807 research outputs found
Dark Matter Antibaryons from a Supersymmetric Hidden Sector
The cosmological origin of both dark and baryonic matter can be explained
through a unified mechanism called hylogenesis where baryon and antibaryon
number are divided between the visible sector and a GeV-scale hidden sector,
while the Universe remains net baryon symmetric. The "missing" antibaryons, in
the form of exotic hidden states, are the dark matter. We study model-building,
cosmological, and phenomenological aspects of this scenario within the
framework of supersymmetry, which naturally stabilizes the light hidden sector
and electroweak mass scales. Inelastic dark matter scattering on visible matter
destroys nucleons, and nucleon decay searches offer a novel avenue for the
direct detection of the hidden antibaryonic dark matter sea.Comment: 33 pages, 10 figures. Minor changes to match published versio
Building school-based cardiovascular health promotion capacity in youth: a mixed methods study
Spin Exchange Rates in Electron-Hydrogen Collisions
The spin temperature of neutral hydrogen, which determines the 21 cm optical
depth and brightness temperature, is set by the competition between radiative
and collisional processes. In the high-redshift intergalactic medium, the
dominant collisions are typically those between hydrogen atoms. However,
collisions with electrons couple much more efficiently to the spin state of
hydrogen than do collisions with other hydrogen atoms and thus become important
once the ionized fraction exceeds ~1%. Here we compute the rate at which
electron-hydrogen collisions change the hydrogen spin. Previous calculations
included only S-wave scattering and ignored resonances near the n=2 threshold.
We provide accurate results, including all partial wave terms through the
F-wave, for the de-excitation rate at temperatures T_K < 15,000 K; beyond that
point, excitation to n>=2 hydrogen levels becomes significant. Accurate
electron-hydrogen collision rates at higher temperatures are not necessary,
because collisional excitation in this regime inevitably produces Lyman-alpha
photons, which in turn dominate spin exchange when T_K > 6200 K even in the
absence of radiative sources. Our rates differ from previous calculations by
several percent over the temperature range of interest. We also consider some
simple astrophysical examples where our spin de-excitation rates are useful.Comment: submitted to MNRAS, 9 pages, 5 figure
Spin Exchange Rates in Proton-Hydrogen Collisions
The spin temperature of neutral hydrogen, which determines the optical depth
and brightness of the 21 cm line, is determined by the competition between
radiative and collisional processes. Here we examine the role of
proton-hydrogen collisions in setting the spin temperature. We use recent fully
quantum mechanical calculations of the relevant cross sections, which allow us
to present accurate results over the entire physically relevant temperature
range 1-10,000 K. For kinetic temperatures T_K>100 K, the proton-hydrogen rate
coefficient exceeds that for hydrogen-hydrogen collisions by about a factor of
two. However, at low temperatures (T_K < 5 K) H-p collisions become several
thousand times more efficient than H-H and even more important than H-e^-
collisions.Comment: submitted to MNRAS, 5 pages, 2 figures, typos correcte
The Spin-Resolved Atomic Velocity Distribution and 21-cm Line Profile of Dark-Age Gas
The 21-cm hyperfine line of atomic hydrogen (HI) is a promising probe of the
cosmic dark ages. In past treatments of 21-cm radiation it was assumed the
hyperfine level populations of HI could be characterized by a
velocity-independent ``spin temperature'' T_s determined by a competition
between 21-cm radiative transitions, spin-changing collisions, and (at lower
redshifts) Lyman-alpha scattering. However we show here that, if the
collisional time is comparable to the radiative time, the spin temperature will
depend on atomic velocity, T_s=T_s(v), and one must replace the usual hyperfine
level rate equations with a Boltzmann equation describing the spin and velocity
dependence of the HI distribution function. We construct here the Boltzmann
equation relevant to the cosmic dark ages and solve it using a basis-function
method. Accounting for the actual spin-resolved atomic velocity distribution
results in up to a 2 per cent suppression of the 21-cm emissivity, and a
redshift and angular-projection dependent suppression or enhancement of the
linear power spectrum of 21-cm fluctuations of up to 5 per cent. The effect on
the 21-cm line profile is more dramatic -- its full-width at half maximum
(FWHM) can be enhanced by up to 60 per cent relative to the
velocity-independent calculation. We discuss the implications for 21-cm
tomography of the dark ages.Comment: 25 pages, 6 figures, submitted to Mon. Not. Roy. Astron. So
Reheating Effects in the Matter Power Spectrum and Implications for Substructure
