Measuring the Expansion of the Universe Through Changes in the CMB Photosphere

The expansion of the universe may be observed in ``realtime’’ by measuring changes in the patterns of the anisotropy in the CMB. As the universe ages, the surface of decoupling–or the CMB photosphere–moves away from us and samples a different gravitational landscape. The response of the CMB to this new landscape results in a different pattern than we observe today. The largest change occurs at l~900. We show that with an array of detectors that we may envision having in a couple of decades, one can in principle measure the change in the anisotropy with two high precision measurements separated by a century.