ratio. Ground-based experiments, such as CBI (Pearson et al 2003; Readhead et al 2004), ACBAR (Arcminute Cosmology Bolometer Array Receiver, Runyan et al 2003; Kuo et al 2004), VSA (Scott et al 2003), and DASI (Halverson et al 2002) have unveiled the high-resolution part of the CMB spectrum. The CMB anisotropy consists of faint features that have to be distinguished from (a) the foregrounds, i.e., mainly our Galaxy and external galaxies, (b) the thermal emission of the Earth and other solar-system objects that are huge warm sources, (c) the self-emission of the spacecraft and the instrument themselves, especially their variation with time and (d) any additional signal of any origin, such as cosmic rays, the local magnetic field, electromagnetic interferences, etc. The stationary part of the noise from the instrument can be evaluated by powerful statistical tools, but its non-Gaussian and non-stationary parts and the other sources of systematics are much more difficult to evaluate and to remove from the data. They have to be reduced as much as possible. This difficulty has been considered as the major design driver of CMB experiments. In addition, an optimal interpre- tation of the C(l) will rely on full sky maps with a pixel size of less than 0.1◦. All these requirements can be met properly only with a space mission with adapted design and mission profile, although very significant observations can be achieved from the ground. This has been undertaken by NASA and ESA with the spacecraft WMAP (Bennett et al 2003) and Planck (The Planck consortia 2005). Both experiments have benefitted from a formal positive decision in spring 1997, a similar date in very different programmatic contexts, NASA favouring immediate start and early launch, while ESA was looking for synergy between Planck and Herschel and therefore a later launch. Consistently, they illustrate very different strategies. WMAP intended to take advantage of the available and proven technology to get major new results as soon as possible, while Planck was designed to perform at the limits set by physics, which required more risky and longer developments. We detail hereafter some of the requirements for a “precision cosmology” mission and the solutions used in each case, which is a good illustration of how two missions can be very similar, because the constraints are the same, yet have different ambitions and schedules.

Common requirements and diﬀerent ambitions

Orbit and general configuration Figure 8.3 shows the WMAP and Planck spacecraft at about the same scale and Table 8.1 gathers the main parameters of the two missions. The Lagrange point L2 of the Sun-Earth system was found to offer major advantages over a low Earth orbit to meet the requirements of a CMB mission. The Earth and the Moon are far enough ( 1.5 106 km) and their thermal radiation does not contaminate the faint signal≈ from× the anisotropies. In addition, they are approximately aligned with the Sun, and a rather natural configuration is obvious: the solar pan- els are used as a screen protecting the spacecraft from the Sun, the Earth and 154 8. The Cosmic Microwave Background

Figure 8.3: WMAP (left, from http://map.gsfc.nasa.gov/mission/observatory.html) and Planck (right) were designed for the same orbit and rather similar attitude with respect to the Sun. the Moon. The telescopes’ optical axes are nearly perpendicular to the Sun. The rotation of the spacecraft around an anti-solar axis ensures a natural coverage of the whole sky in six months, while the spacecraft follows the general movement of the L2 point around the Sun with the Earth. This configuration is extremely favourable for efficient passive cooling: 90 K for WMAP and less than 50 K for Planck, which reduces the thermal emission of the telescopes on the receivers and provides an excellent environment for cooling the WMAP receivers and for the cryogenic coolers of Planck. Finally, this orbit is thermally very stable. All these features minimize the parasitic signals that are the bane of instruments dedicated to measuring statistical properties of the sky. For both spacecraft, full maps of the sky have to be reconstructed from the signal produced by at least six months of data. The noise of the receivers contaminates these maps. Especially, the very slow post-detection noise (the so-called 1/f noise) makes stripes that must be removed by software with an efficiency that depends on the details of the scanning strategy. Map-making software relies on redundant observations of the same part of the sky with different scans, which allows the removal of drifts in the signal of instrumental origin. The WMAP strategy takes advantage of the three-axis attitude control and of the precession of the rotation axis to cover a wide stripe in the sky in one hour, while Planck scans successively the same circles on the sky at one revolution per minute during 45 min. This second option is more demanding in terms of signal stability (see the section on thermal design) but offers powerful and efficient solu- tion to the removal of systematics by comparing 45 successive measurements of the same circle of the sky.

