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