The effect of dark energy on the void-halo perpendicular alignments

We report a numerical discovery that in a more rapidly accelerating spacetime, the galactic halos on void surfaces develop stronger perpendicular alignments with the directions toward the void centers. We utilize the halo catalogs from the AbacusSummit suite of simulations for $10$ different cosmologies that include one Planck $\Lambda$CDM, four $w$CDM and five $w_{0}w_{a}$CDM, which share the identical initial conditions except for the dark energy equation of state. For each cosmology, we identify the voids and void-surface galactic halos at $z=0.1$ and determine the probability density functions of the cosines of the angles, $p(\cos\theta)$, between the shape axes of void-surface galactic halos and the directions toward the void centers. The numerically obtained $p(\cos\theta)$ is fitted to an analytic single-parameter formula derived through an empirical modification of the linear perturbation theory. Eliminating spurious signals caused by the differences in the mass and sphericity distributions of void-surface galactic halos among different cosmologies, we detect a clear net effect of dark energy on the strengths of the perpendicular alignments of void-surface galactic halos, quantified by the single parameter, $d_{t}$. Noting that $d_{t}$ has higher values in the cosmologies where dark energy has more negative pressure and evolves more rapidly, we put forth a bilinear model for the difference in $d_{t}$ between the two cases of $w=-1$ and $w\ne -1$. Demonstrating that this bilinear relation excellently describes the numerical results, we conclude that the perpendicular alignments of void-surface galactic halos should in principle be a powerful independent indicator of the dynamic nature of dark energy.