The Nature versus Nurture of Anisotropies

With the rapidly growing number of cosmic microwave background measurements on various scales, there is real hope that the number of acceptable models for structure formation will be limited to a very few in the near future. Yet any given model can always be saved by introducing and tuning extraneous free parameters. To better understand this question of ``nature versus nurture'' for temperature fluctuations, it is useful to know not only the general features of anisotropy predictions but also their causes. Extracting the physical content of our other works, we present here a {\it simple} account of cosmic microwave background anisotropies on all scales. In particular, we show that analytic approximations can trace the structure of the so-called ``Doppler peaks,'' which arise due to the {\it adiabatic} oscillations in the photon-baryon fluid. We also show how the finite thickness of the last scattering surface and the Silk damping mechanism can be described in a unified way by photon diffusion. In order to present a specific example, we focus on comparing the primordial isocurvature baryon (PIB) model with the standard cold dark matter model (CDM). In particular, we explain why PIB generically predicts larger {\it non}-oscillatory anisotropies from the 1$^\circ$ to 10$^\circ$ scale which may already be in conflict with experiments.