Could PBHs and secondary GWs have originated from squeezed initial states?

International audienceRecently, the production of primordial black holes (PBHs) and secondary gravitational waves (GWs) due to enhanced scalar power on small scales have garnered considerable attention in the literature. Often, the mechanism considered to arrive at such increased power involves a modification of the standard slow roll inflationary dynamics, achieved with the aid of fine-tuned potentials. In this work, we investigate another well known method to generate features in the power spectrum wherein the initial state of the perturbations is assumed to be squeezed states. The approach allows one to generate features even in slow roll inflation with a specific choice for the Bogoliubov coefficients characterizing the squeezed initial states. Also, the method is technically straightforward to implement since the Bogoliubov coefficients can be immediately determined from the form of the desired spectrum with increased scalar power at small scales. It is known that, for squeezed initial states, the scalar bispectrum is strongly scale dependent and the consistency condition governing the scalar bispectrum in the squeezed limit is violated. In fact, the non-Gaussianity parameter characterizing the scalar bispectrum proves to be inversely proportional to the squeezed mode and this dependence enhances its amplitude at large wave numbers making it highly sensitive to even a small deviation from the standard Bunch-Davies vacuum. These aspects can possibly aid in leading to enhanced formation of PBHs and generation of secondary GWs. However, we find that: (i) the desired form of the squeezed initial states may be challenging to achieve from a dynamical mechanism, and (ii) the backreaction due to the excited states severely limits the extent of deviation from the Bunch-Davies vacuum at large wave numbers. We argue that, unless the issue of backreaction is circumvented, squeezed initial states cannot lead to a substantial increase in power on small scales that is required for enhanced formation of PBHs and generation of secondary GWs

Primordial black holes and secondary gravitational waves from ultraslow roll and punctuated inflation

International audienceThe primordial scalar power spectrum is well constrained by the cosmological data on large scales, primarily from the observations of the anisotropies in the cosmic microwave background. Over the last few years, it has been recognized that a sharp rise in power on small scales will lead to the enhanced formation of primordial black holes (PBHs) and also generate secondary gravitational waves (GWs) of higher and, possibly, detectable amplitudes. It is well understood that scalar power spectra with COBE normalized amplitude on the cosmic microwave background scales and enhanced amplitudes on smaller scales can be generated due to deviations from slow roll in single, canonical scalar field models of inflation. In fact, an epoch of so-called ultraslow roll inflation can lead to the desired amplification. We find that scenarios that lead to ultraslow roll can be broadly classified into two types, one wherein there is a brief departure from inflation (a scenario referred to as punctuated inflation) and another wherein such a departure does not arise. In this work, we consider a set of single field inflationary models involving the canonical scalar field that lead to ultraslow roll and punctuated inflation and examine the formation of PBHs as well as the generation of secondary GWs in these models. Apart from considering specific models, we reconstruct potentials from certain functional choices of the first slow roll parameter leading to ultraslow roll and punctuated inflation and investigate their observational signatures. In addition to the secondary tensor power spectrum, we calculate the secondary tensor bispectrum in the equilateral limit in these scenarios. Moreover, we calculate the inflationary scalar bispectrum that arises in all the cases and discuss the imprints of the scalar non-Gaussianities on the extent of PBHs formed and the amplitude of the secondary GWs generated. We conclude with a discussion on the wider implications of our results

Observing nulling of primordial correlations via the 21 cm signal

The 21cm line emitted by neutral hydrogen (HI) during the Dark Ages carries imprints of pristine primordial correlations. In models of inflation driven by a single, canonical scalar field, we show that a phase of ultra-slow-roll can lead to a null in all the primordial correlations at a specific wavenumber $k_\textrm{dip}$. We consider scenarios wherein the null in the correlations occurs over wave numbers $1 \lesssim k_\textrm{dip} \lesssim 10\,\mathrm{Mpc}^{-1}$, and examine the prospects of detecting such a damping in the HI signal due to the nulls at the level of power and bi-spectra in future observational missions