5. COSMOLOGY CONSTRAINS PARTICLE PHYSICS

Limits on particle physics beyond its standard model are mostly sensitive to the bounds imposed on the ⁴He abundance. As described earlier, the ⁴He abundance is predominantly determined by the neutron-to-proton ratio just prior to nucleosynthesis. This ratio is determined by the competition between the weak interaction rates and the universal expansion rate. The latter can be modified from its standard model prediction by the presence of ``new'' particles beyond those known or expected on the basis of the standard model of particle physics. For example, additional neutrino ``flavors'' (N > 0), or other new particles, would increase the total energy density of the Universe, thus increasing the expansion rate (see equations 4 & 5), leaving more neutrons to form more ⁴He. For N sufficiently small, the predicted primordial helium abundance scales nearly linearly with N: Y 0.013 N. As a result, an upper bound to Y_P coupled with a lower bound to (since Y_P increases with increasing baryon abundance) will lead to an upper bound to N and a constraint on particle physics [1].

Figure 5. The number of equivalent massless neutrinos N 3 + N (see eq. 4) as a function of . The region allowed by an assumed primordial he mass fraction in the range 0.228 - 0.248 lies between the two solid curves. The vertical bands bounded by the dashed lines show generous bounds on from ``high'' and ``low'' deuterium.

The constraints on N 3 + N as a function of the baryon-to-photon ratio is shown in Figure 5. For ``low-D/high-'' there is little room for any ``extra'' particles: N 0.3. This would eliminate a new neutrino flavor (N = 1) or a new scalar particle (N = 4/7) provided they were massless or light (m << 1 MeV), and interacted at least as strongly as the ``ordinary'' neutrinos. In contrast, much weaker constraints are found for ``high-D/low-'' where N as large as 5 (N 2) may be allowed (see Fig. 5).