  Provided that matter-antimatter domains are present at the onset of big bang
nucleosynthesis (BBN), the number of allowed additional relativistic species
increases, compared to the standard scenario when matter-antimatter domains are
absent. The extra relativistic species may take the form of massless fermions
or even massless bosons, like relic gravitons. The number of additional degrees
of freedom compatible with BBN depends, in this framework, upon the typical
scale of the domains and the antimatter fraction. Since the presence of
matter-antimatter domains allows a reduction of the neutron to proton ratio
prior to the formation of 4He, large amounts of radiation-like energy density
are allowed. Our results are compared with other constraints on the number of
supplementary relativistic degrees of freedom and with different nonstandard
BBN scenarios where the reduction of the neutron to proton ratio occurs via a
different physical mechanism. The implications of our considerations for the
upper limits on stochastic gravitational waves backgrounds of cosmological
origin are outlined.