Choice of the frequency range and receiver technology In principle, the CMB can be observed from radio frequencies (in the gigahertz range) up to about one terahertz, the frequency at which the 2.7 K black-body emission spectrum vanishes. When increasing the sensitivity, the foregrounds will be detected and start to contaminate the maps. Fortunately, the peak of the CMB 155

Table 8.1: Main features of WMAP and Planck spacecraft. WMAP Planck Sky coverage Full sky Full sky Back-to-back Gregorian, 1.4 m Gregorian, 1.5 m 1.8 m pri- Optical system 1.6 m primaries × mary × Scan angle 70◦ fromrotationaxis 85◦ from rotation axis Detection HEMT± amplifiers HEMT amplifiers + 100 mK bolometers Polarization sensi- I, Q, U Stokes parameters I, Q, U Stokes parameters (not tive at 545 GHz and 857 GHz) HEMT radiometric Sky/sky pseudo-correlation dif- Sky/4 K load differential system ferential Spin modulation 7.57 mHz spacecraft spin 16.6 mHz spacecraft spin Precession modula- ≈ 0.3 mHz spacecraft precession No≈ precession tion ≈ In-flight calibration Amplitude from dipole modula- Amplitude from dipole modulation tion Beam knowledge Calibration on Jupiter Calibration on planets Required tempera- About90K Telescope50K;HEMT20K; ture bolometers 0.1 K Cooling system Passive cooling Passive + H2 J-T + He J-T + 0.1 K Dilution Attitude control 3-axis controlled, 3 wheels, gyros, Spin controlled, thrusters, star star trackers, Sun sensors trackers, gyros Power 419W 2000W Mass 840kg 1450kg Launch DeltaII7425-10on30June2001 Ariane 5 dual launch (with HSO) at 3:46:46.183 EDT – 14 May 2009 Orbit 1◦ to 10◦ Lissajous orbit about Lissajous orbit about second La- second Lagrange point, L2 grange point, L2 Trajectory 3 Earth-Moon phasing loops, lu- Direct way to L2 orbit nar gravity assist to L2 Design lifetime 27 months = 3 month trajectory 17 months = 3 month trajectory +2aatL2 + 14 months at L2 and the minimum of foreground emissions coincide in the 30 GHz to 300 GHz range, with a rather clean window in the 60 GHz to 100 GHz range (Figure 8.4). The choice of the frequency coverage is a major driver of the design. With the sensitivity of Planck, the foregrounds are detectable and constitute a significant source of error if not removed from the data. The Planck choice was to associate in a single mission a High Frequency Instrument (HFI) (Lamarre et al 2003), based on the use of bolometers cooled to 100 mK, and a Low Frequency Instrument (LFI) (Valenziano et al 2007) using HEMT (High Electron Mobility Transistor) receivers at 20 K. The nine channels of Planck (3 LFI and 6 HFI) covering the 30 GHz to 1 THz range make up a self-consistent instrument, providing the measures needed to identify all astrophysical components and remove them from the map to recover the original CMB signal. At frequencies lower than 100 GHz, the best detectors are amplifiers using HEMT that can work at room temperature, although their sensitivity improves at low temperatures. This is the WMAP (Jarosik et al 2003) 156 8. The Cosmic Microwave Background

