ABSTRACT
This chapter considers a universe composed of two relativistic and one non-relativistic components. It also considers fluctuations only in the photon and baryon distributions. Once the background cosmological model is chosen and defined in terms of the density and curvature parameters. The chapter discusses adiabatic fluctuations in the photon and baryon components are expected to be spatially correlated. It discusses two physical mechanisms that do not allow density fluctuations to grow on small scales: pressure forces and diffusion processes. The chapter follows that an initially scale-free power spectrum cannot remain scale-free. There is an alternative way of generating fluctuations that consists of spatially anticorrelating the fluctuations in the photon and baryon components. This is realized by creating at the same comoving position in space, an excess and a deficit, in the baryon and photon densities, respectively.
