  Very light WIMPs (chi), thermal relics that annihilate late in the early
Universe, change the energy and entropy densities at BBN and at recombination.
BBN, in combination with the CMB, can remove some of the degeneracies among
light WIMPs and equivalent neutrinos, constraining the existence and properties
of each. Depending on the nature of the light WIMP (Majorana or Dirac fermion,
real or complex scalar) the joint BBN + CMB analyses set lower bounds to m_chi
in the range 0.5 - 5 MeV (m_chi/m_e > 1 - 10), and they identify best fit
values for m_chi in the range 5 - 10 MeV. The joint BBN + CMB analysis finds a
best fit value for the number of equivalent neutrinos, Delta N_nu ~ 0.65,
nearly independent of the nature of the WIMP. In the absence of a light WIMP
(m_chi > 20 MeV), N_eff = 3.05(1 + Delta N_nu /3). In this case, there is
excellent agreement between BBN and the CMB, but the joint fit reveals Delta
N_nu = 0.40+-0.17, disfavoring standard big bang nucleosynthesis (SBBN) (Delta
N_nu = 0) at ~ 2.4 sigma, as well as a sterile neutrino (Delta N_nu = 1) at ~
3.5 sigma. The best BBN + CMB joint fit disfavors the absence of dark radiation
(Delta N_nu = 0 at ~ 95% confidence), while allowing for the presence of a
sterile neutrino (Delta N_nu = 1 at ~ 1 sigma). For all cases considered here,
the lithium problem persists. These results, presented at the TAUP 2013
Conference, are based on Nollett & Steigman (2013).
