Detection limits for super-Hubble suppression of causal fluctuations - art. no. 123513

We investigate to what extent future microwave background experiments might be able to detect a suppression of fluctuation power on large scales in fl at and open universe models. Such suppression would arise if fluctuations a re generated by causal processes, and a measurement of a small suppression scale would be problematic for inflation models, but consistent with many d efect models. More speculatively, a measurement of a suppression scale of t he order of the present Hubble radius could provide independent evidence fo r a fine-tuned inflation model leading to a low-density universe. We find t hat, depending on the primordial power spectrum, a suppression scale modest ly larger than the visible Horizon can be detected, but that the detectabil ity drops very rapidly with increasing scale. For models with two periods o f inflation, there is essentially no possibility of detecting a causal supp ression scale.