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 T/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:
High resolution (< 30') maps of the background temperature fluctuations will provide a key test of the mechanism by which structure was formed in the early Universe. Inflationary models predict Gaussian fluctuations, whereas non-Gaussian fluctuations are predicted by models in which irregularities are generated by topological defects such as strings, monopoles and textures. By searching for non-Gaussian signatures in the COBRAS/SAMBA maps we can distinguish between these theories, and determine what types of defects are present.
Most theories of the origin of the fluctuations in the universe predict that the potential fluctuations in the early universe should be independent of the scale of the irregularities. A more general power law fluctuation spectrum, in which the square of the fluctuations vary as the power (1-n) of the irregularities, leads to temperature anisotropies which scale with angular size as the power (1-n)/2 for angles larger than about 30' times the square root of the Omega parameter (The Omega parameter is the density of the Universe at the present epoch divided by the critical density). The high sensitivity of COBRAS/SAMBA and especially the extension to high multipoles will allow an accurate measure of the spectral index n. A significant deviation from the n=1 `scale-invariant' prediction of even a few percent would have extremely important consequences for the inflationary paradigm.
Gravitational waves (tensor modes) in addition to the more usual density perturbations (scalar modes) can lead to temperature anisotropies on large angular scales. Inflationary models predict that the ratio of the tensor and scalar mode anisotropies is related to the spectral index n of the fluctuations. This relation, which offers a key test of the inflationary picture, can be tested from the COBRAS/SAMBA maps since the temperature anisotropies from scalar and tensor modes vary with multipole in different ways (see Figure 1).
Temperature anisotropies on angular scales smaller than about 30' (times the square root of Omega0) are expected to arise mainly from the Thomson scattering of photons by moving electrons, rather than from the potential fluctuations that dominate on larger angular scales. The small-scale anisotropies are especially sensitive to the ionization history of the universe and as Figure 1.1 shows, sub-degree scale structure can be erased if the intergalactic medium were reionized at high redshift. Evidence for early reionization would set important constraints on theories of galaxy formation, especially models such as cold dark matter which predict that the first non-linear structures form at recent epochs. The temperature anisotropies on small angular scales depend also on several other factors, e.g. the initial spectrum of irregularities, the baryon density of the Universe, the nature of the dark matter and the geometry of the Universe. The COBRAS/SAMBA maps will provide constraints on these parameters within the context of specific theoretical models.
The temperature anisotropies measured by COBE correspond to present day structures with physical sizes larger than 1000/h Mpc (h is Hubble's constant in units of 100 km/s per Mpc), more than ten times larger than the largest structures observed in the galaxy distribution. The much higher angular resolution of COBRAS/SAMBA will resolve structures comparable in scale to clusters of galaxies (about 10/h Mpc) and so allow a much more direct link between observations of galaxy clustering, galaxy peculiar velocities and temperature anisotropies.
Temperature anisotropies are caused by the frequency change of microwave background photons scattered by hot electrons in the gaseous atmospheres of rich clusters of galaxies (the `Sunyaev-Zeldovich' effect). The effect has been observed in the cores of a handful of rich clusters of galaxies from the ground. COBRAS/SAMBA should measure the Sunyaev-Zeldovich effect in at least 1000 rich clusters; this measurement can be combined with X-ray observations to estimate the Hubble constant. The Sunyaev-Zeldovich effect can be used to constrain the evolution of rich clusters of galaxies.
Using the high sensitivity of COBRAS/SAMBA and the sub-mm bolometer channels it should be possible to provide a statistical detection of temperature fluctuations caused by the peculiar velocities of rich clusters of galaxies, providing a powerful test of theories of structure formation and on the mean mass density of the Universe.