524 research outputs found
On the Abundance of Primordial Helium
We have used recent observations of helium-4, nitrogen and oxygen from some
four dozen, low metallicity, extra-galactic HII regions to define mean
versus , versus and versus relations which are
extrapolated to zero metallicity to determine the primordial mass
fraction . The data and various subsets of the data, selected on the basis
of nitrogen and oxygen, are all consistent with . For
the 2 (statistical) upper bound we find .
Estimating a 2\% systematic uncertainty leads to
a maximum upper bound to the primordial helium mass fraction: . We compare these upper bounds to
with recent calculations of the predicted yield from big bang
nucleosynthesis to derive upper bounds to the nucleon-to-photon ratio
() and the number of equivalent light (\lsim 10
MeV) neutrino species. For (), we find and . If indeed , then BBN
predicts enhanced production of deuterium and helium-3 which may be in conflict
with the primordial abundances inferred from model dependent (chemical
evolution) extrapolations of solar system and interstellar observations. Better
chemical evolution models and more data - especially -absorption in the QSO
Ly- clouds - will be crucial to resolve this potential crisis for BBN.
The larger upper bound, is completelyComment: 21 pages, LaTeX, 6 postscript figures available upon request,
UMN-TH-123
A New Look At Neutrino Limits From Big Bang Nucleosynthesis
We take a fresh look at the limits on the number of neutrino flavors derived
from big bang nucleosynthesis. In particular, recent measurements of the \he4
abundance enable one to estimate the primordial \he4 mass fraction at . For a baryon to photon ratio, ,
consistent with the other light elements, this leads to a best fit for the
number of neutrino flavors (the precise number depends on )
indicating a very strong upper limit to . Here, we derive new upper
limits on , paying special attention to the fact that the best estimate
may lie in an unphysical region ( if all three neutrino flavors are
light or massless; the lower bound to may even be as low as 2, if the
small window for a mass is exploited.) Our resulting upper limits
therefore depend on whether or 3 is assumed. We also explore the
sensitivity of our results to the adopted value of and the assumed
systematic errors in .Comment: 11 pages, latex, four uuencoded ps figures include
Non-BBN Constraints On The Key Cosmological Parameters
Since the baryon-to-photon ratio "eta" is in some doubt at present, we ignore
the constraints on eta from big bang nucleosynthesis (BBN) and fit the three
key cosmological parameters (h, Omega_M, eta) to four other observational
constraints: Hubble parameter, age of the universe, cluster gas (baryon)
fraction, and effective shape parameter "Gamma". We consider open and flat CDM
models and flat "Lambda"-CDM models, testing goodness of fit and drawing
confidence regions by the Delta-chi^2 method. CDM models with Omega_M = 1 (SCDM
models) are accepted only because we allow a large error on h, permitting h <
0.5. Open CDM models are accepted only for Omega_M \gsim 0.4. Lambda-CDM models
give similar results. In all of these models, large eta (\gsim 6) is favored
strongly over small eta, supporting reports of low deuterium abundances on some
QSO lines of sight, and suggesting that observational determinations of
primordial 4He may be contaminated by systematic errors. Only if we drop the
crucial Gamma constraint are much lower values of Omega_M and eta permitted.Comment: 12 pages, Kluwer Latex, 2 Postscript figures, to appear in the
proceedings of the ISSI Workshop, "The Primordial Nuclei and Their Galactic
Evolution" (Bern, May 6-10, 1997), ed. N. Prantzos, M. Tosi, and R. von
Steiger (Kluwer, Dordrecht
Big-bang nucleosynthesis revisited
The homogeneous big-bang nucleosynthesis yields of D, He-3, He-4, and Li-7 are computed taking into account recent measurements of the neutron mean-life as well as updates of several nuclear reaction rates which primarily affect the production of Li-7. The extraction of primordial abundances from observation and the likelihood that the primordial mass fraction of He-4, Y(sub p) is less than or equal to 0.24 are discussed. Using the primordial abundances of D + He-3 and Li-7 we limit the baryon-to-photon ratio (eta in units of 10 exp -10) 2.6 less than or equal to eta(sub 10) less than or equal to 4.3; which we use to argue that baryons contribute between 0.02 and 0.11 to the critical energy density of the universe. An upper limit to Y(sub p) of 0.24 constrains the number of light neutrinos to N(sub nu) less than or equal to 3.4, in excellent agreement with the LEP and SLC collider results. We turn this argument around to show that the collider limit of 3 neutrino species can be used to bound the primordial abundance of He-4: 0.235 less than or equal to Y(sub p) less than or equal to 0.245
Primordial Nucleosynthesis: Theory and Observations
We review the Cosmology and Physics underlying Primordial Nucleosynthesis and
survey current observational data in order to compare the predictions of Big
Bang Nucleosynthesis with the inferred primordial abundances. From this
comparison we report on the status of the consistency of the standard hot big
bang model, we constrain the universal density of baryons (nucleons), and we
set limits to the numbers and/or effective interactions of hypothetical new
"light" particles (equivalent massless neutrinos).Comment: 25 pages, latex, 4 ps figures, to be published in a special memorial
volume of Physics Reports in honor of David Schram
BBN For Pedestrians
The simplest, `standard' model of Big Bang Nucleosynthesis (SBBN) assumes
three light neutrinos (N_nu = 3) and no significant electron neutrino
asymmetry, leaving only one adjustable parameter: the baryon to photon ratio
eta. The primordial abundance of any one nuclide can, therefore, be used to
measure the baryon abundance and the value derived from the observationally
inferred primordial abundance of deuterium closely matches that from current,
non-BBN data, primarily from the WMAP survey. However, using this same estimate
there is a tension between the SBBN-predicted 4He and 7Li abundances and their
current, observationally inferred primordial abundances, suggesting that N_nu
may differ from the standard model value of three and/or that there may be a
non-zero neutral lepton asymmetry (or, that systematic errors in the abundance
determinations have been underestimated or overlooked). The differences are not
large and the allowed ranges of the BBN parameters permitted by the data are
quite small. Within these ranges, the BBN-predicted abundances of D, 3He, 4He,
and 7Li are very smooth, monotonic functions of eta, N_nu, and the lepton
asymmetry. It is possible to describe the dependencies of these abundances (or
powers of them) upon the three parameters by simple, linear fits which, over
their ranges of applicability, are accurate to a few percent or better. The
fits presented here have not been maximized for their accuracy but, for their
simplicity. To identify the ranges of applicability and relative accuracies,
they are compared to detailed BBN calculations; their utility is illustrated
with several examples. Given the tension within BBN, these fits should prove
useful in facilitating studies of the viability of proposals for non-standard
physics and cosmology, prior to undertaking detailed BBN calculations.Comment: Submitted to a Focus Issue on Neutrino Physics in New Journal of
Physics (www.njp.org
Neutrinos And Big Bang Nucleosynthesis
The early universe provides a unique laboratory for probing the frontiers of
particle physics in general and neutrino physics in particular. The primordial
abundances of the relic nuclei produced during the first few minutes of the
evolution of the Universe depend on the electron neutrinos through the
charged-current weak interactions among neutrons and protons (and electrons and
positrons and neutrinos), and on all flavors of neutrinos through their
contributions to the total energy density which regulates the universal
expansion rate. The latter contribution also plays a role in determining the
spectrum of the temperature fluctuations imprinted on the Cosmic Background
Radiation (CBR) some 400 thousand years later. Using deuterium as a baryometer
and helium-4 as a chronometer, the predictions of BBN and the CBR are compared
to observations. The successes of, as well as challenges to the standard models
of particle physics and cosmology are identified. While systematic
uncertainties may be the source of some of the current tensions, it could be
that the data are pointing the way to new physics. In particular, BBN and the
CBR are used to address the questions of whether or not the relic neutrinos
were fully populated in the early universe and, to limit the magnitude of any
lepton asymmetry which may be concealed in the neutrinos.Comment: Accepted for publication in the Proceedings of Nobel Symposium 129,
"Neutrino Physics"; to appear in Physics Scripta, eds., L Bergstrom, O.
Botner, P. Carlson, P. O. Hulth, and T. Ohlsso
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