Figure 8.4: RMS ﬂuctuation spectrum (thermodynamic temperature units) of foreground components at 1◦ resolution (Davies et al 2006). Synchrotron (blue squares), free-free (green triangles) and anomalous dust-correlated emission (magenta circles)

add to the dust (red dot-dashed line). CMB is shown at 70 ñK RMS. Total foregrounds (black solid curve) show a clear minimum around 70 GHz. HEMTs are competitive up to 70 GHz, while bolometers are more sensitive at higher frequencies. and Planck-LFI option (Cuttaia et al 2004) with receivers cooled at 90 K and 20 K, respectively. At higher frequencies, bolometers (Bock et al 1995; Jones et al 2003) oﬀer a sensitivity up to 30 times better, which allows them both to measure more pixels with better angular resolution and to improve the signal to noise for each pixel. Higher frequencies consistently provide higher angular resolutions for a given telescope diameter. Very sensitive bolometers have to be cooled to 100 mK, which had never been done in orbit before Planck and is a major constraint on the system. In most of the bolometer channels of Planck, the sensitivity is limited by the fundamental limits set by the photon statistics of the observed source.

Spatial frequencies of interest and optical systems The CMB anisotropies contain information at all spherical harmonics up to the moment l = 3000, which is a great incentive to measure accurately on all angular scales, from 360◦ to less than 0.1◦. High-moment harmonics gather significantly less energy and are fully measurable with the sensitivity of Planck. In addition, the beam width filters out the highest spatial frequencies, which consistently pre- vents WMAP from measuring harmonic moments larger than about 1000. The main characteristics of all the frequency channels of the two missions are gathered in Table 8.2. The measurement of low-moment harmonics requires the coverage of the whole sky and an efficient rejection of parasitic sources that could be picked up 157

Table 8.2: Channels of WMAP and Planck. WMAP Planck LFI/HFI LFI HFI HEMT amps. x x x x x x x x Bolometers xxxxxx Centre freq. (GHz) 23 33 41 61 94 30 44 70 100 143 217 353 545 857 Bandwidth (GHz) 5.5 7.0 8.3 14 20 6 8.8 14 33 47 72 116 180 283 No. of unpol. dets. 0 0 0 0 0 0 0 0 044444 No. of polarized de- 2 2 4 4 8 4 6 12 888800 tectors Angular resolution 53 40 31 21 13 33 29 14 9.5 7.2 5 5 5 5 (FWHM, arcmin) Average ∆T/Ta 2.5 3.0 5.2 2.5 2.2 4.8 15 NA NA

per pixel (ñK/K) System temp. (K) 29 39 59 92 145 11 16 29 RJ sensitivityb per 800 800 500 600 400 110 120 130 14 12 10 8 10 7 1 2 freq. band (ñK s ) afor a 14 month mission; bRayleigh-Jeans sensitivity by diffraction or in any other way. For these reasons, going to space was a major step towards accurate CMB measurements. The control of far side lobes is an important issue that has to be achieved by an appropriate design of the RF system. At millimetre wavelengths, there is no “black paint” similar to what is used in the optical and infrared ranges to absorb stray radiation and one has to extrapolate the techniques of radio engineering. WMAP and Planck have rather similar telescopes (Fargant et al 2000; Page et al 2003), i.e., off-axis Gregorian-like arrangements were chosen, avoiding any sharp-edged diffracting object in the beam and therefore induced scattering of power in the far side lobes. This design allows for a sufficient focal plane area with a compact configuration that helps fit the fairing envelopes. The cost of compactness is that the beams on the sky are far from circular, which adds a serious difficulty to the data reduction. Both telescopes are coupled to the receivers by corrugated horns producing nearly Gaussian beams. The edge taper, i.e., the response to sources at the edge of the mirrors with respect to that at the centre is exceptionally low for radio systems (–20 dB to –35 dB), which provides a low level of spill-over, i.e., the fraction of the beam sensitive to flux from outside the main mirror is low. This is critical for avoiding contamination of the low-moment harmonics by pick-up in the side lobes of the large structures of the sky, such as our Galaxy. WMAP receivers are cooled down to about 90 K by a rather classical passive cooling, taking advantage of the favourable conditions and exceptional stability offered by the L2 orbit, intending to reduce systematic effects of thermal origin to a minimum. Temperature fluctuations are induced only by the annual variation of the distance to the Sun and by bursts of solar activity. In order to cool its bolometers to 0.1 K, Planck needs a complex chain of coolers. The passive cooling, based on a three-stage V-groove radiator and by a blackened baffle and structure around the telescope (Riti et al 2003), provides a cooling power of 1.5 W at about 60 K. The LFI HEMT amplifiers are cooled to 20 K by Joule-Thomson (J-T) expansion of hydrogen activated by a sorption pump (Prina and Bhandari 2001). The cooling 158 8. The Cosmic Microwave Background