The thermal and expansion history of the Universe before big bang
nucleosynthesis is unknown. We investigate the evolution of cosmological
perturbations through the transition from an early matter era to radiation
domination. We treat reheating as the perturbative decay of an oscillating
scalar field into relativistic plasma and cold dark matter. After reheating, we
find that subhorizon perturbations in the decay-produced dark matter density
are significantly enhanced, while subhorizon radiation perturbations are
instead suppressed. If dark matter originates in the radiation bath after
reheating, this suppression may be the primary cutoff in the matter power
spectrum. Conversely, for dark matter produced nonthermally from scalar decay,
enhanced perturbations can drive structure formation during the cosmic dark
ages and dramatically increase the abundance of compact substructures. For low
reheat temperatures, we find that as much as 50% of all dark matter is in
microhalos with M > 0.1 Earth masses at z=100, compared to a fraction of 1e-10
in the standard case. In this scenario, ultradense substructures may constitute
a large fraction of dark matter in galaxies today.Comment: 20 pages, 13 figures; references added and minor changes made; to
appear in PR
Cosmological Signatures of Interacting Neutrinos
We investigate signatures of neutrino scattering in the Cosmic Microwave
Background (CMB) and matter power spectra, and the extent to which present
cosmological data can distinguish between a free streaming or tightly coupled
fluid of neutrinos. If neutrinos have strong non-standard interactions, for
example, through the coupling of neutrinos to a light boson, they may be kept
in equilibrium until late times. We show how the power spectra for these models
differ from more conventional neutrino scenarios, and use CMB and large scale
structure data to constrain these models. CMB polarization data improves the
constraints on the number of massless neutrinos, while the Lyman--
power spectrum improves the limits on the neutrino mass. Neutrino mass limits
depend strongly on whether some or all of the neutrino species interact and
annihilate. The present data can accommodate a number of tightly-coupled
relativistic degrees of freedom, and none of the interacting-neutrino scenarios
considered are ruled out by current data -- although considerations regarding
the age of the Universe disfavor a model with three annihilating neutrinos with
very large neutrino masses.Comment: 17 pages, 14 figures, minor changes and references added, published
in Phys. Rev.
Chronic wasting disease prions are not transmissible to transgenic mice overexpressing human prion protein
Chronic wasting disease (CWD) is a prion disease that affects free-ranging and captive cervids, including mule deer, white-tailed deer, Rocky Mountain elk and moose. CWD-infected cervids have been reported in 14 USA states, two Canadian provinces and in South Korea. The possibility of a zoonotic transmission of CWD prions via diet is of particular concern in North America where hunting of cervids is a popular sport. To investigate the potential public health risks posed by CWD prions, we have investigated whether intracerebral inoculation of brain and spinal cord from CWD-infected mule deer transmits prion infection to transgenic mice overexpressing human prion protein with methionine or valine at polymorphic residue 129. These transgenic mice have been utilized in extensive transmission studies of human and animal prion disease and are susceptible to BSE and vCJD prions, allowing comparison with CWD. Here, we show that these mice proved entirely resistant to infection with mule deer CWD prions arguing that the transmission barrier associated with this prion strain/host combination is greater than that observed with classical BSE prions. However, it is possible that CWD may be caused by multiple prion strains. Further studies will be required to evaluate the transmission properties of distinct cervid prion strains as they are characterized
Polarizing Bubble Collisions
We predict the polarization of cosmic microwave background (CMB) photons that
results from a cosmic bubble collision. The polarization is purely E-mode,
symmetric around the axis pointing towards the collision bubble, and has
several salient features in its radial dependence that can help distinguish it
from a more conventional explanation for unusually cold or hot features in the
CMB sky. The anomalous "cold spot" detected by the Wilkinson Microwave
Anisotropy Probe (WMAP) satellite is a candidate for a feature produced by such
a collision, and the Planck satellite and other proposed surveys will measure
the polarization on it in the near future. The detection of such a collision
would provide compelling evidence for the string theory landscape.Comment: Published version. 15 pages, 8 figure
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