Proc. Neutrino Astrophysics - MPP Theory Group

Big Bang Nucleosynthesis With Small-Scale
Matter-Antimatter Domains
J.B. Rehm and K. Jedamzik
Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str.1, D-85748 Garching
Big Bang Nucleosynthesis (BBN) is one of the furthest back-reaching cosmological probes
available. By means of comparing the predicted and observationally inferred light element
abundances the physical conditions as early as a few seconds after the Big Bang can be
scrutinized [1]. Nevertheless, presence of matter-antimatter domains during BBN on length
scales comparable in size to the neutron diffusion length at temperatures between 10 and
10 −2 MeV (see e.g. [2]) has so far not been investigated. To our knowledge, the influence
of antimatter on BBN has only been studied [3] with respect to a homogeneous injection of
antiprotons into the primordial plasma after the end of BBN (T < 10 keV). The conclusion
of such studies is that only a small fraction of about 10 −3 antiprotons per proton is allowed
to be injected after the BBN epoch for the abundances of deuterium and 3 He produced in
¯p 4 He reactions not to exceed the observationally inferred values.
In this work [4] we discuss the BBN in a baryo-asymmetric universe with a inhomogeneous
distribution of matter-antimatter domains. For possible scenarios which may yield matterantimatter
domains during an electroweak baryogenesis scenario see e.g. Refs. [5, 6]. As long as
the diffusion length of neutrons and antineutrons is shorter than the size of a typical domain,
matter and antimatter remain segregated. Note that proton diffusion is suppressed due to
Coulomb scattering. Therefore, annihilations may likely only proceed when (anti)neutrons
diffuse out of their domains. We identify three main scenarios: 1. Annihilations before weak
freeze-out: No effect on the light-element abundances because the n/p-ratio, which governs the
abundances, is reset to the equilibrium value by the fast weak interactions. 2. Annihilations
between weak freeze-out (≈ 1 MeV) and the onset of 4 He synthesis (T ≈ 0.08 MeV): The
n/p-ratio is strongly affected. Annihilation of antimatter mostly occurs on neutrons, since
neutrons can both be annihilated by antineutrons diffusing into the matter region and diffuse
themselves into the antimatter region to annihilate there on antiprotons and antineutrons.
Protons on the other hand are confined to the matter region and can only be annihilated by
diffusing antineutrons. By providing this effective sink for neutrons, the 4 He abundance may
be substantially lower than in a standard BBN scenario at the same net entropy-per-baryon,
simply because there are less neutrons left to build up 4 He. In extreme cases, when all neutrons
have annihilated on antimatter before the onset of 4 He synthesis, zero net 4 He production
results. This may well represent the only known BBN scenario in a baryo-asymmetric universe
which precludes the production of any light elements. 3. Annihilations well after BBN: This
scenario was already mentioned above. Annihilations of only small amounts of antimatter on
4 He may easily overproduce the cosmic deuterium and 3 He abundances.
In summary, the figure illustrates that matter-antimatter domains present at the BBN
epoch may have dramatic effects on the net synthesized 4 He abundance. By comparison, the
abundances of other light isotopes are generally less affected. A more detailed analysis of
BBN with matter-antimatter domains will be presented in a forthcoming work [4].
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