More information can be found in recent reviews (e.g. [46, 18]). Here I give a very brief summary.
The next generation of CMB balloon experiments are expected to return data
of much higher quality (and quantity). The new results from BOOMERanG
[47] (as
well as MSAM, MAXIMA and others) are eagerly awaited. BOOMERanG `98 was the
first long-duration balloon flight, and by all accounts was staggeringly
successful. Three immediate questions are expected to be addressed by this
new data-set: do the currently favoured
-dominated cosmologies
continue to be a good fit; what is the precise location of the first peak;
and is there any evidence for other peaks. This last point is perhaps
the most important. Detection of oscillations in the power spectrum, with
tight constraints on the peak spacings, will be a very firm test of the
inflationary paradigm
[48].
The adiabatic, apparently acausal perturbations, generated during inflation, give a series of peaks in the ratio 1:2:3: . . . in l-space. On the other hand, causal, isocurvature perturbations naturally give rise to peaks in the ratio 1:3:5: . . .. So detection of a second peak at roughly half the angular scale of the first, will be a very large step towards `proving inflation'. Failure to observe this will, of course, be even more exciting, since it will demand an entirely new paradigm.
In the short term there are also at least three new interferometer projects (e.g. [49]): DASI at the South Pole, CBI in Chile and the VSA in Tenerife. All of these are nearing completion and the improvement in the available data is expected to increase even more when they return data within the next year or two.
Another direction being pursued from the ground is CMB polarization see [50] for experimental details and [51] for a theory primer. The CMB sky is naturally polarized at the few percent level (the result of the quadrupole term in Compton scattering together with a slightly anisotropic radiation field at z ~ 1000). Measuring the ~ µK signals will be very challenging, but can provide information beyond that contained in the temperature anisotropies alone. Since polarization is such a strong prediction, it better be there, otherwise our whole picture has to change! Furthermore, we can more definitively separate any gravity wave contribution in the CMB (if it is measurable [52]), thus limiting the energy scale of inflation. Large-angle polarization can also constrain the reionization epoch, and details of the polarization power spectrum are a direct probe of physics around the time of last scattering. This is all in addition to the fact that polarization simply gives extra information to better constrain parameters (and to break degeneracies between some combinations of parameters).