Electromagnetic origin of the CMB anisotropy in string cosmology.

In the inflationary scenarios suggested by string theory, the vacuum fluctuations of the electromagnetic field can be amplified by the time-evolution of the dilaton background, and can grow large enough to explain both the origin of the cosmic magnetic fields and of the observed CMB anisotropy. The normalization of the perturbation spectrum is fixed, and implies a relation between the perturbation amplitude at the COBE scale and the spectral index n. Working within a generic two-parameter family of backgrounds, a large scale anisotropy ∆T/T ≃ 10−5 is found to correspond to a spectral index in the range n ≃ 1.11− 1.17. CERN-TH/95-102 April 1995 In the standard inflationary scenario, the anisotropy of the Cosmic Microwave Background (CMB) recently detected by COBE [1, 2] is usually attributed to the cosmological amplification of the quantum fluctuations of the metric. These consist of both tensor (gravitational waves) and scalar perturbations, the latter being coupled to the energy density fluctuations. The observed inhomogeneities of the CMB radiation could also emerge from the vacuum fluctuations of the electromagnetic radiation itself, through their contribution to δρ/ρ. However, the minimal coupling of photons to the metric is conformally invariant in d = 3 spatial dimensions, and it is difficult, in general, to obtain a significant amplification of the electromagnetic fluctuations in the context of the standard inflationary scenario [3]. In the inflationary models based on the low energy limit of critical superstring theory [4, 5, 6], the electromagnetic field Fμν is coupled not only to the metric (gμν) but also to the dilaton (φ) background, according to the dimensionally reduced, effective action [7]