Massless (pseudo-)scalar seeds of CMB anisotropy

A primordial stochastic background of very weakly coupled massless (pseudo-)scalars can seed CMB anisotropy, when large-scale fluctuations of their stress-tensor re-enter the horizon during the matter-dominated era. A general relation between multipole coefficients of the CMB anisotropy and the seed’s energy spectrum is derived. Magnitude and tilt of the observed anisotropies can be reproduced for the nearly scale-invariant axion spectra that are predicted in a particularly symmetric class of string cosmology backgrounds. CERN-TH/98-169 May 1998 In this letter we point out a possible new mechanism for generating large-scale CMB anisotropies. We will show that a cosmologically amplified stochastic background of massless (pseudo-)scalar perturbations, if primordially produced with a properly normalized nearly scale-invariant spectrum, can seed CMB temperature anisotropies in a way consistent with observations [1]. Axionic perturbations with the needed characteristics can be produced [2], [3], [4], for instance, in the so-called pre-big bang scenario [5] of string cosmology. For the sake of generality we define a massless (pseudo-)scalar “seed” field σ through the way it enters the low-energy effective action: Seff = − 1 2 ∫