Wandering in the Lyman-alpha Forest: A Study of Dark Matter-Dark
  Radiation Interactions
  The amplitude of large-scale matter fluctuations inferred from the observed
Sunyaev-Zeldovich (SZ) cluster mass function and from weak gravitational
lensing studies, when taken at face value, is in tension with measurements of
the cosmic microwave background (CMB) and baryon acoustic oscillation (BAO). In
this work, we revisit whether this possible discrepancy can be attributed to
new interactions in the dark matter sector. Focusing on a cosmological model
where dark matter interacts with a dark radiation species until the epoch of
matter-radiation equality, we find that measurements of the Lyman-alpha flux
power spectrum from the Sloan Digital Sky Survey provides no support to the
hypothesis that new dark matter interactions can resolve the possible tension
between CMB and large-scale structure (LSS). Indeed, while the addition of dark
matter-dark radiation interactions leads to an improvement of
$2\Delta\ln\mathcal{L}=12$ with respect to the standard $\Lambda$ cold dark
matter ($\Lambda$CDM) model when only CMB, BAO, and LSS data are considered,
the inclusion of Lyman-alpha data reduces the improvement of the fit to
$2\Delta\ln\mathcal{L}=6$ relative to $\Lambda$CDM. We thus conclude that the
statistical evidence for new dark matter interactions (largely driven by the
Planck SZ dataset) is marginal at best, and likely caused by systematics in the
data. We also perform a Fisher forecast analysis for the reach of a future
dataset composed of a CMB-S4 experiment combined with the Large Synoptic Survey
Telescope galaxy survey. We find that the constraint on the effective number of
fluid-like dark radiation species, $\Delta N_{\rm fluid}$, will be improved by
an order of magnitude compared to current bounds.
