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Observations of the CMB

In 1992, the NASA COBE satellite detected small (T/T ~ 10-5) temperature irregularities in the microwave background radiation temperature on angular scale (Smoot et al. 1992). The COBE results provided the first convincing detection of primordial temperature anisotropies, which had been searched for earnestly by experimenters since the discovery of the microwave background by Penzias & Wilson 1965. COBE has provided low resolution maps of the sky at three frequencies, 30, 53 and 90 GHz and with relatively poor sensitivity (T/T ~ 10-5 at 53 GHz) after four years of observation. The COBE measurements are consistent with ground based measurements from Tenerife at frequencies 10, 15 and 32.5 GHz (Watson et al.1994) and with the balloon-borne Far-infrared Survey (Ganga et al.1993). Thus at large angular scales ( 5°) there is strong evidence that the temperature irregularities are independent of frequency over the range 10--170 GHz, as expected if they are of primordial rather than of Galactic origin.

The COBE discovery has stimulated a concerted effort to detect primordial anisotropies on smaller angular scales ~ 1° (e.g. Fischer et al. 1992, Gaier et al. 1992; Schuster et al. 1993, Meinhold et al. 1993, Wollack et al. 1993, Cheng et al. 1994, de Bernardis et al. 1994, Gundersen et al. 1995, Ruhl et al. 1995, Tanaka et al. 1996, Netterfield et al. 1997, Platt et al. 1997, Tucker et al. 1997; for recent compilations see Lineweaver et al. 1996, Rocha & Hancock 1996). Figure 1.1 shows a recent compilation of CMB experiments (top) and the region in frequancy and multipole space (bottom) covered by these experiments (taken from M. Tegmark's web page).

Figure 1.1 (Top): CMB Experiments as of April 1999. (Bottom): Angular and frequency sensitivity of various experiments. For future multichannel experiments, the crude boxes have been replaced by polygons, the highest l-value at each channel being that at which its effective noise level (see Knox 1995 or Tegmark & Efstathiou 1996) exp(^2 l^2)/w equals that of Qflat=20 microkelvin CMB fluctuations. Likewise, the colored regions show where fluctuations in various foregrounds are expected to exceed those of Qflat=20 microkelvin CMB fluctuations in the cleanest 20% of the sky. The foregrounds are dust (red), point-sources (green), synchrotron (magenta) and free-free emission (cyan), and the estimates are based on Tegmark & Efstathiou 1996.

These experiments utilize either radio frequency techniques at dry, high altitude, sites such as the South Pole, or balloon-borne bolometers sensitive at frequencies 150 GHz. Almost all of these experiments report detections of temperature anisotropies with an amplitude T/T ~ 1 - 4 x 10-5 , but the limited sensitivity and frequency coverage of any single experiment, particularly those at radio frequencies, precludes any firm conclusion that the anisotropies are primordial.

To meet the science goals of this proposal, an experiment with high angular resolution ~ 10', and high sensitivity T/T ~ 10 -6 is required that maps a large fraction of the sky over a wide frequency range. As we will show in this proposal, the latter requirements are essential to measure the statistical properties of the CMB fluctuations to high accuracy and to monitor and remove Galactic and extragalactic foregrounds. These requirements are formidable, and can only be met by a satellite experiment.

It is important to emphasize that balloon-borne experiments planned for the next decade cannot achieve the comprehensive frequency and sky coverage of PLANCK. For example, the TopHat experiment of Kowitt et al. 1995 (frequency range 150-630 GHz, angular resolution of 0.5°) will be mounted on top of a balloon to reduce ground and atmospheric emission, and is expected to measure temperature differences at 40 points on the sky in a single long duration flight of two weeks. The area covered by TopHat per flight is only one four-thousandth of the area of sky covered by PLANCK and would be observed with considerably lower resolution. Furthermore, it is exeedingly difficult to design a balloon-borne experiment free of spurious atmospheric and ground emissions to an accuracy of T/T 1 x 10-5, which is an order of magnitude higher than the sensitivity required for the sientific goals of this proposal. Similar considerations apply to ground based experiments, including interferometric experiments such as the Cambridge Anisotropy Telescope (O'Sullivan et al. 1995). A satellite experiment is the only feasible way of simultaneously achieving a large sky coverage at high angular resolution and sensitivity over a wide frequency range, while eliminating atmospheric and sidelobe contamination. This has been widely recognized by the experimental community, both in Europe and the USA, with proposals for CMB anisotropy satellites under serious consideration by ESA and NASA.

How different will the CMB sky look to PLANCK compared to COBE? The maps in Figure 1.2 show simulations of the CMB anisotropies expected in an inflationary cold dark matter model. The upper figure is constructed at the angular resolution and signal-to-noise of the second year COBE maps (Bennett et al. 1994). The lower figure shows how the sky would appear at the much higher angular and sensitivity of PLANCK. The individual hot and cold spots seen in the PLANCK simulation have physical sizes comparable to those of clusters and superclusters of galaxies observed in the present Universe. Evidently, we expect to see a wealth of fine-scale structure in the microwave sky. The primary goal of PLANCK is to map these structures with high precesion, free of foreground contamination, so enabling us to address the scientific questions described in the rest of this proposal.

Figure 1.2: Simulated maps of the background radiation anisotropies expected in an 0=1 CDM model. The upper picture shows a simulated COBE map with a resolution of FWHM=7° together with random noise as in the COBE two-year data. The lower panel shows the same realization of the sky at the much higher angular resolution and signal-to-noise of PLANCK.

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[last update: 1 August 1999 by P. Fosalba]