Figure 8.5: WMAP data reveal tiny (sub-millikelvin) features that will develop later to become the large structures in the Universe (e.g., clusters of galaxies). Interpreting this image tells a lot about the nature and the history of the Universe and deeply challenges our understanding of physics, as illustrated by the pie chart of the content of the Universe (mass + energy), where “current” matter (atoms) is only 4.6 % of the total (ﬁgures from WMAP web site, http://map.gsfc.nasa.gov/news/).

Table 8.3: Predictions from the ﬁve year data of WMAP best-ﬁt Lambda Cold Dark Matter (ΛCDM) model (Hinshaw et al 2009). Age of the Universe (13.69 0.13) Ga +2±.6 −1 Hubble constant H0 (71.9−2.7) kms /Mpc Mean redshift of CMB photons 1090.51 0.95 Mean time of radiation-matter decoupling (380.08 ±5.84) ka ± Vacuum energy density ΩΛ 0.742 0.030 Amount of baryonic matter (4.41 ±0.30) % Amount of dark matter (21.4 ± 0.2.7) % ±

to 4.5 K of the HFI focal plane unit external box is obtained by J-T expansion of helium driven by mechanical pumps, while 1.6 K and 0.1 K stages are based on a complex 3He/4He dilution cooler (Triqueneaux et al 2006). Temperature ﬂuctua- tions induced by the coolers are a potential source of systematic eﬀects. Extremely stable temperatures are required for the 20 K stage for LFI (100 mK p-p), the 4 K −1/2 −1/2 stage of HFI (10 ñK Hz ) and for the plate of the bolometers (20 nK Hz ), achieved with sophisticated active temperature control (Leroy et al 2006). These features contribute to the complexity of the Planck spacecraft whose development was undertaken by a large international collaboration involving numerous insti- tutes and space agencies (e.g., ESA, NASA, CNES, ASI) while WMAP could be developed within the simpler frame of a NASA MIDEX mission. 159

Figure 8.6: Accurately measuring the C(l) curve should be achieved in the ﬁrst decade of the 21st century, as shown by the 3-year WMAP results (Spergel et al 2007) (lower left), the ACBAR measurements (Reichardt et al 2008) from the South Pole (upper left) and a simulation of the Planck data (lower right). Lower ﬁgures are from Balbi (2007).

A universe to be deciphered

The picture of the Universe given by COBE and WMAP (Spergel et al 2007) is consistent with the Big Bang theory and the Lambda Cold Dark Matter model (ΛCDM, assuming a cosmological constant different from zero), established in the frame of general relativity and the standard model of particle physics. It gives a good knowledge of the main parameters needed to describe the large scale geometry and the history of the Universe inside this frame (see Table 8.3). These results by themselves, consistently with those obtained by large sky surveys and supernovae surveys, converge to describe a reaccelerating Universe with a large cosmological constant and dominated by dark matter and dark energy (Figure 8.5). Dark matter is detected only by its gravitational interaction with visible matter. Dark energy is the energy needed to reaccelerate the Universe although we expect gravitation to slow down its expansion. We do not know the nature of this matter and we ignore everything on dark energy, the way it interacts, its history if any, etc. This opens large and exciting fields of physics that can be tackled by more 160 8. The Cosmic Microwave Background accurate measurements of the CMB, as planned with the spacecraft Planck and also ground-based experiments (Reichardt et al 2008 and Figure 8.6). Accuracies expected from Planck will open new windows to check the validity of the most accepted or unconventional models. Accurate measurements of polarization, expected both from Planck and ground-based experiments, should give independent confirmation of what temperature maps tell us, and also provide entirely new tests on the physics of inflation, by the detection of the signature of gravitational waves on the last scattering surface.

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