781 research outputs found
Tensor Detection Severely Constrains Axion Dark Matter
The recent detection of B-modes by BICEP2 has non-trivial implications for
axion dark matter implied by combining the tensor interpretation with
isocurvature constraints from Planck. In this paper the measurement is taken as
fact, and its implications considered, though further experimental verification
is required. In the simplest inflation models implies . If the axion decay constant constraints on
the dark matter (DM) abundance alone rule out the QCD axion as DM for (where accounts for theoretical
uncertainty). If then vacuum fluctuations of the axion field
place conflicting demands on axion DM: isocurvature constraints require a DM
abundance which is too small to be reached when the back reaction of
fluctuations is included. High QCD axions are thus ruled out. Constraints
on axion-like particles, as a function of their mass and DM fraction, are also
considered. For heavy axions with we find
, with stronger constraints on heavier
axions. Lighter axions, however, are allowed and (inflationary)
model-independent constraints from the CMB temperature power spectrum and large
scale structure are stronger than those implied by tensor modes.Comment: 6 pages, 1 figure. v2: Some discussion and references added. v3
Update on QCD discussion. Version accepted for publication in Physical Review
Letter
Future CMB tests of dark matter: ultra-light axions and massive neutrinos
Measurements of cosmic microwave background (CMB) anisotropies provide strong
evidence for the existence of dark matter and dark energy. They can also test
its composition, probing the energy density and particle mass of different
dark-matter and dark-energy components. CMB data have already shown that
ultra-light axions (ULAs) with mass in the range compose a fraction of the cosmological critical
density. Here, the sensitivity of a proposed CMB-Stage IV (CMB-S4) experiment
(assuming a 1 arcmin beam and noise levels over a sky
fraction of 0.4) to the density of ULAs and other dark-sector components is
assessed. CMB-S4 data should be times more sensitive to the ULA
energy-density than Planck data alone, across a wide range of ULA masses
, and will probe axion decay constants of
, at the grand unified scale. CMB-S4 could
improve the CMB lower bound on the ULA mass from to
, nearing the mass range probed by dwarf galaxy abundances
and dark-matter halo density profiles. These improvements will allow for a
multi- detection of percent-level departures from CDM over a wide range
of masses. Much of this improvement is driven by the effects of weak
gravitational lensing on the CMB, which breaks degeneracies between ULAs and
neutrinos. We also find that the addition of ULA parameters does not
significantly degrade the sensitivity of the CMB to neutrino masses. These
results were obtained using the axionCAMB code (a modification to the CAMB
Boltzmann code), presented here for public use.Comment: 16 pages, 12 figures. The axionCAMB code will be available online at
http://github.com/dgrin1/axionCAMB from 1 August 201
Axion constraints in non-standard thermal histories
It is usually assumed that dark matter is produced during the radiation
dominated era. There is, however, no direct evidence for radiation domination
prior to big-bang nucleosynthesis. Two non-standard thermal histories are
considered. In one, the low-temperature-reheating scenario, radiation
domination begins as late as 1 MeV, and is preceded by significant entropy
generation. Thermal axion relic abundances are then suppressed, and
cosmological limits to axions are loosened. For reheating temperatures less
than 35 MeV, the large-scale structure limit to the axion mass is lifted. The
remaining constraint from the total density of matter is significantly relaxed.
Constraints are also relaxed for higher reheating temperatures. In a kination
scenario, a more modest change to cosmological axion constraints is obtained.
Future possible constraints to axions and low-temperature reheating from the
helium abundance and next-generation large-scale-structure surveys are
discussed.Comment: 10 pages, 7 figures, revised to match version published in Phys. Rev.
D. Fig. 7 and Eq. (20) modifie
Mars: Noachian hydrology by its statistics and topology
Discrimination between fluvial features generated by surface drainage and subsurface aquifer discharges will provide clues to the understanding of early Mars' climatic history. Our approach is to define the process of formation of the oldest fluvial valleys by statistical and topological analyses. Formation of fluvial valley systems reached its highest statistical concentration during the Noachian Period. Nevertheless, they are a scarce phenomenom in Martian history, localized on the craterized upland, and subject to latitudinal distribution. They occur sparsely on Noachian geological units with a weak distribution density, and appear in reduced isolated surface (around 5 x 10(exp 3)(sq km)), filled by short streams (100-300 km length). Topological analysis of the internal organization of 71 surveyed Noachian fluvial valley networks also provides information on the mechanisms of formation
Separable Structure of Many-Body Ground-State Wave Function
We have investigated a general structure of the ground-state wave function
for the Schr\"odinger equation for identical interacting particles (bosons
or fermions) confined in a harmonic anisotropic trap in the limit of large .
