The COBE satellite

5.1 The COBE satellite

The Cosmic Background Explorer satellite (COBE) was launched on 18th November 1989. Primordial anisotropy measurements are made using the DMR experiment, which consists of six differential microwave radiometers, two at each of 31.5 GHz, 53.0 GHz and 90.0 GHz. The first-year COBE observations provided convincing statistical evidence for the existence of CMB fluctuations. It was not however, possible to see individual CMB features, on the scale of the beam size, in the DMR maps, because even combining all of the maps together, the noise level per beam area was $∼ 45 μK$ and the signal to noise remained less than one.

The results of the analysis of all four years of DMR data have now become available. A convenient summary of all the results is given in Bennett et al. (1996) [9 ]. On a statistical level, the results can be used to constrain the normalisation of a power law primordial spectrum. For a given slope $n$ , normalisation is usually expressed via the implied amplitude of the quadrupole component of the power spectrum, $C2$ , as

where $T$ is the mean CMB temperature. (Note that this $Qrms− ps$ value need not be the same as the actual quadrupole component. The fit is to a whole power spectrum as parameterised by a given $n$ ). For an assumed value of $n = 1$ , (the Harrison–Zel’dovich value), Bennett et al. quote $Qrms −ps = 18 ± 1.6 μK$ . The joint best fit values of $n$ and $Qrms −ps$ are $n = 1.2 ± 0.3$ and $Qrms −ps = 15.3 μK$ . This restriction on the value of $n$ is of course of great interest in the context of inflationary predictions that $n = 1$ . It is also of interest that inflation predicts Gaussian fluctuations, and while this is much harder to test for than finding the amplitude and slope of the spectrum, the data are also consistent with this prediction. Specifically, Bennett et al. state ‘statistical tests prefer Gaussian over other toy statistical models by a factor of $∼$ 5’.

With the accumulation of four years of data, the individual anisotopy features within the maps on the scale of the beam size are now becoming statistically significant. Figure 11 shows the all-sky maps at each frequency taken from Bennett et al. [9 ]. Some of the features in these maps away from the Galactic plane are expected to be real CMB fluctuations, since the signal to noise in these regions is now about 2 sigma per 10 degree sky patch. Indeed, features which repeat well between the different frequencies are now clearly visible.

Figure 11: The COBE DMR 4 year data displayed as all-sky maps.