Evan Grohs

The surprising influence of late charged current weak interactions on Big Bang Nucleosynthesis

The weak interaction charged current processes (νe+n↔p+e−; ν¯e+p↔n+e+; n↔p+e−+ν¯e) interconvert neutrons and protons in the early universe and have significant influence on Big Bang Nucleosynthesis (BBN) light-element abundance yields, particularly that for 4He. We demonstrate that the influence of these processes is still significant even when they operate well below temperatures T∼0.7 MeV usually invoked for “weak freeze-out,” and in fact down nearly into the alpha-particle formation epoch (T≈0.1 MeV). This physics is correctly captured in commonly used BBN codes, though this late-time, low-temperature persistent effect of the isospin-changing weak processes, and the sensitivity of the associated rates to lepton energy distribution functions and blocking factors are not widely appreciated. We quantify this late-time influence by analyzing weak interaction rate dependence on the neutron lifetime, lepton energy distribution functions, entropy, the proton–neutron mass difference, and Hubble expansion rate. The effects we point out here render BBN a keen probe of any beyond-standard-model physics that alters lepton number/energy distributions, even subtly, in epochs of the early universe all the way down to near T=100 keV.

Lepton asymmetry, neutrino spectral distortions, and big bang nucleosynthesis

PHYSICAL REVIEW D 95, 063503 (2017) Lepton asymmetry, neutrino spectral distortions, and big bang nucleosynthesis E. Grohs, 1 George M. Fuller, 2 C. T. Kishimoto, 2,3 and Mark W. Paris 4 Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA Department of Physics, University of California, San Diego, La Jolla, California 92093, USA Department of Physics and Biophysics, University of San Diego, San Diego, California 92110, USA Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA (Received 9 December 2016; published 3 March 2017) We calculate Boltzmann neutrino energy transport with self-consistently coupled nuclear reactions through the weak-decoupling-nucleosynthesis epoch in an early universe with significant lepton numbers. We find that the presence of lepton asymmetry enhances processes which give rise to nonthermal neutrino spectral distortions. Our results reveal how asymmetries in energy and entropy density uniquely evolve for different transport processes and neutrino flavors. The enhanced distortions in the neutrino spectra alter the expected big bang nucleosynthesis light element abundance yields relative to those in the standard Fermi-Dirac neutrino distribution cases. These yields, sensitive to the shapes of the neutrino energy spectra, are also sensitive to the phasing of the growth of distortions and entropy flow with time/scale factor. We analyze these issues and speculate on new sensitivity limits of deuterium and helium to lepton number. DOI: 10.1103/PhysRevD.95.063503 I. INTRODUCTION In this paper we use the BURST neutrino-transport code [1] to calculate the baseline effects of out-of-equilibrium neutrino scattering on nucleosynthesis in an early universe with a nonzero lepton number, i.e., an asymmetry in the numbers of neutrinos and antineutrinos. Our baseline includes a strong, electromagnetic, and weak nuclear reaction network; modifications to the equation of state for the primeval plasma; and a Boltzmann neutrino energy transport network. We do not include neutrino flavor oscillations in this work. Our intent is to provide a coupled Boltzmann transport and nuclear reaction calculation to which future oscillation calculations can be compared. In fact, the outstanding issues in achieving ultimate precision in big bang nucleosynthesis (BBN) simulations will revolve around oscillations and plasma physics effects. These issues exist in both the zero and nonzero lepton-number cases, but are more acute in the presence of an asymmetry. We self-consistently follow the evolution of the neutrino phase-space occupation numbers through the weak- decoupling-nucleosynthesis epoch. There are many studies of the effects of lepton numbers on light element, BBN abundance yields. Early work [2,3] briefly explored the changes in the helium-4 ( 4 He) abundance in the presence of large neutrino degeneracies. Later work considered how lepton numbers could influence the 4 He yield [4,5] through neutrino oscillations. In addition, other works employed lepton numbers to constrain the cosmic microwave back- ground (CMB) radiation energy density [6,7] or the sum of the light neutrino masses [8]. References [9,10] simulta- neously investigated BBN abundances and CMB quantities using lepton numbers. The most recent work has used the primordial abundances to constrain lepton numbers which have been invoked to produce sterile neutrinos through matter-enhanced Mikheyev-Smirnow-Wolfenstein reso- nances [11–13]. Currently, our best constraints on these lepton numbers come from comparing the observationally inferred primordial abundances of either 4 He or deuterium (D) with the predicted yields of 4 He and D calculated in these models. Previous BBN calculations with neutrino asymmetry have made the assumption that the neutrino energy dis- tribution functions have thermal, Fermi-Dirac (FD) shaped forms. In fact, we know that neutrino scattering with electrons, positrons, and other neutrinos and electron- positron annihilation produce nonthermal distortions in these energy distributions, with concomitant effects on BBN abundance yields [1]. Though the nucleosynthesis changes induced with self-consistent transport are small, they nevertheless may be important in the context of high precision cosmology. Anticipated Stage-IV CMB measure- ments [14,15] of primordial helium and the relativistic energy density fraction at photon decoupling, coupled with the expected high precision deuterium measurements made with future 30-m class telescopes [16–20] will provide new probes of the relic neutrino history. In the standard cosmology with zero lepton numbers, neutrino oscillations act to interchange the populations of electron neutrinos and antineutrinos (ν e , ν ¯ e ) with those of muon and tau species (ν μ , ν ¯ μ , ν τ , ν ¯ τ ) [21]. Once we posit that there are asymmetries in the numbers of neutrinos and antineutrinos in one or more neutrino flavors, then neutrino

