Running with BICEP2: Implications for Small-Scale Problems in CDM

TL;DR

The paper investigates how a running spectral index, inspired by BICEP2 results, can resolve small-scale structure issues in the DM model, showing it reduces halo densities and subhalo counts more effectively than other models.

Contribution

It demonstrates that a BICEP2-inspired running spectral index model better alleviates small-scale problems in DM than Planck-based or WDM models, through detailed simulations.

Findings

Reduces central densities of dwarf halos

Decreases small subhalo counts by ~50%

Lowers dark matter annihilation boost factors

Abstract

The BICEP2 results, when interpreted as a gravitational wave signal and combined with other CMB data, suggest a roll-off in power towards small scales in the primordial matter power spectrum. Among the simplest possibilities is a running of the spectral index. Here we show that the preferred level of running alleviates small-scale issues within the $\Lambda$CDM model, more so even than viable WDM models. We use cosmological zoom-in simulations of a Milky Way-size halo along with full-box simulations to compare predictions among four separate cosmologies: a BICEP2-inspired running index model ($\alpha_s$ = -0.024), two fixed-tilt $\Lambda$CDM models motivated by Planck, and a 2.6 keV thermal WDM model. We find that the running BICEP2 model reduces the central densities of large dwarf-size halos ($V_\mathrm{max}$ ~ 30 - 80 km s$^{-1}$) and alleviates the too-big-to-fail problem significantly compared to our adopted Planck and WDM cases. Further, the BICEP2 model suppresses the count of small subhalos by ~50% relative to Planck models, and yields a significantly lower "boost" factor for dark matter annihilation signals. Our findings highlight the need to understand the shape of the primordial power spectrum in order to correctly interpret small-scale data.