Could PBHs and secondary GWs have originated from squeezed initial states?. (arXiv:2011.09938v1 [astro-ph.CO])

Could PBHs and secondary GWs have originated from squeezed initial states?. (arXiv:2011.09938v1 [astro-ph.CO])
<a href="http://arxiv.org/find/astro-ph/1/au:+Ragavendra_H/0/1/0/all/0/1">H. V. Ragavendra</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sriramkumar_L/0/1/0/all/0/1">L. Sriramkumar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Silk_J/0/1/0/all/0/1">Joseph Silk</a>

Recently, 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 enhanced 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. This approach is free from the fine-tuning
challenges faced in constructing inflationary potentials to achieve the
enhancement of 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 $f_{_{rm NL}}$ 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 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.

Recently, 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 enhanced 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. This approach is free from the fine-tuning
challenges faced in constructing inflationary potentials to achieve the
enhancement of 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 $f_{_{rm NL}}$ 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 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.

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