Since the announcement in 1992 of the first results from the Cosmic Background Explorer (COBE ), there has been an enthusiastic resurgence of interest in the study of the anisotropies of the cosmic microwave background (CMB), fueled by the realization that the CMB can set very significant constraints on our current cosmological paradigm. The main objective of the COBRAS/SAMBA mission is to build on the pioneering work of COBE and fulfill this potential.
The special status of CMB observations is well illustrated by one of COBE's major achievements, i.e. the absolute measurement of the spectrum of theCMB to an accuracy of 10E-3 per spectral element. The CMB, which represents 99% of the electromagnetic content of the Universe, was generated at an early time when the Universe was nearly uniform and close to thermodynamic equilibrium, a time when all the relevant physical processes are linear and easily understood. Consequently, high precision measurements can be used to constrain cosmological parameters to correspondingly high accuracy.
Another major achievement of COBE was the detection of the anisotropies of the CMB on large angular scales at a level of T/T ~ 1E-5. This detection, although very important, was obtained with scant signal to noise (~1) and angular resolution (~7 degrees), and is still far from the fundamental limits to the precision achievable in such a measurement; limits which are set by confusion due to emission from other astrophysical sources (e.g. galactic foregrounds, and background levels due to extragalactic point sources). True imaging of the CMB fluctuations with a precision approaching the limits set by astrophysics still remains to be accomplished, but can be achieved by an experiment which combines high angular resolution, high sensitivity, wide frequency coverage, and excellent rejection of systematic effects. This combination of requirements cannot be met by either ground-based or balloon-borne observations, but rather demands a space mission such as COBRAS/SAMBA.
Detailed computations of the properties of the CMB anisotropies predict that most of the cosmological information in the CMB is contained in angular scales of order and larger than 10 arcminutes. Detailed analysis and simulations of the foreground contaminants show that it is possible to map the CMB fluctuations with a sensitivity in 10 arcminute pixels which is 10 times better than the one obtained by COBE. Thus the basic scientific goal of the COBRAS/SAMBA mission is to measure the CMB anisotropies at all angular scales larger than 10 arcminutes, with an accuracy set by astrophysical limits.
One immediate result of such a measurement is the determination of the angular power spectrum and of the statistical properties of the fluctuations originated in the early universe. Since particle physics at the extremely large energies involved (10E16 GeV) cannot be tested in any accelerator, this determination constitutes the most direct way of testing not only the physics of the early Universe, but also fundamental theories of high-energy physics. In particular, it will establish the nature of the primordial fluctuations, e.g. whether they are due to topological defects or quantum fluctuations.
In either case, fluctuations at angular scales larger than one degree depend primarily on the primordial spectrum, but as smaller angular scales are probed, the power spectrum depends more on physical processes which are sensitive to most of the fundamental cosmological parameters. Very accurate measurements of the CMB anisotropies at high angular resolutions can therefore be used to constrain strongly (to a few percent) basic parameters such as the Hubble constant, the geometry of the universe as characterized by the total density parameter, the cosmological constant, the baryon content and the nature of the dark matter. Observational cosmology has been struggling for more than 50 years to constrain these parameters from local measurements, but the remaining uncertainties are still very large: e.g. the Hubble constant is uncertain by a factor of 2 and the density parameter by a factor of 10. The astrophysical processes involved in these local measurements are for the most part nonlinear (evolution of galaxies, supernovae, giant HII regions, hot gas in clusters, cluster dynamical mass, etc), and they offer no hope of reaching the accuracies achievable with measurements of the CMB.
A critical issue that is extensively discussed in this report is the ability to separate the observed microwave signal into the various astrophysical components that contribute to it. The galactic foregrounds around 100 GHz are much weaker than the CMB in the cleanest 50% of the sky. In this region, COBRAS/SAMBA will be able to determine the amplitude of CMB fluctuations to an uncertainty better than T/T ~ 2E-6 at all angular scales larger than ~10 arcminutes. To achieve this sensitivity level on the CMB measurements, the amplitude of the foregrounds has to be determined to a relatively low precision of ~10%; this will be accomplished using the information on the spectral behavior of the foregrounds which is contained in measurements at frequencies where they are much stronger than the CMB. A major simulation effort has been carried out during this study that demonstrates the feasibility of the separation process with the required accuracy.
