634 research outputs found
On the Relation between Solar Activity and Clear-Sky Terrestrial Irradiance
The Mauna Loa Observatory record of direct-beam solar irradiance measurements
for the years 1958-2010 is analysed to investigate the variation of clear-sky
terrestrial insolation with solar activity over more than four solar cycles.
The raw irradiance data exhibit a marked seasonal cycle, extended periods of
lower irradiance due to emissions of volcanic aerosols, and a long-term
decrease in atmospheric transmission independent of solar activity. After
correcting for these effects, it is found that clear-sky terrestrial irradiance
typically varies by about 0.2 +/- 0.1% over the course of the solar cycle, a
change of the same order of magnitude as the variations of the total solar
irradiance above the atmosphere. An investigation of changes in the clear-sky
atmospheric transmission fails to find a significant trend with sunspot number.
Hence there is no evidence for a yet unknown effect amplifying variations of
clear-sky irradiance with solar activity.Comment: 16 pages, 7 figures, in press at Solar Physics; minor changes to the
text to match final published versio
Solutions to the cosmological constant problems
We critically review several recent approaches to solving the two
cosmological constant problems. The "old" problem is the discrepancy between
the observed value of and the large values suggested by particle
physics models. The second problem is the "time coincidence" between the epoch
of galaxy formation and the epoch of -domination t_\L. It is
conceivable that the "old" problem can be resolved by fundamental physics
alone, but we argue that in order to explain the "time coincidence" we must
account for anthropic selection effects. Our main focus here is on the
discrete- models in which can change through nucleation of
branes. We consider the cosmology of this type of models in the context of
inflation and discuss the observational constraints on the model parameters.
The issue of multiple brane nucleation raised by Feng {\it et. al.} is
discussed in some detail. We also review continuous-\L models in which the
role of the cosmological constant is played by a slowly varying potential of a
scalar field. We find that both continuous and discrete models can in principle
solve both cosmological constant problems, although the required values of the
parameters do not appear very natural. M-theory-motivated brane models, in
which the brane tension is determined by the brane coupling to the four-form
field, do not seem to be viable, except perhaps in a very tight corner of the
parameter space. Finally, we point out that the time coincidence can also be
explained in models where is fixed, but the primordial density
contrast is treated as a random variable.Comment: 30 pages, 3 figures, two notes adde
Gravitational Violation of R Parity and its Cosmological Signatures
The discrete R-parity () usually imposed on the Supersymmetric (SUSY)
models is expected to be broken at least gravitationally. If the neutralino is
a dark matter particle its decay channels into positrons, antiprotons and
neutrinos are severely constrained from astrophysical observations. These
constraints are shown to be violated even for Planck-mass-suppressed
dimension-five interactions arising from gravitational effects. We perform a
general analysis of gravitationally induced violation and identify two
plausible and astrophysically consistent scenarios for achieving the required
suppression.Comment: 10 pages, no figure
Matrix models as solvable glass models
We present a family of solvable models of interacting particles in high
dimensionalities without quenched disorder. We show that the models have a
glassy regime with aging effects. The interaction is controlled by a parameter
. For we obtain matrix models and for `tensor' models. We
concentrate on the cases which we study analytically and numerically.Comment: 10 pages + 2 figures, Univ.Roma I, 1038/94, ROM2F/94/2
Inducible nitric oxide synthase, Nos2, does not mediate optic neuropathy and retinopathy in the DBA/2J glaucoma model
<p>Abstract</p> <p>Background</p> <p>Nitric oxide synthase 2 (NOS2) contributes to neural death in some settings, but its role in glaucoma remains controversial. NOS2 is implicated in retinal ganglion cell degeneration in a rat glaucoma model in which intraocular pressure (IOP) is experimentally elevated by blood vessel cauterization, but not in a rat glaucoma model where IOP was elevated by injection of hypertonic saline. To test the importance of NOS2 for an inherited glaucoma, in this study we both genetically and pharmacologically decreased NOS2 activity in the DBA/2J mouse glaucoma model.</p> <p>Methods</p> <p>The expression of <it>Nos2 </it>in the optic nerve head was analyzed at both the RNA and protein levels at different stages of disease pathogenesis. To test the involvement of <it>Nos2 </it>in glaucomatous neurodegeneration, a null allele of <it>Nos2 </it>was backcrossed into DBA/2J mice and the incidence and severity of glaucoma was assessed in mice of each <it>Nos2 </it>genotype. Additionally, DBA/2J mice were treated with the NOS2 inhibitor aminoguanidine and the disease compared to untreated mice.</p> <p>Results</p> <p>Optic nerve head <it>Nos2 </it>RNA levels varied and increased during moderate but decreased at early and severe stages of disease. Despite the presence of a few NOS2 positive cells in the optic nerve head, NOS2 protein was not substantially increased during the glaucoma. Genetic deficiency of <it>Nos2 </it>or aminoguanidine treatment did not alter the IOP profile of DBA/2J mice. Additionally, neither <it>Nos2 </it>deficiency nor aminoguanidine had any detectable affect on the glaucomatous optic nerve damage.</p> <p>Conclusion</p> <p>Glaucomatous neurodegeneration in DBA/2J mice does not require NOS2 activity. Further experiments involving various models are needed to assess the general importance of <it>Nos2 </it>in glaucoma.</p
