Measuring the Anisotropy in the CMB: Current Status & Future Prospects

The CMB is perhaps the cleanest cosmological observable. Given a cosmology model, the angular spectrum of the CMB can be computed to percent accuracy. On the observational side, as far as we know, there is little that stands in the way between an accurate measurement and a rigorous confrontation with theory. In this article, we review the state of the data and indicate future directions. The data clearly show a rise in the angular spectrum to a peak of roughly $\delta T_l=(l(l+1)C_l/2\pi)^{1/2}\approx 85~\mu$K at $l\approx 200$ and a fall at higher $l$. In particular, $\delta T_l$ at $l=400$ is significantly less than at $l=200$. This is shown by a combined analysis of data sets and by the TOCO data alone. In the simplest open models with $\Omega_m = 0.35$, one expects a peak in the angular spectrum near $l=400$. For spatially flat models, a peak near $l=200$ is indicated and thus this model is preferred by the data. The combination of this, along with the growing body of evidence that $\Omega_m \approx 0.3$, suggests a cosmological constant is required. Further evidence for a cosmological constant is provided by the height of the peak. This conclusion is independent of the supernovae data.