Origin of Delta N_eff as a Result of an Interaction between Dark
  Radiation and Dark Matter
  Results from the Wilkinson Microwave Anisotropy Probe (WMAP), Atacama
Cosmology Telescope (ACT) and recently from the South Pole Telescope (SPT) have
indicated the possible existence of an extra radiation component in addition to
the well known three neutrino species predicted by the Standard Model of
particle physics. In this paper, we explore the possibility of the apparent
extra dark radiation being linked directly to the physics of cold dark matter
(CDM). In particular, we consider a generic scenario where dark radiation, as a
result of an interaction, is produced directly by a fraction of the dark matter
density effectively decaying into dark radiation. At an early epoch when the
dark matter density is negligible, as an obvious consequence, the density of
dark radiation is also very small. As the Universe approaches matter radiation
equality, the dark matter density starts to dominate thereby increasing the
content of dark radiation and changing the expansion rate of the Universe. As
this increase in dark radiation content happens naturally after Big Bang
Nucleosynthesis (BBN), it can relax the possible tension with lower values of
radiation degrees of freedom measured from light element abundances compared to
that of the CMB. We numerically confront this scenario with WMAP+ACT and
WMAP+SPT data and derive an upper limit on the allowed fraction of dark matter
decaying into dark radiation.
