arXiv subjects

Bahar Nikbakht

Publications and source records attributed to Bahar Nikbakht.

4 recordsLinked to original sources

arXiv2025

Geometry of non-Gaussianity in transient non-attractor inflation

Inflationary models predicting abundant primordial black holes (PBHs) and large amplitude of scalar-induced gravitational waves (SIGWs) often rely on amplified fluctuations over limited scales, typically driven by phase transitions, particle production, or departures from slow-roll evolution. While the power spectrum of these models has been extensively studied, higher-order correlations are much less understood. Motivated by the complex physics involved and the fact that PBH and SIGW formation are both sensitive to non-linearities, we present a detailed study of the bispectrum as the leading non-linear effect in these scenarios. We refer to the scale- and shape-dependence of the bispectrum collectively as its geometry; and define a scale-dependent shape correlator to disentangle the two dependencies. Generally, we find that for the scales most affected by phase transitions and particle production (including the power spectrum peak), the bispectrum is strongest near the equilateral configuration, while non-attractor phases tend to produce correlations near the squeezed configuration. We further propose a simplified bispectrum estimator, resembling local-type non-Gaussianity but with scale-dependent amplitude, that captures the main features of the full bispectrum. As an implication of our results, we show that incorporating the bispectrum significantly broadens the range of scales with a substantial probability of large smoothed density contrasts compared to linear analysis. This suggests that non-linearities can alter not only PBH abundance and SIGW amplitude but also their mass and frequency spectra. In particular, and in contrast with the usual assumption, our results hint that the second-highest peak of the power spectrum may produce more PBHs than the highest peak.

astro-ph.CO↗

arXiv2025

Hamiltonians to all Orders in Perturbation Theory and Higher Loop Corrections in Single Field Inflation with PBHs Formation

We calculate the action and the interaction Hamiltonians to all orders in perturbation theory in the model of single field inflation with a transient ultra slow-roll phase. Employing the formalism of EFT of inflation, we obtain a compact non-perturbative expression for the interaction Hamiltonian in terms of the Goldstone field $\pi$ in the decoupling limit. In addition, we also present a non-linear relation between $\pi$ and the curvature perturbations to all orders in perturbation theory. These are powerful results which enable us to calculate the cosmological correlators and loop corrections to any order in perturbation theory. As a non-trivial example, we calculate the $L$-loop corrections on long CMB scale perturbations in the USR models which are used for PBHs formation. We show that the loop corrections scale like $(\Delta N {\cal P}_e L) ^L$ in which ${\cal P}_e$ is the peak of the power spectrum and $\Delta N$ is the duration of the USR phase. This indicates that the loop corrections grow quickly out of perturbative control for large values of $L$. In the conventional USR setup for PBHs formation with $\Delta N \simeq 2.5$, this happens at $L=4$.

astro-ph.CO↗

arXiv2025

Non-Perturbative Hamiltonian and Higher Loop Corrections in USR Inflation

Calculating the action and the interaction Hamiltonian at higher orders in cosmological perturbation theory is a cumbersome task. We employ the formalism of EFT of inflation in the decoupling limit for single-field ultra slow-roll (USR) inflation and obtain a non-perturbative Hamiltonian in terms of the Goldstone field $\pi$. To complete the dictionary, a non-linear relation between the curvature perturbations and $\pi$ is presented. Using these results, we compute higher-order loop corrections in USR models with a sharp transition to the attractor phase, relevant for PBHs formation. It is shown that in the idealized picture in which the transition from the USR phase to SR phase is instantaneous and sharp, the loop corrections on long CMB scales increase rapidly with the number of loops $L$ and the setup may go out of perturbative control.

astro-ph.CO↗

arXiv2024

Non-Gaussianity consistency relations and their consequences for the peaks

Strong deviations from scale invariance and the appearance of high peaks in the primordial power spectrum have been extensively studied for generating primordial black holes (PBHs) or gravitational waves (GWs). It is also well-known that the effect of non-linearities can be significant in both phenomena. In this paper, we advocate the existence of a general single-field consistency relation that relates the amplitude of non-Gaussianity in the squeezed limit $f_{\text{NL}}$ to the power spectrum and remains valid when almost all other consistency relations are violated. In particular, it is suitable for studying scenarios where scale invariance is strongly violated. We discuss the general and model-independent consequences of the consistency relation on the behavior of $f_{\text{NL}}$ at different scales. Specifically, we study the size, sign and slope of $f_{\text{NL}}$ at the scales where the power spectrum peaks and argue that generally the peaks of $f_{\text{NL}}$ and the power spectrum occur at different scales. As an implication of our results, we argue that non-linearities can shift or extend the range of scales responsible for the production of PBHs or GWs, relative to the window as determined by the largest peak of the power spectrum, and may also open up new windows for both phenomena.

astro-ph.CO↗