Some Implications of a Cosmological Phase Transition

http://cds.cern.ch/record/538390/files/0202122.pdf

Neutrinos in cosmology

The Hubble expansion of galaxies, the 2.73 K black-body radiation background and the cosmic abundances of the light elements argue for a hot, dense origin of the universe - the standard big bang cosmology - and enable its evolution to be traced back fairly reliably to the nucleosynthesis era when the temperature was of O(1) MeV corresponding to an expansion age of O(1) s. All particles, known and hypothetical, would have been created at higher temperatures in the early universe and analyses of their possible effects on the abundances of the synthesized elements enable many interesting constraints to be obtained on particle properties. These arguments have usefully complemented laboratory experiments in guiding attempts to extend physics beyond the standard model, incorporating ideas such as supersymmetry, compositeness and unification. We first present a pedagogical account of relativistic cosmology and primordial nucleosynthesis, discussing both theoretical and observational aspects, and then proceed to examine such constraints in detail, in particular those pertaining to new massless particles and massive unstable particles. Finally, in a section aimed at particle physicists, we illustrate applications of such constraints to models of new physics.

https://iopscience.iop.org/article/10.1088/0034-4885/59/12/001/pdf

Big bang nucleosynthesis and physics beyond the standard model

The relic cosmic background neutrinos accompanying the cosmic microwave background (CMB) photons may hide a universal lepton asymmetry orders of magnitude larger than the universal baryon asymmetry. At present, the only direct way to probe such an asymmetry is through its effect on the abundances of the light elements produced during primordial nucleosynthesis. The relic light element abundances also depend on the baryon asymmetry, parameterized by the baryon density parameter () and on the early-universe expansion rate, parameterized by the expansion rate factor () or, equivalently, by the effective number of neutrinos (). We use data from the CMB (and large scale structure: LSS) along with the observationally inferred relic abundances of deuterium and helium-4 to provide new bounds on the universal lepton asymmetry, finding for ηL, the analog of ηB,0.072 ± 0.053 if it is assumed that Nν = 3 and, 0.115 ± 0.095 along with Nν = 3.3−0.6+0.7, if Nν is free to vary.

http://iopscience.iop.org/article/10.1088/1475-7516/2008/08/011/pdf

Constraining the universal lepton asymmetry

The relic cosmic background neutrinos accompanying the cosmic microwave background (CMB) photons may hide a universal lepton asymmetry orders of magnitude larger than the universal baryon asymmetry. At present, the only direct way to probe such an asymmetry is through its effect on the abundances of the light elements produced during primordial nucleosynthesis. The relic light element abundances also depend on the baryon asymmetry, parameterized by the baryon density parameter (eta_B = n_B/n_gamma = 10^(-10)*eta_10), and on the early-universe expansion rate, parameterized by the expansion rate factor (S = H'/H) or, equivalently by the effective number of neutrinos (N_nu = 3 + 43(S^2 - 1)/7). We use data from the CMB (and Large Scale Structure: LSS) along with the observationally-inferred relic abundances of deuterium and helium-4 to provide new bounds on the universal lepton asymmetry, finding for eta_L, the analog of eta_B, 0.072 +/- 0.053 if it is assumed that N_nu = 3 and, 0.115 +/- 0.095 along with N_nu = 3.3^{+0.7}_{-0.6}, if N_nu is free to vary.

Constraining The Universal Lepton Asymmetry

Big-bang nucleosynthesis per se cannot decide if the universe is open or not. Present gauge theories of elementary particles favour, on several grounds, low values (much less than unity) of the universal lepton numbers. These values, in the context of nucleosynthesis compatibility, still suggest an open Universe (13

H. Reeves

Papers

Weak interactions in the early Universe: is the Universe open?