3. The flat universe
Fluctuations in the temperature of the cosmic microwave background plotted as a function of angle for three experiments: MAT/TOCO (triangles), BOOMERANG (squares) and MAXIMA (circles). The position of the peak in this spectrum depends on the geometry of the universe. (The height of the peaks in the three datasets are different due to instrument-calibration effects.) Recent observations confirm that the peak occurs at the position predicted for a flat universe (blue). In an open universe the peak would be to the left (red), and in a closed universe it would be on the right (green).