Simran Yadav | Searcharxiv

arXiv2025

Sub-Horizon Amplification of Curvature Perturbations: A New Route to Primordial Black Holes and Gravitational Waves

The enhanced primordial scalar power spectrum is a widely studied mechanism for generating primordial gravitational waves (PGWs), also referred to as scalar-induced gravitational waves (SIGWs). This process also plays a pivotal role in facilitating the formation of primordial black holes (PBHs). Traditionally, the ultra slow-roll (USR) mechanism has been the predominant approach used in the early universe. In this framework, the second slow-roll parameter $\epsilon_2$, is typically set to $-6$ or lower for a brief period -- marking a significant departure from the standard slow-roll condition where $\epsilon_2 \simeq 0$. Such conditions often emerge in models with inflection points or localized features, such as bumps in the potential. In this paper, we challenge the conventional assumption that $\epsilon_2 \lesssim -6$ is a prerequisite for substantial amplification of the scalar power spectrum. We demonstrate that any negative value of the second slow-roll parameter can indeed enhance the scalar power spectrum through sub-horizon growth, establishing this as a necessary and sufficient condition for amplification. Consequently, this mechanism facilitates the generation of both PGWs and PBHs. To illustrate this, we examine a standard scenario where a brief USR phase is embedded between two slow-roll (SR) phases. By systematically varying $\epsilon_{2}$ values from $-1$ to $-10$ in the USR region, we investigate the amplification of the power spectrum and its implications for PGWs and PBHs production, particularly in the context of ongoing and future cosmological missions.

astro-ph.CO↗

arXiv2024

Precision Inflationary Predictions: Impact of Accurate End-of-Inflation Dynamics

The precision era of cosmology demands accurate theoretical predictions from inflationary models. In quantitative reheating analyses, inflationary observables depend sensitively on the number of e-folds between horizon exit and the end of inflation, $N_k$, whose determination relies on slow-roll approximations near the end of inflation. Since inflation ends when the first slow-roll parameter reaches unity, even modest inaccuracies in this approximation can shift the end of inflation and thereby alter $N_k$, leading to modifications in predicted observables -- including those evaluated at leading-order. While such effects are implicit in standard treatments, their quantitative impact on observable constraints has not been systematically assessed. In this work, we first re-evaluate leading-order slow-roll predictions using an improved determination of $N_k$ within a simple quantitative reheating framework, and then incorporate higher-order slow-roll corrections consistently with the revised background evolution. Applying this framework to the Starobinsky model, we find that improved end-of-inflation dynamics alone can induce shifts of order $\Delta n_s \sim 10^{-3}$, while higher-order slow-roll corrections provide additional refinements at the $\sim 4 \times 10^{-4}$ level. The cumulative effect yields a maximum shift of $\Delta n_s \sim 1.2 \times 10^{-3}$ within the allowed reheating range. To our knowledge, this is the first systematic decomposition of end-of-inflation corrections and their individual contributions to $n_s$ in the Starobinsky model, with implications for model discrimination in next-generation CMB surveys. These results demonstrate that an accurate determination of the end of inflation is essential for precision tests of inflationary models.

astro-ph.CO↗