Inflation with $0\le c_{\mathrm{s}}\le 1$

Abstract

We investigate a novel single field inflationary scenario which allows a transition between a slow-roll k-inflation with $c_{\mathrm{s}}$ of order unity and a ghost inflation with $c_{\mathrm{s}}\simeq 0$, where $c_{\mathrm{s}}$ is the sound speed for the curvature perturbations. We unify the two phases smoothly by appropriately taking into account a higher derivative scordatura term, which is always there from the effective field theory point of view but which becomes important only in the $c_{\mathrm{s}}\simeq 0$ regime. The model achieves the whole range of $0\le c_{\mathrm{s}}\le 1$ avoiding strong coupling and gradient instability, and allows us to access the $c_{\mathrm{s}}\simeq 0$ regime in a self-consistent manner. We also discuss implications to the formation of primordial black holes.

29 pages, 8 figures, matches published version

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• Generalized Regge-Wheeler Equation from Effective Field Theory of Black Hole Perturbations with a Timelike Scalar Profile
• Generation of primordial black holes from an inflation model with modified dispersion relation
• Approximately stealth black hole in higher-order scalar-tensor theories
• Primordial Black Hole Formation in Non-Standard Post-Inflationary Epochs
• Growth of power spectrum due to decrease of sound speed during inflation
• Black hole perturbations in higher-order scalar-tensor theories: initial value problem and dynamical stability
• Are $f(R,\mathrm{Matter})$ theories really relevant to cosmology?
• Unique gravitational wave signatures of GLPV scalar-tensor theories
• Primordial black holes and scalar induced gravitational waves from sound speed resonance in non-minimal derivative coupling inflation model