647 research outputs found
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
Anisotropies in the gamma-ray sky from millisecond pulsars
Pulsars emerge in the Fermi era as a sizable population of gamma-ray sources. Millisecond
pulsars (MSPs) constitute an older subpopulation whose sky distribution extends to high
Galactic latitudes, and it has been suggested that unresolved members of this class may
contribute a significant fraction of the measured large-scale isotropic gamma-ray background
(IGRB). We investigate the possible energy-dependent contribution of unresolved MSPs to
the anisotropy of the Fermi-measured IGRB. For observationally motivated MSP population
models, we show that the preliminary Fermi anisotropy measurement places an interesting
constraint on the abundance of MSPs in the Galaxy and the typical MSP flux, about an order of
magnitude stronger than constraints on this population derived from the intensity of the IGRB
alone. We also examine the possibility of an MSP component in the IGRB mimicking a dark
matter signal in anisotropy-based searches, and conclude that the energy dependence of an
anisotropy signature would distinguish MSPs from all but very light dark matter candidates
Development of non-antibiotic-resistant, chromosomally based, constitutive and inducible expression systems for aroA-attenuated Salmonella enterica serovar typhimurium
Live-vaccine delivery systems expressing two model antigens from Mycoplasma hyopneumoniae, F2(P97) (Adh) and NrdF, were constructed using Salmonella enterica serovar Typhimurium aroA (STM-1), and immunogenicity in mice was evaluated. Recombinant plasmid-based expression (PBE) and chromosomally based expression (CBE) systems were constructed. The PBE system was formed by cloning both antigen genes into pJLA507 to create an operon downstream of temperature-inducible promoters. Constitutive CBE was achieved using a promoter-trapping technique whereby the promoterless operon was stably integrated into the chromosome of STM-1, and the expression of antigens was assessed. The chromosomal position of the operon was mapped in four clones. Inducible CBE was obtained by using the in vivo-induced sspA promoter and recombining the expression construct into aroD. Dual expression of the antigens was detected in all systems, with PBE producing much larger quantities of both antigens. The stability of antigen expression after in vivo passage was 100% for all CBE strains recovered. PBE and CBE strains were selected for comparison in a vaccination trial. The vaccine strains were delivered orally into mice, and significant systemic immunoglobulin M(IgM) and IgG responses against both antigens were detected among all CBE groups. No significant immune response was detected using PBE strains. Expression of recombinant antigens in S. enterica serovar Typhimurium aroA from chromosomally located strong promoters without the use of antibiotic resistance markers is a reliable and effective method of inducing a significant immune response
APS Neutrino Study: Report of the Neutrino Astrophysics and Cosmology Working Group
In 2002, Ray Davis and Masatoshi Koshiba were awarded the Nobel Prize in
Physics ``for pioneering contributions to astrophysics, in particular for the
detection of cosmic neutrinos.'' However, while astronomy has undergone a
revolution in understanding by synthesizing data taken at many wavelengths, the
universe has only barely been glimpsed in neutrinos, just the Sun and the
nearby SN 1987A. An entire universe awaits, and since neutrinos can probe
astrophysical objects at densities, energies, and distances that are otherwise
inaccessible, the results are expected to be particularly exciting. Similarly,
the revolution in quantitative cosmology has heightened the need for very
precise tests that depend on the effects of neutrinos, and prominent among them
is the search for the effects of neutrino mass, since neutrinos are a small but
known component of the dark matter. In this report, we highlight some of the
key opportunties for progress in neutrino astrophysics and cosmology, and the
implications for other areas of physics
Direct X-ray Constraints on Sterile Neutrino Warm Dark Matter
Warm dark matter (WDM) might more easily account for small scale clustering
measurements than the heavier particles typically invoked in Lambda cold dark
matter (LCDM) cosmologies. In this paper, we consider a Lambda WDM cosmology in
which sterile neutrinos nu_s, with a mass m_s of roughly 1-100 keV, are the
dark matter. We use the diffuse X-ray spectrum (total minus resolved point
source emission) of the Andromeda galaxy to constrain the rate of sterile
neutrino radiative decay: nu_s -> nu_{e,mu,tau} + gamma. Our findings demand
that m_s < 3.5 keV (95% C.L.) which is a significant improvement over the
previous (95% C.L.) limits inferred from the X-ray emission of nearby clusters,
m_s < 8.2 keV (Virgo A) and m_s < 6.3 keV (Virgo A + Coma).Comment: 8 pages, 2 figures, minor revisions, accepted for publication in
Physical Review
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
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Mixed waste contamination selects for a mobile genetic element population enriched in multiple heavy metal resistance genes
Mobile genetic elements (MGEs) like plasmids, viruses, and transposable elements can provide fitness benefits to their hosts for survival in the presence of environmental stressors. Heavy metal resistance genes (HMRGs) are frequently observed on MGEs, suggesting that MGEs may be an important driver of adaptive evolution in environments contaminated with heavy metals. Here, we report the meta-mobilome of the heavy metal-contaminated regions of the Oak Ridge Reservation subsurface. This meta-mobilome was compared with one derived from samples collected from unimpacted regions of the Oak Ridge Reservation subsurface. We assembled 1615 unique circularized DNA elements that we propose to be MGEs. The circular elements from the highly contaminated subsurface were enriched in HMRG clusters relative to those from the nearby unimpacted regions. Additionally, we found that these HMRGs were associated with Gamma and Betaproteobacteria hosts in the contaminated subsurface and potentially facilitate the persistence and dominance of these taxa in this region. Finally, the HMRGs were associated with conjugative elements, suggesting their potential for future lateral transfer. We demonstrate how our understanding of MGE ecology, evolution, and function can be enhanced through the genomic context provided by completed MGE assemblies
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