2,938 research outputs found
Ultra-Light Scalar Fields and the Growth of Structure in the Universe
Ultra-light scalar fields, with masses of between m=10^{-33} eV and
m=10^{-22} eV, can affect the growth of structure in the Universe. We identify
the different regimes in the evolution of ultra-light scalar fields, how they
affect the expansion rate of the universe and how they affect the growth rate
of cosmological perturbations. We find a number of interesting effects, discuss
how they might arise in realistic scenarios of the early universe and comment
on how they might be observed.Comment: 12 pages, 11 figure
Quintessence in a quandary: prior dependence in dark energy models
The archetypal theory of dark energy is quintessence: a minimally coupled
scalar field with a canonical kinetic energy and potential. By studying random
potentials we show that quintessence imposes a restricted set of priors on the
equation of state of dark energy. Focusing on the commonly-used
parametrisation, , we show that there is a natural
scale and direction in the plane that distinguishes quintessence
as a general framework. We calculate the expected information gain for a given
survey and show that, because of the non-trivial prior information, it is a
function of more than just the figure of merit. This allows us to make a
quantitative case for novel survey strategies. We show that the scale of the
prior sets target observational requirements for gaining significant
information. This corresponds to a figure of merit FOM, a
requirement that future galaxy redshift surveys will meet.Comment: 5 pages, 3 figures. For the busy reader, Fig. 1 is the money plot.
v2: Minor changes, matches published version. Code open source at
gitorious.org/random-quintessenc
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
Galaxy UV-luminosity function and reionization constraints on axion dark matter
If the dark matter (DM) were composed of axions, then structure formation in
the Universe would be suppressed below the axion Jeans scale. Using an analytic
model for the halo mass function of a mixed DM model with axions and cold dark
matter, combined with the abundance-matching technique, we construct the
UV-luminosity function. Axions suppress high- galaxy formation and the
UV-luminosity function is truncated at a faintest limiting magnitude. From the
UV-luminosity function, we predict the reionization history of the universe and
find that axion DM causes reionization to occur at lower redshift. We search
for evidence of axions using the Hubble Ultra Deep Field UV-luminosity function
in the redshift range -, and the optical depth to reionization,
, as measured from cosmic microwave background polarization. All probes
we consider consistently exclude from
contributing more than half of the DM, with our strongest constraint ruling
this model out at more than significance. In conservative models of
reionization a dominant component of DM with is in
tension with the measured value of , putting pressure on an
axion solution to the cusp-core problem. Tension is reduced to for
the axion contributing only half of the DM. A future measurement of the
UV-luminosity function in the range - by JWST would provide further
evidence for or against . Probing still higher masses
of will be possible using future measurements of the
kinetic Sunyaev-Zel'dovich effect by Advanced ACTPol to constrain the time and
duration of reionization.Comment: 17 pages, 8 figures, 2 tables. v2: Minor Changes. References added.
Published in MNRA
The Grassmannian Sigma Model in SU(2) Yang-Mills Theory
Spin-charge separation in pure SU(2) Yang-Mills theory was recently found to
involve the dynamics of an O(3) non-linear sigma model and, seemingly, a
Grassmannian non-linear sigma model. In this article we explicitly construct
the Grassmannian sigma model of the form appearing in the the spin-charge
separated SU(2) theory through a quaternionic decomposition of the manifold,
thus verifying its relevance in this context. The coupling between this model
and the O(3) non-linear sigma model is further commented upon.Comment: 11 pages, undergraduate research project; version published in J.
Phys.
A failure management prototype: DR/Rx
This failure management prototype performs failure diagnosis and recovery management of hierarchical, distributed systems. The prototype, which evolved from a series of previous prototypes following a spiral model for development, focuses on two functions: (1) the diagnostic reasoner (DR) performs integrated failure diagnosis in distributed systems; and (2) the recovery expert (Rx) develops plans to recover from the failure. Issues related to expert system prototype design and the previous history of this prototype are discussed. The architecture of the current prototype is described in terms of the knowledge representation and functionality of its components
The Axiverse Extended: Vacuum Destabilisation, Early Dark Energy and Cosmological Collapse
A model is presented in the philosophy of the "String Axiverse" of Arvanitaki
et al (arXiv:0905.4720v2 [hep-th]) that incorporates a coupling of ultralight
axions to their corresponding moduli through the mass term. The light fields
roll in their potentials at late times and contribute to the dark sector energy
densities in the cosmological expansion. The addition of a coupling and extra
field greatly enrich the possible phenomenology of the axiverse. There are a
number of interesting phases where the axion and modulus components behave as
Dark Matter or Dark Energy and can have considerable and distinct effects on
the expansion history of the universe by modifying the equation of state in the
past or causing possible future collapse of the universe. In future such a
coupling may help to alleviate fine tuning problems for cosmological axions. We
motivate and present the model, and briefly explore its cosmological
consequences numerically.Comment: 13 pages, 17 figures, published in PRD. v3: corrected SUSY
interpretation of axion potential scal
Metal-dependent assembly of a protein nano-cage
Short, alpha-helical coiled coils provide a simple, modular method to direct the assembly of proteins into higher order structures. We previously demonstrated that by genetically fusing de novo-designed coiled coils of the appropriate oligomerization state to a natural trimeric protein, we could direct the assembly of this protein into various geometrical cages. Here, we have extended this approach by appending a coiled coil designed to trimerize in response to binding divalent transition metal ions and thereby achieve metal ion-dependent assembly of a tetrahedral protein cage. Ni2+, Co2+, Cu2+, and Zn2+ ions were evaluated, with Ni2+ proving the most effective at mediating protein assembly. Characterization of the assembled protein indicated that the metal ion-protein complex formed discrete globular structures of the diameter expected for a complex containing 12 copies of the protein monomer. Protein assembly could be reversed by removing metal ions with ethylenediaminetetraacetic acid or under mildly acidic conditions.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151280/1/pro3676_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151280/2/pro3676-sup-0001-supinfo.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151280/3/pro3676.pd
NuSTAR hard X-ray observation of a sub-A class solar flare
We report a NuSTAR observation of a solar microflare, SOL2015-09-01T04.
Although it was too faint to be observed by the GOES X-ray Sensor, we estimate
the event to be an A0.1 class flare in brightness. This microflare, with only 5
counts per second per detector observed by RHESSI, is fainter than any hard
X-ray (HXR) flare in the existing literature. The microflare occurred during a
solar pointing by the highly sensitive NuSTAR astrophysical observatory, which
used its direct focusing optics to produce detailed HXR microflare spectra and
images. The microflare exhibits HXR properties commonly observed in larger
flares, including a fast rise and more gradual decay, earlier peak time with
higher energy, spatial dimensions similar to the RHESSI microflares, and a
high-energy excess beyond an isothermal spectral component during the impulsive
phase. The microflare is small in emission measure, temperature, and energy,
though not in physical size; observations are consistent with an origin via the
interaction of at least two magnetic loops. We estimate the increase in thermal
energy at the time of the microflare to be 2.4x10^27 ergs. The observation
suggests that flares do indeed scale down to extremely small energies and
retain what we customarily think of as "flarelike" properties.Comment: Status: Accepted by the Astrophysical Journal, 2017 July 1
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
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