Primordial Black Holes from a tiny bump/dip in the Inflaton potential

Abstract: Scalar perturbations during inflation can be substantially amplified by tiny features in the inflaton potential. A bump-like feature behaves like a local speed-breaker and lowers the speed of the scalar field, thereby locally enhancing the scalar power spectrum. A bump-like feature emerges naturally if the base inflaton potential $V_b(\phi)$ contains a local correction term such as $V_b(\phi)\left[1+\varepsilon(\phi)\right]$ at $\phi=\phi_0$. The presence of such a localised correction term at $\phi_0$ leads to a large peak in the curvature power spectrum and to an enhanced probability of black hole formation. Remarkably this does not significantly affect the scalar spectral index $n_{S}$ and tensor to scalar ratio $r$ on CMB scales. Consequently such models can produce higher mass primordial black holes ($M{\rm PBH}\geq 1 M_{\odot}$) in contrast to models with `near inflection-point potentials' in which generating higher mass black holes severely affects $n_{S}$ and $r$. With a suitable choice of the base potential - such as the string theory based (KKLT) inflation or the $\alpha$-attractor models - the amplification of primordial scalar power spectrum can be as large as $10^7$ which leads to a significant contribution of primordial black holes (PBHs) to the dark matter density today, $f{\rm PBH} = \Omega_{0,\rm PBH}/\Omega_{0,\rm DM} \sim O(1)$. Interestingly, our results remain valid if the bump is replaced by a dip. In this case the base inflaton potential $V_b(\phi)$ contains a negative local correction term such as $V_b(\phi)\left[1-\varepsilon(\phi)\right]$ at $\phi=\phi_0$ which leads to an enhanced probability of PBH formation. We conclude that primordial black holes in the mass range $10^{-17} M_{\odot} \leq M_{\rm PBH} \leq 100\, M_{\odot}$ can easily form in single field inflation in the presence of small bump-like and dip-like features in the inflaton potential.