COBRAS/SAMBA will not only yield CMB anisotropies, but also near-all-sky maps of all the major sources of microwave emission, opening a broad expanse of astrophysical topics to scrutiny (see the accompanying Table). These maps will constitute a product which is comparable to the IRAS and COBE-DIRBE maps at shorter wavelengths. The IRAS data have been in use by the community for over 10 years with a scientific output which has remained roughly constant throughout this period. The COBRAS/SAMBA data set will have a similar impact on many areas of astrophysics. In particular, the physics of dust at long wavelengths and the relative distribution of interstellar matter (neutral and ionized) and magnetic fields will be investigated using dust, free-free and synchrotron maps. In the field of star formation, COBRAS/SAMBA will provide a systematic search of the sky for dense, cold condensations which are the first stage in the star formation process. One specific and local distortion of the CMB which will be mapped by COBRAS/SAMBA is the Sunyaev-Zeldovich (SZ) effect arising from the Compton interaction of CMB photons with the hot gas of clusters of galaxies. The very well defined spectral shape of the SZ effect allows it to be cleanly separated from the primordial anisotropy. The physics of gas condensation in cluster-size potential wells is an important element in the quest to understand the physics of structure formation and ultimately of galaxy formation. The COBRAS/SAMBA data set will be an extremely useful complement to X-ray data from the XMM observatory for such studies: the COBRAS/SAMBA SZ measurements are in fact more sensitive than XMM for the detection of clusters at redshift larger than 0.5, and to detect the gas in the outskirts of the clusters, but X-ray data will be needed to determine the redshift, the gas temperature, and for studies of the physics of the central cores of clusters. From the SZ data can also be extracted a signal which is sensitive to deviations of cluster velocities from the Hubble flow: the sensitivity of COBRAS/SAMBA will allow the determination of the large scale peculiar velocity fields as traced by ensembles of clusters.
Finally the survey will detect several thousands of extragalactic sources in a frequency range little observed so far. It will certainly find many new sources and considerably increase our knowledge of the spectra of star burst galaxies, AGNs, radio galaxies and quasars in the millimetre and submillimetre wavelength range.
Achieving the scientific objectives of COBRAS/SAMBA presents a significant challenge to the design of its payload, but one that can be met. This study shows that technological progress since the time when the COBE satellite was defined allows the large required increase in both angular resolution and sensitivity. In addition, COBRAS/SAMBA can be developed within the boundary conditions of the call for proposals of the ESA's M3 mission.
It is now possible to achieve instrumental sensitivities of microKelvin magnitude thanks to the recent rapid progress in detector technology and the resulting increase in their performance. Amplifiers using High Electron Mobility Transistors (HEMT) now operate above 100 GHz with good noise figures at temperatures which can be reached by passive cooling. New types of bolometers cooled to 100 mK have noise figures a thousand times better than e.g. those used in the COBE/FIRAS experiment, and provide a total power output with negligible 1/f noise over time scales of minutes. A dilution cooler that can reach these very low temperatures in space is now available and can be coupled to active space-qualified cryocoolers. Low-mass high-accuracy reflectors made of carbon fiber epoxy material have been developed and fulfill the requirements of the mission.
The model payload consists of a 1.5 meter off-axis telescope with an optical design which minimizes off-axis distortions and two focal plane arrays of detectors sharing the focal plane. The low frequencies (30 to 125 GHz) are covered by 56 tuned radio receivers grouped into four channels and the high frequencies are covered by 56 bolometers divided among 5 channels. The straylight level as well as the thermal stability of the telescope are controlled by a system of optical baffles.
To minimize the contributions of the strong sources of radiation present in the sky (Earth, Sun and Moon), and to reach more easily the required temperature and thermal stability, the satellite will be placed into orbit around the outer Lagrange point of the Sun-Earth system. The technical requirements on the satellite are very moderate by today's standards, and can be implemented in a spin-stabilized spacecraft which minimizes its cost.
The sky scanning strategy is simple. The optical axis of the telescope is offset by 70 degrees from the rotation axis, and scans one full circle on the sky every minute. Near-full sky coverage is achieved by periodically displacing the rotation axis to remain within 15 degrees of the anti-solar direction. The nominal mission duration calls for two coverages of the sky, to be achieved within 14-15 months of routine operations. Very simple spacecraft operations will consist of a daily set of twelve preplanned manoeuvers automatically carried out by the on-board computer.
Planning of the science operations, as well as data reduction, will be carried out by a Science Team (ST). The ST will oversee and direct the work carried out by three Consortia of scientists, centered respectively around three hardware teams, one for each of the two focal plane instruments, and the third for the telescope. The Consortia will include subteams responsible for data processing and the production and delivery of the various scientific products which will be distributed to the astronomical community. Although conceived as a PI mission, the structure of the scientific operations scenario allows enough flexibility to include the active participation of the astronomical community in a wide range of activities, from hardware development to the generation of a large number of diverse scientific products.
The primary output of the mission will be 9 calibrated all-sky maps ranging in frequency from 30 GHz to 900 GHz and in angular resolution from 30 to 4.5 arcminutes. These maps will be made available to the astronomical community within one year after the end of the nominal 18-month mission. In addition to this basic product, the result of the first component separation analysis will be made available at the same time in the form of all-sky maps of the major physical processes: CMB anisotropies, Compton parameter, dust, free-free and synchrotron emission. Finally, the calibrated data stream and attitude reconstruction will also be made available for reanalysis by other groups.
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