Distinguishing Causal Seeds from Inflation

Causal seed models, such as cosmological defects, generically predict a distinctly different structure to the CMB power spectrum than inflation, due to the behavior of the perturbations outside the horizon. We provide a general analysis of their causal generation from isocurvature initial conditions by analyzing the role of stress perturbations and conservation laws in the causal evolution. As a special case, we consider the class of scaling models recently proposed by Turok. Energy-momentum conservation implies pathologies in the gravitational sector for this class unless the anisotropic stresses are tuned to avoid them. We propose and motivate a simpler cure for the pathologies that illustrates why such models tend to follow the canonical isocurvature pattern. Thus they can easily be distinguished from inflation by CMB experiments currently underway.