It is shown that the ground-state wave function can be written in a separable
form. As an example of its applications, this form is used to obtain the
ground-state wave function describing collective dynamics for trapped
bosons interacting via contact forces.Comment: J. Phys. B: At. Mol. Opt. Phys. 33 (2000) (accepted for publication
Neutron-diffraction study of field-induced transitions in the heavy-fermion compound Ce2RhIn8
We present neutron diffraction measurements in high magnetic fields (0 to
14.5 T) and at low temperatures (2.5, 2.3, 0.77 and 0.068 K) on single crystals
of the tetragonal heavy fermion antiferromagnet Ce2RhIn8. For B//[110] the
field dependence of selected magnetic and nuclear reflections reveals that the
material undergoes several transitions, the temperature dependence of which
suggests a complex B-T phase diagram. We present the detailed evolution of the
integrated intensities of selected reflections and discuss the associated
field-induced transitions.Comment: 12 pages, 3 figures Proceeding Euro-conference "Properties of
Condensed Matter probed by x-ray and neutron scattering"; to appear in
Physica
Crystal-field effects in the mixed-valence compounds Yb2M3Ga9 (M= Rh, Ir)
Magnetic susceptibility, heat capacity, and electrical resistivity
measurements have been carried out on single crystals of the intermediate
valence compounds Yb2Rh3Ga9 and Yb2Ir3Ga9. These measurements reveal a large
anisotropy due apparently to an interplay between crystalline electric field
(CEF) and Kondo effects. The temperature dependence of magnetic susceptibility
can be modelled using the Anderson impurity model including CEF within an
approach based on the Non-Crossing Approximation.Comment: Accepted to Phys. Rev.
What Influences the Diffusion of Grassroots Innovations for Sustainability? Investigating Community Currency Niches
Community action for sustainability is a promising site of socio-technical innovation. Here we test the applicability of co-evolutionary niche theories of innovation diffusion (Strategic Niche Management, SNM) to the context of ‘grassroots innovations’. We present new empirical findings from an international study of 12 community currency niches (such as LETS, time banks, local currencies). These are parallel systems of exchange, designed to operate alongside mainstream money, meeting additional sustainability needs. Our findings confirm SNM predictions that niche-level activity correlates with diffusion success, but we highlight additional or confounding factors, and how niche theories might be adapted to better fit civil-society innovations. In so doing, we develop a model of grassroots innovation niche diffusion which builds on existing work and tailors it to this specific context. The paper concludes with a series of theoretically-informed recommendations for practitioners and policymakers to support the development and potential of grassroots innovations
A search for ultra-light axions using precision cosmological data
Ultra-light axions (ULAs) with masses in the range 10^{-33} eV <m <10^{-20}
eV are motivated by string theory and might contribute to either the
dark-matter or dark-energy density of the Universe. ULAs could suppress the
growth of structure on small scales, or lead to an enhanced integrated
Sachs-Wolfe effect on large-scale cosmic microwave-background (CMB)
anisotropies. In this work, cosmological observables over the full ULA mass
range are computed, and then used to search for evidence of ULAs using CMB data
from the Wilkinson Microwave Anisotropy Probe (WMAP), Planck satellite, Atacama
Cosmology Telescope, and South Pole Telescope, as well as galaxy clustering
data from the WiggleZ galaxy-redshift survey. In the mass range 10^{-32} eV < m
<10^{-25.5} eV, the axion relic-density \Omega_{a} (relative to the total
dark-matter relic density \Omega_{d}) must obey the constraints
\Omega_{a}/\Omega_{d} < 0.05 and \Omega_{a}h^{2} < 0.006 at 95%-confidence. For
m> 10^{-24} eV, ULAs are indistinguishable from standard cold dark matter on
the length scales probed, and are thus allowed by these data. For m < 10^{-32}
eV, ULAs are allowed to compose a significant fraction of the dark energy.Comment: 31 pages, 16 figures, 1 table, updated to have same figure
line-types/language as version published in Phys. Rev. D, grammatical
corrections made, references added, results unchange
Axiverse cosmology and the energy scale of inflation
Ultra-light axions (eV), motivated by string theory,
can be a powerful probe of the energy scale of inflation. In contrast to
heavier axions the isocurvature modes in the ultra-light axions can coexist
with observable gravitational waves. Here it is shown that large scale
structure constraints severely limit the parameter space for axion mass,
density fraction and isocurvature amplitude. It is also shown that radically
different CMB observables for the ultra-light axion isocurvature mode
additionally reduce this space. The results of a new, accurate and efficient
method to calculate this isocurvature power spectrum are presented, and can be
used to constrain ultra-light axions and inflation.Comment: 4 pages, 3 figures, v3 some references added, matches version
published in Physical Review
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