Constraining Inflation Models with Spinning Voids

TL;DR

This work proposes and validates a novel void-spin diagnostic based on the maximum redshift asymmetry $\Delta z_{\max}$ of galaxies inside voids, observed in 2D redshift space. Using a large suite of AbacusSummit $N$-body simulations, the authors show that the redshift-asymmetry distribution $p(\Delta z_{\max})$ is well described by a generalized Gamma model with a nearly universal shape parameter $k$ and a scale parameter $\theta$ that depends bilinearly on $\Omega_m$ and $\sigma_8$ via $\theta_{\rm model}(\Omega_m,\sigma_8) = \eta_1\Omega_m + \eta_2\sigma_8 + \eta_3$. The key finding is that this universality breaks when the running of the scalar spectral index $\alpha_s$ is nonzero, with positive $\alpha_s$ reducing $\theta_{\rm model}$ and negative $\alpha_s$ increasing it, thereby providing a new, independent route to constrain inflationary physics on void scales. The method offers a practical, redshift-based probe that complements CMB and Lyman-$\alpha$ constraints and could help discriminate among inflation models by testing the scale dependence of $n_s$ on intermediate cosmological scales.

Abstract

We present a powerful new diagnostics by which the running of scalar spectral index of primordial density fluctuations can be tightly and independently constrained. This new diagnostics utilizes coherent rotation of void galaxies, which can be observed as redshift asymmetry in opposite sides dichotomized by the projected spin axes of hosting voids. Comparing the numerical results from the AbacusSummit of cosmological simulations, we derive a non-parametric model for the redshift asymmetry distribution of void galaxies, which turns out to be almost universally valid for a very broad range of cosmologies including dynamic dark energy models with time-dependent equation of states as well as the $Λ$CDM models with various initial conditions. We discover that the universality of this model breaks down only if the running of scalar spectral index deviates from zero, detecting a consistent trend that a more positive (negative) running yields a lower (higher) redshift asymmetry of voids than the model predictions. Given that non-standard inflations usually predict non-zero runnings of the spectral index and that the redshift asymmetry distribution of voids is a readily observable quantity, we conclude that this new diagnostics will pave another path toward understanding the true mechanism of inflation.

Figures (6)

• Figure 1: Configurations of three classes of $44$ different background cosmologies in 2D spaces spanned by $\Omega_{m}$-$\sigma_{8}$ (left panel), by $w_{0}$-$w_{a}$ (middle panel) and by $n_{s}$-$\alpha_{s}$ (right panel).
• Figure 2: Illustration of the dependence of redshift asymmetry of void galaxies on the orientation of the bisector axes.
• Figure 3: Original and controlled distributions of the void sizes for the three classes of the background cosmologies in the left and right panels, respectively. Note that the controlled size distributions of void are identical over the three classes.
• Figure 4: $\Omega_{m}$ and $\sigma_{8}$ variations of two parameters, $k$ and $\theta$, of the generalized $\Gamma$-model for the void redshift asymmetry distribution in the left and right panels, respectively.
• Figure 5: Numerically obtained probability density distributions of void redshift asymmetry (filled black circles) compared with the best-fit $\Gamma$-models (red solid line) for the three different classes of the cosmologies.