33 research outputs found
Neutrinos and Big-Bang Nucleosynthesis
Observations of clusters and super clusters of galaxies have indicated that
the Universe is more dominated by baryons than ever estimated in the
homogeneous cosmological model for primordial nucleosynthesis. Recent
detections of possibly low deuterium abundance in Lyman- clouds along
the line of sight to high red-shift quasars have raised another potential
difficulty that \he4 is overproduced in any cosmological models which satisfy
the low deuterium abundance constraint. We show that the inhomogeneous
cosmological model with degenerate electron-neutrino can resolve these two
difficulties.Comment: 7 pages, latex, 3 figures. To appear in Nucl. Phys. A62
A High Deuterium Abundance at z=0.7
Of the light elements, the primordial abundance of deuterium, (D/H)_p,
provides the most sensitive diagnostic for the cosmological mass density
parameter Omega_B. Recent high redshift (D/H) measurements are highly
discrepant, although this may reflect observational uncertainties. The larger
(D/H) values, which imply a low Omega_B and require the Universe to be
dominated by non-baryonic matter (dynamical studies indicate a higher total
density parameter), cause problems for galactic chemical evolution models since
they have difficulty in reproducing the large decline down to the lower
present-day (D/H). Conversely, low (D/H) values imply an Omega_B greater than
derived from ^7Li and ^4He abundance measurements, and may require a deuterium
abundance evolution that is too low to easily explain. Here we report the first
measurement at intermediate redshift, where the observational difficulties are
smaller, of a gas cloud with ideal characteristics for this experiment. Our
analysis of the z = 0.7010 absorber toward 1718+4807 indicates (D/H) = 2.0 +/-
0.5 x 10^{-4} which is in the high range. This and other independent
observations suggests there may be a cosmological inhomogeneity in (D/H)_p of
at least a factor of ten.Comment: 6 pages, 1 figur
Deuteronomy and Numbers
Four light isotopes - D, ^3He, ^4He and ^7Li - were produced by nuclear
reactions a few seconds after the big bang. New measurements of ^3He in the ISM
by Gloeckler and Geiss and of deuterium in high redshift hydrogen clouds by
Tytler and his collaborators provide further confirmation of big-bang
nucleosynthesis and new insight about the density of ordinary matter (baryons).Comment: 6 pages LaTeX with 1 eps Figur
Low Mass Stars and the He3 Problem
The prediction of standard chemical evolution models of higher abundances of
He3 at the solar and present-day epochs than are observed indicates a possible
problem with the yield of He3 for stars in the range of 1-3 solar masses.
Because He3 is one of the nuclei produced in Big Bang Nucleosynthesis (BBN), it
is noted that galactic and stellar evolution uncertainties necessarily relax
constraints based on He3. We incorporate into chemical evolution models which
include outflow, the new yields for He3 of Boothroyd & Malaney (1995) which
predict that low mass stars are net destroyers of He3. Since these yields do
not account for the high \he3/H ratio observed in some planetary nebulae, we
also consider the possibility that some fraction of stars in the 1 - 3 solar
mass range do not destroy their He3 in theirpost main-sequence phase. We also
consider the possibility that the gas expelled by stars in these mass ranges
does not mix with the ISM instantaneously thus delaying the He3 produced in
these stars, according to standard yields, from reaching the ISM. In general,
we find that the Galactic D and He3 abundances can be fit regardless of whether
the primordial D/H value is high (2 x 10^{-4}) or low (2.5 x 10^{-5}).Comment: 20 pages, latex, 9 ps figure
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 Deuteron Confronts Big Bang Nucleosynthesis
Recent determinations of the deuterium abundance, H/H, in high redshift
Lyman limit hydrogen clouds challenge the usual picture of primordial
nucleosynthesis based on \lq\lq concordance\rq\rq\ of the calculated light
element (H, He, He, Li) nucleosynthesis yields with the
observationally-inferred abundances of these species. Concordance implies that
all light element yields can be made to agree with the observationally-inferred
abundances (within errors) for single global specifications of the
baryon-to-photon ratio, ; lepton number; neutron lifetime; and expansion
rate (or equivalently, effective number of light neutrino degrees of freedom
). Though one group studying Lyman limit systems obtains a high value
of H/H (), another group finds consistently low
values (). In the former case, concordance for is readily attained for the current observationally-inferred abundances of
He and Li. But if the latter case represents the primordial deuterium
abundance, then concordance for {\it any} is impossible unless the
primordial value of Li/H is considerably larger than the abundance of
lithium as measured in old, hot Pop II halo stars. Furthermore, concordance
with is possible for low H/H only if either (1) the primordial
He abundance has been significantly underestimated, or (2) new neutrino
sector physics is invoked. We argue that systematic underestimation of both the
Li and He primordial abundances is the likely resolution of this
problem, a conclusion which is strengthened by new results on He.Comment: To be published in Nucl. Phys. B (Proc. Suppl.), in the proceedings
of "Sources and Detection of Dark Matter in the Universe", held in Santa
Monica, Feb. 14-16 1996. 5 pages. Replaced version has a TeX command removed
that apparently caused some latex compilers to fai
The Statistics of Density Peaks and the Column Density Distribution of the Lyman-Alpha Forest
We develop a method to calculate the column density distribution of the
Lyman-alpha forest for column densities in the range . The Zel'dovich approximation, with appropriate smoothing, is used to
compute the density and peculiar velocity fields. The effect of the latter on
absorption profiles is discussed and it is shown to have little effect on the
column density distribution. An approximation is introduced in which the column
density distribution is related to a statistic of density peaks (involving its
height and first and second derivatives along the line of sight) in real space.