FUTURE MACHINE: Making Myths & Designing Technology for a Responsible Future: Making Myths and Entanglement: Community engagement at the edge of participatory design and user experience
This paper explores the unique methods and strategies employed by a team of artists, in collaboration with engineers, programmers, a climate scientist, researchers and members of the public, who have come together to create the Future Machine, with the aim to continue this longitudinal project for 30 years. We explore the process of designing Future Machine – the provenance and sustainability of the materials, the participatory design process, how Future Machine's interactivity is evolving over time and how the technology might be sustained for 30 years. We investigate the role that Future Machine plays in engaging the human populations across these diverse places, exploring how the artists used strategies of entanglement and myth-making to support the interactions between the ecologies, technologies, non-human inhabitants, narratives and neighbourly relations that are emerging as the project evolves over time. In conclusion we propose that the artist strategies of entanglement and encouraging myth-making reveal new approaches to experiential design. We propose that these strategies provide opportunities to open up a set of disruptive/radical/novel design challenges for HCI, that encourage sustainable and positive forms of interactions for a world increasingly impacted by anthropogenic climate and environmental change
Global structure of exact cosmological solutions in the brane world
We find the explicit coordinate transformation which links two exact
cosmological solutions of the brane world which have been recently discovered.
This means that both solutions are exactly the same with each other. One of two
solutions is described by the motion of a domain wall in the well-known
5-dimensional Schwarzshild-AdS spacetime. Hence, we can easily understand the
region covered by the coordinate used by another solution.Comment: Latex, 9 pages including 5 figures; references add, accepted for
publication in Physical Review
Studies of the motion and decay of axion walls bounded by strings
We discuss the appearance at the QCD phase transition, and the subsequent
decay, of axion walls bounded by strings in N=1 axion models. We argue on
intuitive grounds that the main decay mechanism is into barely relativistic
axions. We present numerical simulations of the decay process. In these
simulations, the decay happens immediately, in a time scale of order the light
travel time, and the average energy of the radiated axions is for . is found to increase
approximately linearly with . Extrapolation of this behaviour
yields in axion models of interest. We find that the
contribution to the cosmological energy density of axions from wall decay is of
the same order of magnitude as that from vacuum realignment, with however large
uncertainties. The velocity dispersion of axions from wall decay is found to be
larger, by a factor or so, than that of axions from vacuum realignment
and string decay. We discuss the implications of this for the formation and
evolution of axion miniclusters and for the direct detection of axion dark
matter on Earth. Finally we discuss the cosmology of axion models with in
which the domain wall problem is solved by introducing a small U(1)
breaking interaction. We find that in this case the walls decay into
gravitational waves.Comment: 37 pages, 10 figures, a minor mistake was corrected, several
references and comments were adde
X-ray emission during the muonic cascade in hydrogen
We report our investigations of X rays emitted during the muonic cascade in
hydrogen employing charge coupled devices as X-ray detectors. The density
dependence of the relative X-ray yields for the muonic hydrogen lines (K_alpha,
K_beta, K_gamma) has been measured at densities between 0.00115 and 0.97 of
liquid hydrogen density. In this density region collisional processes dominate
the cascade down to low energy levels. A comparison with recent calculations is
given in order to demonstrate the influence of Coulomb deexcitation.Comment: 5 pages, Tex, 4 figures, submitted to Physical Review Letter
Top-squark searches at the Tevatron in models of low-energy supersymmetry breaking
We study the production and decays of top squarks (stops) at the Tevatron
collider in models of low-energy supersymmetry breaking. We consider the case
where the lightest Standard Model (SM) superpartner is a light neutralino that
predominantly decays into a photon and a light gravitino. Considering the
lighter stop to be the next-to-lightest Standard Model superpartner, we analyze
stop signatures associated with jets, photons and missing energy, which lead to
signals naturally larger than the associated SM backgrounds. We consider both
2-body and 3-body decays of the top squarks and show that the reach of the
Tevatron can be significantly larger than that expected within either the
standard supergravity models or models of low-energy supersymmetry breaking in
which the stop is the lightest SM superpartner. For a modest projection of the
final Tevatron luminosity, L = 4 fb-1, stop masses of order 300 GeV are
accessible at the Tevatron collider in both 2-body and 3-body decay modes. We
also consider the production and decay of ten degenerate squarks that are the
supersymmetric partners of the five light quarks. In this case we find that
common squark masses up to 360 GeV are easily accessible at the Tevatron
collider, and that the reach increases further if the gluino is light.Comment: 32 pages, 9 figures; references adde
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