Detecting the expansion of the universe through changes in the CMB photosphere

The expansion of the universe may be observed in "real time" by measuring changes in the patterns of the anisotropy in the cosmic microwave background (CMB). As the universe ages, the surface of decoupling-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 what we observe today. The largest change occurs at / ≈ 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.