We show that the slope of the column density distribution is determined by the
temperature-density relation as well as the power spectrum on scales . An expression relating the three is given. We
find very good agreement between the column density distribution obtained by
applying the Voigt-profile-fitting technique to the output of a full
hydrodynamic simulation and that obtained using our approximate method for a
test model. This formalism then is applied to study a group of CDM as well as
CHDM models. We show that the amplitude of the column density distribution
depends on the combination of parameters , which is not well-constrained by independent observations. The
slope of the distribution, on the other hand, can be used to distinguish
between different models: those with a smaller amplitude and a steeper slope of
the power spectrum on small scales give rise to steeper distributions, for the
range of column densities we study. Comparison with high resolution Keck data
is made.Comment: match accepted version; discussion added: the effect of the shape of
the power spectrum on the slope of the column density distributio
Limits on Active-Sterile Neutrino Mixing and the Primordial Deuterium Abundance
Studies of limits on active-sterile neutrino mixing derived from big bang
nucleosynthesis considerations are extended to consider the dependance of these
constraints on the primordial deuterium abundance. This study is motivated by
recent measurements of D/H in quasar absorption systems, which at present yield
discordant results. Limits on active-sterile mixing are somewhat relaxed for
high D/H. For low D/H (), no active-sterile neutrino
mixing is allowed by currently popular upper limits on the primordial He
abundance . For such low primordial D/H values, the observational inference
of active-sterile neutrino mixing by upcoming solar neutrino experiments would
imply that has been systematically underestimated, unless there is new
physics not included in standard BBN.Comment: 10 pages + 2 figures, uses revtex macros, submitted to Phys. Rev. D.
Corrected figure captions and an added referenc
Influence of Gamma-Ray Emission on the Isotopic Composition of Clouds in the Interstellar Medium
We investigate one mechanism of the change in the isotopic composition of
cosmologically distant clouds of interstellar gas whose matter was subjected
only slightly to star formation processes. According to the standard
cosmological model, the isotopic composition of the gas in such clouds was
formed at the epoch of Big Bang nucleosynthesis and is determined only by the
baryon density in the Universe. The dispersion in the available cloud
composition observations exceeds the errors of individual measurements. This
may indicate that there are mechanisms of the change in the composition of
matter in the Universe after the completion of Big Bang nucleosynthesis. We
have calculated the destruction and production rates of light isotopes (D, 3He,
4He) under the influence of photonuclear reactions triggered by the gamma-ray
emission from active galactic nuclei (AGNs). We investigate the destruction and
production of light elements depending on the spectral characteristics of the
gamma-ray emission. We show that in comparison with previous works, taking into
account the influence of spectral hardness on the photonuclear reaction rates
can increase the characteristic radii of influence of the gamma-ray emission
from AGNs by a factor of 2-8. The high gamma-ray luminosities of AGNs observed
in recent years increase the previous estimates of the characteristic radii by
two orders of magnitude. This may suggest that the influence of the emission
from AGNs on the change in the composition of the medium in the immediate
neighborhood (the host galaxy) has been underestimated.Comment: 13 pages, 13 figures, 3 table