Observational Imprints of Enhanced Scalar Power on Small Scales in Ultra Slow Roll Inflation and Associated Non-Gaussianities

Abstract

The discovery of gravitational waves from merging binary black holes has generated considerable interest in examining whether these black holes could have a primordial origin. If a significant number of black holes have to be produced in the early universe, the primordial scalar power spectrum should have an enhanced amplitude on small scales, when compared to the COBE normalized values on the large scales that is strongly constrained by the anisotropies in the cosmic microwave background. In the inflationary scenario driven by a single, canonical scalar field, such power spectra can be achieved in models that permit a brief period of ultra slow roll inflation during which the first slow roll parameter decreases exponentially. In this review, we shall consider a handful of such inflationary models as well as a reconstructed scenario and examine the extent of formation of primordial black holes and the generation of secondary gravitational waves in these cases. We shall also discuss the strength and shape of the scalar bispectrum and the associated non-Gaussianity parameter that arise in such situations. We shall conclude with an outlook wherein we discuss the wider implications of the increased strengths of the non-Gaussianities on smaller scales.

Primordial black holes dark matter and secondary gravitational waves from warm Higgs-G inflation

“…In order to produce PBHs by the gravitational collapse, the amplitude of primordial curvature power spectrum at small scales has to be ∼ O(10 −2 ). Such an enhancement by several orders can be achieved in different models, such as the hybrid inflation [74][75][76], running-mass inflation [77][78][79][80], hilltop inflation [81], inflating curvaton [82], axion curvaton inflation [83,84], double inflation [85], thermal inflation [86], multifield inflation [87,88], single field inflation with a broken scale invariance [89], or by introducing an inflection point in the potential [90][91][92][93][94][95][96][97], or bump/dip in the potential [98], running of the spectral index [99][100][101], suppression of the sound speed [102,103], modified dispersion relations [104], or resonant instability [105][106][107], modified gravity theories [108][109][110][111], etc. PBHs are crucial probes of the small scale features of the primordial power spectrum and hence different inflationary models [112][113][114]…”

Loop contributions to the scalar power spectrum due to quartic order action in ultra slow roll inflation

“…However, in inflationary models with a phase of USR, the scalar power can rise by seven to eight orders of magnitude on small scales (for a recent review on the topic, see ref. [58]). In such situations, it is conceivable that the higher order contributions from loops become comparable to the quantities arrived at from the linear theory, thus violating the validity of perturbative expansion (for the ongoing debate on this issue, see refs.…”

Multi-field inflation with large scalar fluctuations: non-Gaussianity and perturbativity

“…Usually, the production of a peak in the scalar power spectrum is associated with departures from slow-roll [15]. In the context of single-field inflation this can be achieved when the field slows down rapidly on its potential, for example because an inflection point leads to a phase of ultra-slow roll [16][17][18][19][20][21][22][23][24]. 1 In this work we focus on an alternative mechanism, based on the (likely) possibility that many fields were playing a role during inflation [27].…”