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These are exciting times for observational cosmology. In 1992, the Cosmic Background Explorer (COBE) satellite detected the existence of temperature irregularities in the cosmic microwave background (CMB) radiation field (Smoot et al. 1992). These irregularities (usually referred to as anisotropies) were imprinted on the CMB by primordial perturbations generated within 10-35 seconds of the Big Bang, and provide us with a probe of the ultra-high energy conditions in the early Universe. COBE has opened up the field of microwave background research, and propelled it into a state of hectic activity. Experimental groups are devoting much effort on the design of new ground-based, balloon-borne and satellite experiments to improve on the poor angular resolution and sensitivity of COBE. This effort has been complemented by theoretical calculations that have revealed a wide horizon of new science that can be extracted from high precision measurements of the CMB anisotropies, science that ranges in time and scope from the physics of the early Universe to that of our present astrophysical environment.

We here summarize the case for a new satellite, PLANCK, that will provide a definitive high-angular resolution mapping of the microwave background over at least 95% of the sky and over a wide frequency range. PLANCK has been designed to have 10 times the sensitivity and more than 50 times the angular resolution of the COBE satellite. The simultaneous mapping of the sky over a wide frequency range by PLANCK will permit a separation of Galactic and extragalactic foregrounds from the primordial cosmological signal to exquisite precision over much of the sky. PLANCK will far exceed the performance of balloon-borne and ground-based experiments and will be superior to any other proposed satellite mission. PLANCK will operate in a survey mode, scanning at least 95% of the sky twice over in 12 months. High resolution (FWHM~5'-30') maps of the sky in 9 frequency bands in the range 30--1000 GHz will be made generally available within a year of the end of the survey, providing the community with near all-sky data of unprecedented quality at frequencies that have hitherto been little explored.

Very little is known about the Universe prior to the epoch of nucleosynthesis. For example, we do not yet know whether the large-scale uniformity of the Universe owes its origin to an early period of rapid expansion known as inflation. Neither do we know the origin of the primordial irregularities required to form galaxies and other structure in the Universe; e.g. are galaxies and clusters the product of quantum fluctuations generated during an inflationary phase or of topological defects, such as cosmic strings, created at an ultra-high energy phase transition? What is the spatial curvature of the Universe? What is the nature of the dark matter that dominates the present Universe? What is the relationship between primordial irregularities and the large-scale structure -- the chains, filaments and voids -- observed in galaxy surveys? These are some of the key scientific questions that PLANCK has been designed to answer. PLANCK will revolutionize our understanding of Cosmology, from fundamental questions concerning the origin and early phases of the Universe to the astrophysics of galaxy formation. The unprecedented quality of the maps provided by PLANCK will, however, open an even wider range of scientific problems: e.g. with certain general assumptions we show that it is possible to determine the fundamental cosmological parameters, in particular, the total density 0, the baryon density b, and the Hubble constant H0 to a precision of a few percent -- an accuracy that is far beyond what is possible with conventional astronomical techniques. Such high precision measurements will have a major impact on practically every aspect of astronomy, including the physics of the early Universe, determinations of the extragalactic distance scale, primordial nucleosynthesis, stellar ages, dynamical measurements of the mean mass density and the origin of large-scale structure in the Universe. Many thousands of individual extragalactic and Galactic sources will be detected by PLANCK via their spectral signature, including clusters of galaxies, infrared luminous galaxies and Galactic star forming regions. Furthermore, the PLANCK maps will revolutionize our understanding of diffuse background emissions, again with wide ranging implications extending from the contribution of primordial galaxies to the far-infrared background, to the origin of Galactic synchrotron emission and the nature of Galactic dust. It is important to emphasize that PLANCK, while primarily a mission designed to solve cosmological problems, will provide data that will be invaluable to an extremely wide astronomical community.

PLANCK will also have a strong impact on particle physics. Observations of the CMB anisotropies are one of the very few ways of testing physics at energies greater than ~ 1015 GeV and are of crucial importance in developing fundamental theories of high energy physics (e.g. supersymmetry, superstrings and quantum gravity). Accurate investigations of the CMB anisotropies are thus complementary to the next generation of accelerator experiments such as those planned with the Large Hadron Collider (LHC) at CERN and provide a critical link between high energy physics and the formation of structures -- quasars, galaxies, clusters and superclusters of galaxies -- that can be observed by astronomers.
The key scientific objectives of PLANCK are as follows:

The main science goals of PLANCK are described in the following Sections. However Figure 1.1 shows one specific example of the dramatic impact on cosmology that we can expect from PLANCK. PLANCK will produce maps of the CMB anisotropies with about 4000 times as many resolution elements as the COBE maps and with more than an order of magnitude improvement in the sensitivity/pixel. From this vast increase in information, we show in Section Determining Fundamental Cosmological Parameters) that it is possible to estimate fundamental cosmological parameters which determine the properties of the CMB anisotropies, in particular 0, H0, and b to a precision of a few percent or better. There is enough information in the fine scale structure of the CMB anisotropies to remove degeneracies between these and other parameters, such as the cosmological constant, , the residual optical depth to Thomson scattering, etc, which are also extremely uncertain at present. Figure 1.1 shows the joint probability distribution of H0 and 0 derivable from PLANCK, illustrating that it will be possible to constrain these parameters to a precision of about a percent, independently of the values of other cosmological parameters. The only assumptions involved in deriving these constraints are that the CMB anisotropies arise from primordial adiabatic fluctuations (which can be verified from the PLANCK maps of the CMB) and that secondary CMB anisotropies, Galactic, and extragalactic foregrounds can be removed to an accuracy of T/T ~ 2x10-6. Such high accuracies are realistic, since the relevant physics describing the temperature anisotropies is linear and extremely well understood, a situation that is rare in astronomy. A high sensitivity CMB experiment thus offers the exciting prospect of determining fundamental cosmological parameters to accuracies unimaginable with conventional astronomical techniques.

Figure 1.1: The joint probability distribution of the cosmological density parameter, 0, and the Hubble constant H0, expected from fitting models of the primary CMB anisotropies to the COBRAS/SAMBA maps. The heavy lines show the 50% and 5% contours of the distribution, and have been projected into the H0 and 0 plane in the upper part of the figure. These estimates are derived under the general assumption that the CMB anisotropies arise from adiabatic perturbations in the early universe. In this specific example, we have assumed that 0=1 and H0= 50 km/s/Mpc.

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