13,018 research outputs found
Insights into neutrino decoupling gleaned from considerations of the role of electron mass
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.Comment: 32 pages, 8 figures, 1 table. Version accepted by Nuclear Physics
The surprising influence of late charged current weak interactions on Big Bang Nucleosynthesis
The weak interaction charged current processes (, , ) interconvert neutrons and protons in the early universe and
have significant influence on Big Bang Nucleosynthesis (BBN) light-element
abundance yields, particulary that for . We demonstrate that the
influence of these processes is still significant even when they operate well
below temperatures usually invoked for "weak freeze-out,"
and in fact down nearly into the alpha-particle formation epoch (). 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 .Comment: 27 pages, 8 figure
Presupernova collapse models with improved weak-interaction rates
Improved values for stellar weak interaction rates have been recently
calculated based upon a large shell model diagonalization. Using these new
rates (for both beta decay and electron capture), we have examined the
presupernova evolution of massive stars in the range 15-40 Msun. Comparing our
new models with a standard set of presupernova models by Woosley and Weaver, we
find significantly larger values for the electron-to-baryon ratio Ye at the
onset of collapse and iron core masses reduced by approximately 0.1 Msun. The
inclusion of beta-decay accounts for roughly half of the revisions, while the
other half is a consequence of the improved nuclear physics. These changes will
have important consequences for nucleosynthesis and the supernova explosion
mechanism.Comment: 4 pages, 2 figure
The genealogy of judgement: towards a deep history of academic freedom
The classical conception of academic freedom associated with Wilhelm von Humboldt and the rise of the modern university has a quite specific cultural foundation that centres on the controversial mental faculty of 'judgement'. This article traces the roots of 'judgement' back to the Protestant Reformation, through its heyday as the signature feature of German idealism, and to its gradual loss of salience as both a philosophical and a psychological concept. This trajectory has been accompanied by a general shrinking in the scope of academic freedom from the promulgation of world-views to the offering of expert opinion
Artificial atmosphere control system
Two-gas control system has been developed which uses existing hardware. Three systems are used for control, monitoring, and safety backup. Pure oxygen will be supplied to maintain safe pressure level should something go wrong
Probing neutrino physics with a self-consistent treatment of the weak decoupling, nucleosynthesis, and photon decoupling epochs
We show that a self-consistent and coupled treatment of the weak decoupling,
big bang nucleosynthesis, and photon decoupling epochs can be used to provide
new insights and constraints on neutrino sector physics from high-precision
measurements of light element abundances and cosmic microwave background
observables. Implications of beyond-standard-model physics in cosmology,
especially within the neutrino sector, are assessed by comparing predictions
against five observables: the baryon energy density, helium abundance,
deuterium abundance, effective number of neutrinos, and sum of the light
neutrino mass eigenstates. We give examples for constraints on dark radiation,
neutrino rest mass, lepton numbers, and scenarios for light and heavy sterile
neutrinos.Comment: 29 pages, 10 figure
Majorana Neutrino Magnetic Moment and Neutrino Decoupling in Big Bang Nucleosynthesis
We examine the physics of the early universe when Majorana neutrinos
(electron neutrino, muon neutrino, tau neutrino) possess transition magnetic
moments. These extra couplings beyond the usual weak interaction couplings
alter the way neutrinos decouple from the plasma of electrons/positrons and
photons. We calculate how transition magnetic moment couplings modify neutrino
decoupling temperatures, and then use a full weak, strong, and electromagnetic
reaction network to compute corresponding changes in Big Bang Nucleosynthesis
abundance yields. We find that light element abundances and other cosmological
parameters are sensitive to magnetic couplings on the order of 10^{-10} Bohr
magnetons. Given the recent analysis of sub-MeV Borexino data which constrains
Majorana moments to the order of 10^{-11} Bohr magnetons or less, we find that
changes in cosmological parameters from magnetic contributions to neutrino
decoupling temperatures are below the level of upcoming precision observations.Comment: 19 pages, 9 figure
Evidence for an Intense Neutrino Flux during -Process Nucleosynthesis?
We investigate the possibility that neutrino capture on heavy nuclei competes
with beta decay in the environment where the -Process elements are
synthesized. We find that such neutrino capture is not excluded by existing
abundance determinations. We show that inclusion of significant neutrino
capture on the (neutron number) N=82 waiting point nuclei can allow the
inferred abundances of these species to provide a good fit to steady weak (beta
decay plus neutrino capture) flow equilibrium. In fact, for particular choices
of neutrino flux conditions, this fit is improved over the case where nuclei
change their charge by beta decay alone. However, this improved fit can be
realized only if neutrino capture plays a negligible role in nuclear decay back
toward stability. We discuss the implications of these considerations for
current proposed sites and models for -Process nucleosynthesis.Comment: 10 pages, plain tex, submitted to ApJ
Neutrino energy transport in weak decoupling and big bang nucleosynthesis
We calculate the evolution of the early universe through the epochs of weak
decoupling, weak freeze-out and big bang nucleosynthesis (BBN) by
simultaneously coupling a full strong, electromagnetic, and weak nuclear
reaction network with a multi-energy group Boltzmann neutrino energy transport
scheme. The modular structure of our code provides the ability to dissect the
relative contributions of each process responsible for evolving the dynamics of
the early universe in the absence of neutrino flavor oscillations. Such an
approach allows a detailed accounting of the evolution of the ,
, , , , energy
distribution functions alongside and self-consistently with the nuclear
reactions and entropy/heat generation and flow between the neutrino and
photon/electron/positron/baryon plasma components. This calculation reveals
nonlinear feedback in the time evolution of neutrino distribution functions and
plasma thermodynamic conditions (e.g., electron-positron pair densities), with
implications for: the phasing between scale factor and plasma temperature; the
neutron-to-proton ratio; light-element abundance histories; and the
cosmological parameter \neff. We find that our approach of following the time
development of neutrino spectral distortions and concomitant entropy production
and extraction from the plasma results in changes in the computed value of the
BBN deuterium yield. For example, for particular implementations of quantum
corrections in plasma thermodynamics, our calculations show a increase
in deuterium. These changes are potentially significant in the context of
anticipated improvements in observational and nuclear physics uncertainties.Comment: 37 pages, 12 Figures, 6 Table
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