Scale invariance of the primordial tensor power spectrum

Abstract

Future cosmic microwave background polarization experiments will search for evidence of primordial tensor modes at large angular scales, in the multipole range $4\le \ell \le 50.$ Because in that range there is some mild evidence of departures from scale invariance in the power spectrum of primordial curvature perturbations, one may wonder about the possibility of similar deviations appearing in the primordial power spectrum of tensor modes. Here we address this issue and analyze the possible presence of features in the tensor spectrum resulting from the dynamics of primordial fluctuations during inflation. We derive a general, model independent, relation linking features in the spectra of curvature and tensor perturbations. We conclude that even with large deviations from scale invariance in the curvature power spectrum, the tensor spectrum remains scale invariant for all observational purposes.

22 pages, 4 figures; v2: added references and clarifying comments; v3: added reference and few more comments. Matches published version

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• Constraints on inflation with LSS surveys: features in the primordial power spectrum
• Knot reconstruction of the scalar primordial power spectrum with Planck, ACT, and SPT CMB data
• Primordial blackhole formation: Exploring chaotic potential with a sharp step via the GLMS perspective
• Cross-tests of CMB features in the primordial spectra