11.2. The number of Relativistic Particles and their Decays
The main idea here is that the 4He abundance depends on the number of relativistic particles during BBN. Extra particles, such as neutrinos or supersymmetric particles, which are relativistic during BBN, lead to faster expansion, larger n/p and a larger Yp.
Steigman, Schramm & Gunn  calculated that BBN limited the number of families to N < 5 to match the 4He abundance. The range allowed by SBBN and laboratory measurements have both narrowed over the years and agree well today , . A recent update  gives N < 3.20 (95%) from SBBN, although a larger range is obtained if a wider variety of measured abundances are accepted . March-Russell et al.  note that additional relativistic degrees of freedom are allowed if there is a large compensating asymmetry in the electron neutrino number.
Shi, Fuller & Abazajian  follow the time evolution of a lepton number asymmetry arising from active - sterile neutrino transformations during BBN. For e mixing with s, Yp was allowed to change from -1% to +9%, while or µ mixing with s allowed -2% to +5%. Hence the Yp predicted by low D/H in SBBN could be lowered to 0.241, which is between the high and low measurements.
For many years past, observations suggested that Yp was smaller than expected for the low D/H in the ISM and now QSOs. It is hard to make Yp lower, since this requires fewer, not more, particles than in SBBN. Holtmann et al. ,  proposed decays of neutrinos, but this is nearly ruled out by the Kamiokande results on atmospheric neutrinos .
Lindley  found that massive particles decaying into photons must have lifetimes in excess of a few thousand seconds, to avoid the destruction of BBN D. Audouze, Lindley & Silk  noted that such radiative decays could photodisintegrate 4He and make D and 3He , removing the upper bound on b . Other references are given by Holtmann et al. , who discuss weakly interacting massive (100 GeV) particles which decay of order 106 s after BBN. The authors give limits on the abundance and lifetimes of gravitinos and neutralinos, for a wide range of light nuclei primordial abundances.
Kohri & Yokoyama  give limits on the mass fraction in primordial black holes with masses 108 - 3 x 1010 g which evaporate during BBN and change the abundances.
López-Suárez & Canal  combine inhomogeneous nucleosynthesis and particles which decay at a late time to reassess the limits on b . They find parameters which allow b < 0.13 - 0.18 h70-2 (h70-2 is the Hubble constant in units of 70 km s-1 Mpc-1). Such high b might appear to remove the need for non-baryonic dark matter, but there would then be conflicts with other measures of b , especially the baryon fraction in clusters of galaxies, if all those baryons were observable today.