Dark Radiation and interacting scenarios

TL;DR

The paper investigates how a dark radiation component interacting with dark matter affects cosmological constraints on the radiation abundance and its clustering properties. Using a modified Boltzmann framework and MCMC analyses with current CMB and large-scale structure data, the authors find that while bounds on the extra radiation density, $\Delta N_{ extrm{eff}}$, remain robust to coupling, the constraints on the clustering parameters $c_{ m eff}^2$ and $c_{ m vis}^2$ become substantially weaker due to degeneracies with the interaction strength $Q_0$. Forecasts for Planck and COrE show that neglecting possible interactions can bias inferred clustering parameters ($c_{ m eff}^2$ biased high; $c_{ m vis}^2$ biased low) though $\Delta N_{ extrm{eff}}$ is less affected. The results emphasize the importance of including dark radiation–dark matter interactions in analyses of upcoming CMB data to avoid biased conclusions about the properties of additional relativistic species.

Abstract

An extra dark radiation component can be present in the universe in the form of sterile neutrinos, axions or other very light degrees of freedom which may interact with the dark matter sector. We derive here the cosmological constraints on the dark radiation abundance, on its effective velocity and on its viscosity parameter from current data in dark radiation-dark matter coupled models. The cosmological bounds on the number of extra dark radiation species do not change significantly when considering interacting schemes. We also find that the constraints on the dark radiation effective velocity are degraded by an order of magnitude while the errors on the viscosity parameter are a factor of two larger when considering interacting scenarios. If future Cosmic Microwave Background data are analysed assuming a non interacting model but the dark radiation and the dark matter sectors interact in nature, the reconstructed values for the effective velocity and for the viscosity parameter will be shifted from their standard 1/3 expectation, namely ceff=0.34 (+0.006 -0.003) and cvis=0.29 (+0.002 -0.001) at 95% CL for the future COrE mission data.

Figures (3)

• Figure 1: Upper panel: The magenta lines depict the CMB temperature power spectra $C_l^{TT}$ for the best fit parameters for a $\Lambda$CDM model from the WMAP seven year data set. The dotted curve shows the scenario with a constant interacting cross section with $Q_0 = 10^{-32}$cm$^2$ MeV$^{-1}$ for $\Delta N_{\textrm{eff}} = 1$ and assuming canonical values for $c_{\rm eff}^2=c_{\rm vis}^2=1/3$. The dashed (dot dashed) curve illustrates the same interacting scenario but with $c_{\rm eff}^2=0$ and $c_{\rm vis}^2=1/3$ ($c_{\rm eff}^2=1/3$ and $c_{\rm vis}^2=0$). We depict as well the data from the WMAP and SPT experiments, see text for details. Lower panel: matter power spectrum for the different models described in the upper panel. The data correspond to the clustering measurements of luminous red galaxies from SDSS II DR7 beth.
• Figure 2: Left panel: $68\%$ and $95\%$ CL contours in the ($c_{\rm eff}^2$, $\Delta N_{\textrm{eff}}$) plane arising from the MCMC analysis of WMAP, SPT, BAO and HST/SNLS3 data. The green (yellow) contours refer to the case of WMAP, SPT, BAO and SNLS3 data sets with (without) interaction between the dark radiation and dark matter fluids. The magenta (red) contours refer to the case of WMAP, SPT, BAO and HST data sets with (without) interaction between the dark radiation and dark matter sectors. Right panel: as in the left panel but in the ($c_{\rm vis}^2$, $\Delta N_{\textrm{eff}}$) plane.
• Figure 3: Left panel: $68\%$ and $95\%$ CL contours in the ($c_{\rm eff}^2$, $\Delta N_{\textrm{eff}}$) arising from the MCMC analysis of Planck (red contours) and COrE (blue contours) CMB mock data. The mock data are generated adding an interaction between the dark radiation and dark matter sectors with $Q_0=10^{-32}$cm$^2$ MeV$^{-1}$, assuming one dark radiation interacting species $\Delta N_{\textrm{eff}}$=1 and standard clustering and viscosity parameters for the dark radiation. The CMB mock data is then fitted to a non interacting cosmology but allowing the dark radiation parameters $c_{\rm eff}^2$ and $c_{\rm vis}^2$ to have non standard values. Right panel: as in the left panel but in the ($c_{\rm vis}^2$, $\Delta N_{\textrm{eff}}$) plane.