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

Understand

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.