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Primordial black holes from single-field inflation: a fine-tuning audit
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abstract
All single-field inflationary models invoke varying degrees of tuning in order to account for cosmological observations. Mechanisms that generate primordial black holes (PBHs) from enhancement of primordial power at small scales posit inflationary potentials that transiently break scale invariance and possibly adiabaticity over a range of modes. This requires additional tuning on top of that required to account for observations at scales probed by cosmic microwave background (CMB) anisotropies. In this paper we study the parametric dependence of various single-field models of inflation that enhance power at small scales and quantify the degree to which coefficients in the model construction have to be tuned in order for certain observables to lie within specified ranges. We find significant tuning: changing the parameters of the potentials by between one part in a hundred and one part in $10^8$ (depending on the model) is enough to change the power spectrum peak amplitude by an order one factor. The fine-tuning of the PBH abundance is larger still by 1-2 orders of magnitude. We highlight the challenges imposed by this tuning on any given model construction. Furthermore, polynomial potentials appear to require significant additional fine-tuning to also match the CMB observations.
Forward citations
Cited by 8 Pith papers
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Superhorizon curvature perturbations in hybrid inflation revisited
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In Randall-Sundrum warped extra dimension models, the supercooled radion phase transition can form primordial black holes that account for all of dark matter for IR scales 10 TeV to 10^4 TeV, with correlated gravitati...
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A kinetic cubic Horndeski coupling creates an ultra-slow-roll phase that amplifies curvature perturbations enough to produce asteroid-mass primordial black holes, potentially 90% of dark matter.
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Are Primordial Black Holes Truly Fine-Tuned?
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A scaling invariance of k-inflation background equations allows parameter redefinitions that normalize the curvature perturbation spectrum at the CMB pivot scale without altering its shape.
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