The Sachs-Wolfe Plateau

The large angular scales as measured by COBE are particular interesting to theoretical cosmologists as they correspond directly to the ``frozen'' part of the primordial power spectrum waiting to enter the horizon scale so they can grow. These cause local metric variations at the last-scattering surface which give rise to temperature fluctuations through gravitational red-shift via the Sachs-Wolfe effect[8]. The big success of COBE has been in establishing the amplitude and slope of the first 10 or so components thus fixing the normalizing quadruple component for the power spectrum Q_rms-PS = 17^+2.5_-2.3 with spectral index $n=1.15\pm 0.2$in Banday et al[12]. This is consistent with the predictions of inflation of n=1. Another mechanism which could affect these angular scales in that of tensor modes generated in inflation appearing as long wavelength gravitational waves, which distort the distribution of passing CMB photons. These will be discussed in detail in the section three.