The Scientific Programme of Planck

Abstract

For 40 years, the cosmic microwave background (CMB) has been the most important source of information about the geometry and contents of the Universe. Even so, only a small fraction of the information available in the CMB has been extracted to date. Planck, the third space CMB mission after COBE and WMAP, is designed to extract essentially all of the information in the CMB temperature anisotropies. Planck will also measure to high accuracy the polarization of CMB anisotropies, which encodes not only a wealth of cosmological information but also provides a unique probe of the thermal history of the Universe during the time when the first stars and galaxies formed. Polarization measurements may also detect the signature of a stochastic background of gravitational waves generated during inflation, 10^(-35) s after the Big Bang. This book describes the expected scientific output of the Planck mission, both cosmological and non-cosmological. Chapter 1 summarizes the experimental concept and the operation of the satellite. Chapter 2 covers the core cosmological science of the mission, describing the measurements that Planck will make, what we expect to learn from them about the geometry and contents of the Universe and about fundamental physics, and the combination of CMB data with other data to provide additional insights. Although the primary goal of Planck is cosmology, it will survey the whole sky with an unprecedented combination of frequency coverage, angular resolution, and sensitivity, providing data valuable for a broad range of astrophysics. Chapters 3, 4, and 5 describe non-cosmological astrophysical uses of the Planck data. This book can also be downloaded directly from http://www.rssd.esa.int/Planck .

Figures (151)

• Figure 1: - Main elements of Planck. The instrument focal plane unit (barely visible) contains both LFI and HFI detectors. The function of the large baffle surrounding the telescope is to control the far sidelobe level of the radiation pattern as seen from the detectors. The specular conical shields (often called "V-grooves") thermally decouple the Service Module (which contains all warm elements of the satellite) from the Payload Module. The satellite spins around the indicated axis, such that the solar array is always exposed to the Sun, and shields the payload from solar radiation. Figures courtesy of Alcatel Space (Cannes).
• Figure 2: -Planck focal plane unit. The HFI is inserted into the ring formed by the LFI horns, and includes thermal stages at$18 \mathrm{~K}, 4 \mathrm{~K}, 2 \mathrm{~K}$ and 0.1 K . The cold LFI unit ( 20 K ) is attached by bipods to the telescope structure.
• Figure 3: - Spectrum of the CMB, and the frequency coverage of the Planck channels. Also indicated are the spectra of other sources of fluctuations in the microwave sky. Dust, synchrotron, and free-free temperature fluctuation (i.e., unpolarized) levels correspond to the WMAP Kp2 levels ($85 \%$ of the sky; Bennett et al. 2003). The CMB and Galactic fluctuation levels depend on angular scale, and are shown for $\sim 1^{\circ}$. On small angular scales, extragalactic sources dominate. The minimum in diffuse foregrounds and the clearest window on CMB fluctuations occurs near 70 GHz . The highest HFI frequencies are primarily sensitive to dust.
• Figure 4: -Planck orbit at the 2nd Lagrangian point of the Earth-Sun system ($L_{2}$ ). The spin axis is pointed near the Sun, with the solar panel shading the payload, and the telescope sweeps the sky in large circles at 1 rpm .
• Figure 5: -The LFI radiometer array assembly (left), with details of the front-end and back-end units (right). The front-ends are based on wide-band low-noise amplifiers, fed by corrugated feedhorns which collect the radiation from the telescope. The waveguides transport the amplified signals from the front-end (at 20 K ) to the back-end (at 300 K ). They are designed to meet simultaneously radiometric, thermal, and mechanical requirements, and are thermally linked to the three V-groove thermal shields of the Planck payload module. The back-end unit, located on top of the Planck service module, contains additional amplification as well as the detectors, and is interfaced to the data acquisition electronics. The HFI is inserted into and attached to the frame of the LFI focal-plane unit.