Scientific Rationale

CMB Anisotropy Scientific Rationale

One of the most challenging problems in cosmology is the identification of primordial density perturbations which grew to form the structures that we see in the Universe today. Such perturbations left imprints as small temperature anisotropies in the cosmic microwave background radiation; by studying these temperature irregularities we can learn about the ultra-high energy conditions in the very early universe. The detection of temperature anisotropies by the COBE satellite has opened up the field of microwave background research. However, the COBE are limited by sensitivity, and restricted to angular scales greater than 7 degrees,which is much more extended than the precursors of any of the structures observed in the Universe today. The COBE results are compatible with a wide range of cosmological theories. Temperature anisotropy measurements at high sensitivity on angular scales of one degree or less are required to discriminate among the various theories, and to obtain strong constraints on: (i) the existence and characteristics of a very early phase of exponential expansion (inflation) of the Universe; (ii) the possible presence of primordial topological defects, seeds of the observed large scale structure; (iii) the origin of galaxies and clusters of galaxies, and (iv) the nature of the dark matter.

We here describe a new satellite, COBRAS/SAMBA, that will provide decisive high-angular resolution mapping of the microwave background anisotropies over almost all of the sky and over a wide frequency range. COBRAS/SAMBA will have 10 times the sensitivity and more than 50 times the resolution of the COBE satellite, and will provide fundamental data with which to test theories of the early universe and the origin of structure. COBRAS/SAMBA will far exceed the performance of balloon-borne and ground-based experiments. COBRAS/SAMBA will also provide information on the hot gas and motions of rich clusters of galaxies and valuable information on the small and large scale structure of the interstellar medium.

COBRAS/SAMBA will use tuned radio receivers over the frequency range 30 -125 GHz and bolometers over the range 100 - 800 GHz. The detectors will be mounted in the focal plane of a Gregorian optical system. The angular resolution of COBRAS/SAMBA will vary from 30' at the lowest frequency to 3' at the highest frequency; the final anisotropy maps will have a resolution as high as 7', much better than the 7 degrees afforded by COBE.

It is the combination of high sensitivity, large sky coverage, high angular resolution and wide frequency range, allowing for the accurate removal of foreground contamination, that makes COBRAS/SAMBA more powerful than any other cosmic background anisotropy experiment planned for the next decade.

COBRAS/SAMBA will provide a near all-sky map of the background anisotropies in 8 channels over the frequency range 30 - 800 GHz, with a peak sensitivity of deltaT/T ~ 1E-6 per pixel (1 sigma) in the frequency range 100 to 300 GHz. In addition, the observing strategy is designed to provide higher sensitivity (a factor of 3 better than that achieved in the large scale survey) in targeted areas covering about 2% of the sky. The low and high frequency channels will be used to map the foreground emissions, in order to distinguish them from the primordial anisotropy structure. The COBRAS/SAMBA maps will provide a detailed picture of the background fluctuations in which individual hot and cold spots should be visible well above the statistical noise level. Furthermore, COBRAS/SAMBA will provide a measure of multipoles of the temperature anisotropies from l=1 (dipole) up to l=5000 (corresponding to the resolution limit of 7'), in contrast to COBE which provides no useful information on multipoles l> 20.

The COBRAS/SAMBA maps of the background anisotropies will allow us to answer the following fundamental questions: