408 research outputs found
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
The Primordial Abundance of He4: An Update
We include new data in an updated analysis of helium in low metallicity
extragalactic HII regions with the goal of deriving the primordial abundance of
He4 (Y_P). We show that the new observations of Izotov et al (ITL) are
consistent with previous data. However they should not be taken in isolation to
determine (Y_P) due to the lack of sufficiently low metallicity points. We use
the extant data in a semi-empirical approach to bounding the size of possible
systematic uncertainties in the determination of (Y_P). Our best estimate for
the primordial abundance of He4 assuming a linear relation between He4 and O/H
is Y_P = 0.230 \pm 0.003 (stat) based on the subset of HII regions with the
lowest metallicity; for our full data set we find Y_P = 0.234 \pm 0.002 (stat).
Both values are entirely consistent with our previous results. We discuss the
implications of these values for standard big bang nucleosynthesis (SBBN),
particularly in the context of recent measurements of deuterium in high
redshift, low metallicity QSO absorption-line systems.Comment: 26 pages, latex, 6 ps figure
The Evolution of Helium and Hydrogen Ionization Corrections as HII Regions Age
Helium and hydrogen recombination lines observed in low-metallicity,
extragalactic, HII regions provide the data used to infer the primordial helium
mass fraction, Y_P. In deriving abundances from observations, the correction
for unseen neutral helium or hydrogen is usually assumed to be absent; i.e.,
the ionization correction factor is taken to be unity (icf = 1). In a previous
paper (VGS), we revisited the question of the icf, confirming a "reverse"
ionization correction: icf < 1. In VGS the icf was calculated using more nearly
realistic models of inhomogeneous HII regions, suggesting that the published
values of Y_P needed to be reduced by an amount of order 0.003. As star
clusters age, their stellar spectra evolve and so, too, will their icfs. Here
the evolution of the icf is studied, along with that of two, alternate,
measures of the "hardness" of the radiation spectrum. The differences between
the icf for radiation-bounded and matter-bounded models are also explored,
along with the effect on the icf of the He/H ratio (since He and H compete for
some of the same ionizing photons). Particular attention is paid to the amount
of doubly-ionized helium predicted, leading us to suggest that observations of,
or bounds to, He++ may help to discriminate among models of HII regions ionized
by starbursts of different ages and spectra. We apply our analysis to the
Izotov & Thuan (IT) data set utilizing the radiation softness parameter, the
[OIII]/[OI] ratio, and the presence or absence of He++ to find 0.95 < icf <
0.99. This suggests that the IT estimate of the primordial helium abundance
should be reduced by Delta-Y = 0.006 +- 0.002, from 0.244 +- 0.002 to 0.238 +-
0.003.Comment: 27 double-spaced pages, 11 figures, 5 equations; revised to match the
version accepted for publication in the Ap
Cosmic Microwave Background Polarization
Cosmic microwave background (CMB) anisotropy is our richest source of
cosmological information; the standard cosmological model was largely
established thanks to study of the temperature anisotropies. By the end of the
decade, the Planck satellite will close this important chapter and move us
deeper into the new frontier of polarization measurements. Numerous
ground--based and balloon--borne experiments are already forging into this new
territory. Besides providing new and independent information on the primordial
density perturbations and cosmological parameters, polarization measurements
offer the potential to detect primordial gravity waves, constrain dark energy
and measure the neutrino mass scale. A vigorous experimental program is
underway worldwide and heading towards a new satellite mission dedicated to CMB
polarization.Comment: Review given at TAUP 2005; References added; Additional reference
Ribbons on the CBR Sky: A Powerful Test of a Baryon Symmetric Universe
If the Universe consists of domains of matter and antimatter, annihilations
at domain interfaces leave a distinctive imprint on the Cosmic Background
Radiation (CBR) sky. The signature is anisotropies in the form of long, thin
ribbons of width , separated by angle where L is the characteristic domain size, and
y-distortion parameter . Such a pattern could potentially be
detected by the high-resolution CBR anisotropy experiments planned for the next
decade, and such experiments may finally settle the question of whether or not
our Hubble volume is baryon symmetric.Comment: LaTeX, 10 pages, 4 figures in epsf. Revised version corrects a couple
of relevant mistake
Big Bang Nucleosynthesis in Crisis?
A new evaluation of the constraint on the number of light neutrino species
(N_nu) from big bang nucleosynthesis suggests a discrepancy between the
predicted light element abundances and those inferred from observations, unless
the inferred primordial 4He abundance has been underestimated by 0.014 +/-
0.004 (1 sigma) or less than 10% (95%C.L.) of 3He survives stellar processing.
With the quoted systematic errors in the observed abundances and a conservative
chemical evolution parameterization, the best fit to the combined data is N_nu
= 2.1 +/- 0.3 (1 sigma) and the upper limit is N_nu < 2.6 (95% C.L.). The data
are inconsistent with the Standard Model (N_nu = 3) at the 98.6% C.L.Comment: To be published in Phys. Rev. Lett. Revised version to reflect
referee comments and criticisms by Copi, Schramm, and Turner of robustness of
D/He-3 analysis. Small quantitative changes but qualitative conclusions
unchanged. Question mark added to title. Entire ps file available at
ftp://upenn5.hep.upenn.edu/pub/hata/papers/bbn.ps.Z See also astro-ph/941208
Increasing Neff with particles in thermal equilibrium with neutrinos
Recent work on increasing the effective number of neutrino species (Neff) in
the early universe has focussed on introducing extra relativistic species
(`dark radiation'). We draw attention to another possibility: a new particle of
mass less than 10 MeV that remains in thermal equilibrium with neutrinos until
it becomes non-relativistic increases the neutrino temperature relative to the
photons. We demonstrate that this leads to a value of Neff that is greater than
three and that Neff at CMB formation is larger than at BBN. We investigate the
constraints on such particles from the primordial abundance of helium and
deuterium created during BBN and from the CMB power spectrum measured by ACT
and SPT and find that they are presently relatively unconstrained. We forecast
the sensitivity of the Planck satellite to this scenario: in addition to
dramatically improving constraints on the particle mass, in some regions of
parameter space it can discriminate between the new particle being a real or
complex scalar.Comment: 10 pages, 5 figures v2 matches version to appear in JCA
Ionization Corrections For Low-Metallicity H II Regions and the Primordial Helium Abundance
Helium and hydrogen recombination lines observed in low-metallicity,
extragalactic H II regions provide the data used to infer the primordial helium
mass fraction, Y_P. The ionization corrections for unseen neutral helium (or
hydrogen) are usually assumed to be absent; i.e., the ionization correction
factor is taken to be unity (icf = 1). In this paper we revisit the question of
the icf for H II regions ionized by clusters of young, hot, metal-poor stars.
Our key result is that for the H II regions used in the determination of Y_P,
there is a ``reverse'' ionization correction: icf < 1. We explore the effect on
the icf of more realistic inhomogeneous H II region models and find that for
those regions ionized by young stars, with ``hard'' radiation spectra, the icf
is reduced further below unity. In Monte Carlos using H II region data from the
literature (Izotov and Thuan 1998) we estimate a reduction in the published
value of Y_P of order 0.003, which is roughly twice as large as the quoted
statistical error in the Y_P determination.Comment: 23 pages, 2 postscript figures; ApJ accepted; minor change
- …