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4. MAP and Planck

Two satellite missions are currently planned to study the CMB from space, where the whole sky can be imaged, far from the complicating effects of the atmosphere. The NASA Microwave Anisotropy Probe (MAP [53]) is due for launch in November 2000. It will travel to the Earth-Sun outer Lagrange point, L2, where it will map the sky at 5 frequencies between 22 and 90 GHz, reaching to l ~ 800 in the power spectrum. The careful control of systematics possible with an extended space mission means that MAP should represent a very large improvement over the data available from the Earth-based experiments.

The ESA mission Planck can be thought of as the third generation CMB satellite, mapping at 9 separate frequencies between 30 and 850 GHz, with both radiometer and bolometer technologies, and measuring the Cls to beyond l of 2000. Thus Planck is expected to measure essentially all of the primordial CMB power spectrum (see Figure 4), and cover all the frequencies required to measure and remove the foreground signals. The Planck data set should enable cosmological parameters to be constrained with exquisite precision - or, to put it another way, the power spectrum should be measured at a level of several million sigma. In addition Planck will measure the polarization (and cross-correlation with temperature) power spectra, providing even more information.

Figure 4

Figure 4. An estimate for how well the power spectrum might be measured by Planck. This is a realization of a CDM power spectrum, assuming the Planck instrumental sensitivity over two thirds of the sky. Planck should supply us with essentially cosmic-variance limited information on all the angular scales relevant to primary anisotropies, over the full range of relevant frequencies.

In terms of particle physics, there will be constraints on anything which could potentially affect the anisotropies. This is just like Big Bang Nucleosynthesis constraining strange things occurring at ~ MeV energies, or ~ minute timescales. There have already been many studies (too many to list in detail) estimating how well various things could be limited by Planck data. These include: variation of fundamental constants; decaying particles; Lambda, Quintessence, rolling scalar fields, Dark Energy, etc. equation of state; alternative gravity models; parity violation; extra relativistic degrees of freedom; and just about anything else you can think of which isn't already ruled out.

Certainly cosmological parameters will be constrained. And definitely some messy astrophysical details will be uncovered (in the foregrounds, as well as through some weak processing effects occurring between z = 0 and 1000). And whatever the basic paradigm, there will surely be some clues to fundamental physics lurking in there, since the CMB anisotropies provide the cleanest information about the initial conditions and the largest scale properties of the Universe. What is therefore clear is that

We will learn a great deal about cosmology, astrophysics and particle physics from MAP and Planck

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