Production of heavy elements in inhomogeneous cosmologies

Abstract

Baryon density inhomogeneities during big bang nucleosynthesis can result from a variety of possible causes (e.g., quantum chromodynamic and electroweak phase transitions; cosmic strings). We present here the consequences of such inhomogeneities with special emphasis on the production of heavy elements in a parameter study, varying the global baryon-to-photon ratio eta (which is related to the baryon density and the Hubble constant via eta<SUB>10</SUB> = 64.94 Omega<SUB>b</SUB>(H<SUB>0</SUB>/50)<SUP>2</SUP> and the length scale of the density inhomogeneities. The production of heavy elements beyond Fe can only occur in neutron-rich environments; thus, we limit our study to neutron-rich zones, originating from neutron diffusion into low-density regions. In this first calculation including elements heavier than Si, we prove an earlier hypothesis that under such conditions r-process elements can be produced, strongly enhanced by the process of fission cycling. Primordial r-process abundances are, however, very sensitive to the choice of eta. Significant amounts, comparable to or larger than the (permitted) floor of heavy-element abundances found in low-metallicity stars at the onset of galactic evolution, can only be obtained for values in excess of eta<SUB>10</SUB> = 133 (i.e., Omega<SUB>b</SUB>(h<SUB>50</SUB>)<SUP>2</SUP> = 2.0; e.g., Omega<SUB>b</SUB> = 1, H<SUB>0</SUB> = 71, or Omega<SUB>b</SUB> = 0.5, H<SUB>0</SUB> = 100) and large length scales of inhomogeneities, which minimize the back-diffusion of neutrons into proton-rich regions. Recent investigations analyzing the primordial abundances of light elements seem to set tighter limits, eta<SUB>10</SUB> less than 26 to 39 (Omega b)(h<SUB>50</SUB>)<SUP>2</SUP> less than 0.4 to 0.6, from He-4 and apparently considerably lower values based on Li, Be, and B. Under such conditions the predicted abundances of heavy elements are a factor of 10<SUP>5</SUP> or more below presently observable limits.