Effect of neutrino rest mass on ionization equilibrium freeze-out

Author(s): Grohs, E; Fuller, GM; Kishimoto, CT; Paris, MW | Abstract:

Insights into neutrino decoupling gleaned from considerations of the role of electron mass

Abstract We present calculations showing how electron rest mass influences entropy flow, neutrino decoupling, and Big Bang Nucleosynthesis (BBN) in the early universe. To elucidate this physics and especially the sensitivity of BBN and related epochs to electron mass, we consider a parameter space of rest mass values larger and smaller than the accepted vacuum value. Electromagnetic equilibrium, coupled with the high entropy of the early universe, guarantees that significant numbers of electron–positron pairs are present, and dominate over the number of ionization electrons to temperatures much lower than the vacuum electron rest mass. Scattering between the electrons–positrons and the neutrinos largely controls the flow of entropy from the plasma into the neutrino seas. Moreover, the number density of electron–positron-pair targets can be exponentially sensitive to the effective in-medium electron mass. This entropy flow influences the phasing of scale factor and temperature, the charged current weak-interaction-determined neutron-to-proton ratio, and the spectral distortions in the relic neutrino energy spectra. Our calculations show the sensitivity of the physics of this epoch to three separate effects: finite electron mass, finite-temperature quantum electrodynamic (QED) effects on the plasma equation of state, and Boltzmann neutrino energy transport. The ratio of neutrino to plasma–component energy scales manifests in Cosmic Microwave Background (CMB) observables, namely the baryon density and the radiation energy density, along with the primordial helium and deuterium abundances. Our results demonstrate how the treatment of in-medium electron mass (i.e., QED effects) could translate into an important source of uncertainty in extracting neutrino and beyond-standard-model physics limits from future high-precision CMB data.

Constraints on vacuum energy from structure formation and Nucleosynthesis

This paper derives an upper limit on the density

On the habitability of universes without stable deuterium

\rho_{\scriptstyle\Lambda}

Sensitivity of carbon and oxygen yields to the triple-alpha resonance in massive stars

of dark energy based on the requirement that cosmological structure forms before being frozen out by the eventual acceleration of the universe. By allowing for variations in both the cosmological parameters and the strength of gravity, the resulting constraint is a generalization of previous limits. The specific parameters under consideration include the amplitude

Neutrino energy transport in weak decoupling and big bang nucleosynthesis

Q

Stellar helium burning in other universes: A solution to the triple alpha fine-tuning problem

of the primordial density fluctuations, the Planck mass

Nuclear processes in other universes: Varying the strength of the weak force

M_{\rm pl}