An important side effect is that if we cannot see beyond the horizon, then neither can we be affected by any physical effect from beyond the horizon. Regions of space in the Universe which are separated in distance by more than the horizon simply do not know about each other, and cannot influence each other's physical conditions. If we calculate the size of the horizon in the sky for the Universe at the epoch of decoupling, it turns out to be approximately one degree (about twice the angular diameter of the Moon). The fact that the spectrum and intensity of the CMBR are essentially the same for patches much larger than this size is very hard to explain since our scenario does not allow these patches to communicate with each other and conspire to determine their physical characteristics. 

A solution to this dilemma was found in the early 1980's and is called 'inflation' theory. The concept, motivated by a merging of high energy particle physics theory with cosmology, is that within the first 10^-34 seconds of the Big Bang, the Universe went through a period of extremely rapid expansion. This causes our entire Universe to be derived from a very small region of space (before inflation), which would have very uniform properties because it was small and its constituent parts could indeed communicate with each other. Thus, when we observe the CMBR from parts of the sky which are separated by more than one degree, the intensity variations are those imposed by the conditions in the pre-inflationary era (when the Universe was less than 10^-34 seconds old!). After inflation, there is simply no way for physical processes to modify these variations. At angular sizes less than one degree, however, there has been sufficient time for physical interactions to modify the intensity of the CMBR at the epoch of decoupling, and the resulting variations in intensity depend on the details of the theory of such interactions. 

Recently, GSFC's COBE satellite made some very fundamental measurements of the CMBR. The FIRAS instrument determined that the spectrum matches the theoretical curve for thermal equilibrium to within 0.03%, lending unequivocal support to the Big Bang scenario, and confirming the belief that the early Universe was indeed in thermal equilibrium. The COBE/DMR instrument measured the anisotropy of the CMBR and made the first detection of the variations of CMBR intensity at the level of one part in 100,000 of the CMBR intensity. Both of these instruments have a beam size of 7 degrees, which means that they can only see structures which are larger than about 14 Moon diameters in the sky. As noted above, any anisotropy measurement at resolutions coarser than one degree will probe the fundamental fluctuations from the pre-inflation era, but could not possibly be the result of any interactions at a later time. It is also hard to relate these measurements directly to modern structures. The reason is that when we look at 7 degree size for a source at the distance of the era of decoupling, it corresponds to a size (after expanding by a factor of 1000) larger than the largest of structures which we can see in the Universe today. The scale sizes on which we have data for the Universe today correspond to patches of only about a half degree when we look back at the decoupling epoch. Thus, in order to relate the observed CMBR anisotropy to the structures that we see today (which are, in a sense, today's anisotropy), it is convenient to have measurements on smaller angular scales than COBE/DMR. 

In anticipation of this need, GSFC started the Medium-Scale Anisotropy Measurement (MSAM) project in 1988. Working at wavelengths between 0.5 and 3 mm, MSAM is a balloon-borne measurement of the CMBR anisotropy on angular scales between 0.5 and 3 degrees. The experiment uses a one meter, off-axis telescope, coupled with a four spectral band radiometer. The pointing for the experiment is controlled by a platform which was originally built in the early 1980's by a group at GSFC to map the Galactic plane. The payload is launched by NASA's National Scientific Balloon Facility in Palestine, Texas for overnight flights of 10 to 12 hours duration. Approximately 6 to 8 hours of scientific data are returned for each flight, giving sufficient sensitivity to measure changes in the CMBR at the one part in 100,000 level. MSAM had its first flight in June 1992. This flight returned outstanding results, and detected anisotropy at the 3 parts per 100,000 level, which is consistent with reasonable theoretical extrapolations from the COBE measurements at larger angular scales.  After three very successful flights, the original MSAM configuration was retired after its last flight in June 1995. A new radiometer formed the basis for a new, higher sensitivity measurement, the MSAM2.  This configuration had its initial flight in June 1997. 

The anisotropy detection by MSAM is at a level which is only a few times the minimum detectable level by the instrument. In order to provide higher reliability measurements, we have started a new project, TopHat, which will increase sensitivity by about a factor of thirty. The concept is to place a receiver and a small (one meter) telescope on top of a balloon. With long duration ballooning, which currently provides two week flights, such a configuration can provide an extremely stable, low-cost platform for CMBR anisotropy measurements. Placement on top of the balloon gives the telescope a completely unobstructed view of the sky. This top-mounted design will also prove to be a very cost-effective way to test hardware for future space missions. The TopHat will fly in Antarctica at the end of 2000. 

The study of CMBR anisotropy is still in its infancy, but has clearly entered a new phase. With the COBE/DMR discovery, the sensitivity level at which we need to work has been set. We have entered a measurement and characterization phase during which the smaller angular scales as probed by MSAM, TopHat, and future space missions will be key to uncovering the mystery of the origin of structure in the Universe. 

For more information on our research program, see our brochure.  For a more detailed explanation of CMBR physics, please visit Wayne Hu's excellent website.

For a broad and entertaining introduction to cosmology, please visit Ned Wright's website.