5,662 research outputs found
Background cosmological dynamics in gravity and observational constraints
In this paper, we carry out a study of viable cosmological models in
-gravity at the background level. We use observable parameters like
and to form autonomous system of equations and show that the
models under consideration exhibit two different regimes in their time
evolution, namely, a phantom phase followed by a quintessence like behavior. We
employ statefinder parameters to emphasize a characteristic discriminative
signature of these models.Comment: 6 pages, Latex style, 9 eps figures, replaced versions with new
references added, Submitted to Phys.Rev.
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Lessons Learned and Next Steps in Energy Efficiency Measurement and Attribution: Energy Savings, Net to Gross, Non-Energy Benefits, and Persistence of Energy Efficiency Behavior
This white paper examines four topics addressing evaluation, measurement, and attribution of direct and indirect effects to energy efficiency and behavioral programs: Estimates of program savings (gross); Net savings derivation through free ridership / net to gross analyses; Indirect non-energy benefits / impacts (e.g., comfort, convenience, emissions, jobs); and, Persistence of savings
ALMA detection of dark chromospheric holes in the quiet Sun
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations
of a quiet-Sun region at a wavelength of 3 mm, obtained during the first solar
ALMA cycle on April 27, 2017, and compare them with available chromospheric
observations in the UV and visible as well as with photospheric magnetograms.
ALMA images clearly reveal the presence of distinct particularly dark/cool
areas in the millimeter maps having temperatures as low as 60% of the normal
quiet Sun at 3 mm, which are not seen in the other data. We speculate that ALMA
is sensing cool chromospheric gas, whose presence had earlier been inferred
from infrared CO spectra.Comment: 9 pages, 3 figures, accepted for publication in ApJ
The relationship between chromospheric emissions and magnetic field strength
Aims. We analyze observational data from 4 instruments to study the
correlations between chromospheric emission, spanning the heights from the
temperature minimum region to the middle chromosphere, and photospheric
magnetic field. Methods: The data consist of radio images at 3.5 mm from the
Berkeley-Illinois-Maryland Array (BIMA), UV images at 1600 A from TRACE, Ca II
K-line filtergrams from BBSO, and MDI/SOHO longitudinal photospheric
magnetograms. For the first time interferometric millimeter data with the
highest currently available resolution are included in such an analysis. We
determine various parameters of the intensity maps and correlate the
intensities with each other and with the magnetic field. Results: The
chromospheric diagnostics studied here show a pronounced similarity in their
brightness structures and map out the underlying photospheric magnetic field
relatively well. We find a power law to be a good representation of the
relationship between photospheric magnetic field and emission from
chromospheric diagnostics at all wavelengths. The dependence of chromospheric
brightness on magnetic field is found to be different for network and
internetwork regions.Comment: 13 pages, 14 figures, 3 table
The chromosphere above sunspots at millimeter wavelengths
Aims: The aim of this paper is to demonstrate that millimeter wave data can
be used to distinguish between various atmospheric models of sunspots, whose
temperature structure in the upper photosphere and chromosphere has been the
source of some controversy. Methods: We use observations of the temperature
contrast (relative to the quiet Sun) above a sunspot umbra at 3.5 mm obtained
with the Berkeley-Illinois-Maryland Array (BIMA), complemented by submm
observations from Lindsey & Kopp (1995) and 2 cm observations with the Very
Large Array. These are compared with the umbral contrast calculated from
various atmospheric models of sunspots. Results: Current mm and submm
observational data suggest that the brightness observed at these wavelengths is
low compared to the most widely used sunspot models. These data impose strong
constraints on the temperature and density stratifications of the sunspot
umbral atmosphere, in particular on the location and depth of the temperature
minimum and the location of the transition region. Conclusions: A successful
model that is in agreement with millimeter umbral brightness should have an
extended and deep temperature minimum (below 3000 K). Better spatial resolution
as well as better wavelength coverage are needed for a more complete
determination of the chromospheric temperature stratification above sunspot
umbrae.Comment: 9 pages, 11 figures.
http://www.aanda.org/articles/aa/abs/2014/01/aa21321-13/aa21321-13.htm
Phantom Field and the Fate of Universe
In this paper we analyze the cosmological dynamics of phantom field in a
variety of potentials unbounded from above. We demonstrate that the nature of
future evolution generically depends upon the steepness of the phantom
potential and discuss the fate of Universe accordingly.Comment: RevTeX4, 5 pages and 3 eps figures; references update
Loop cosmological dynamics and dualities with Randall-Sundrum braneworlds
The discrete quantum geometric effects play an important role in dynamical
evolution in the loop quantum cosmology. These effects which are significant at
the high energies lead to the quadratic energy density modifications to the
Friedmann equation, as in the Randall-Sundrum braneworld scenarios but with a
negative sign. We investigate the scalar field dynamics in this scenario and
show the existence of a phase of super-inflation independent of the inverse
scale factor modifications as found earlier. In this regime the scalar field
mimics the dynamics of a phantom field and vice versa. We also find various
symmetries between the expanding phase, the contracting phase and the phantom
phase in the loop quantum cosmology. We then construct the scaling solutions in
the loop quantum cosmology and show their dual relationship with those of the
Randall-Sundrum cosmology.Comment: Revtex4, 9 page
Galileon gravity and its relevance to late time cosmic acceleration
We consider the covariant galileon gravity taking into account the third
order and fourth order scalar field Lagrangians L_3(\pi) and L_4(\pi)
consisting of three and four 's with four and five derivatives acting on
them respectively. The background dynamical equations are set up for the system
under consideration and the stability of the self accelerating solution is
demonstrated in general setting. We extended this study to the general case of
the fifth order theory. For spherically symmetric static background, we spell
out conditions for suppression of fifth force effects mediated by the galileon
field . We study the field perturbations in the fixed background and
investigate conditions for their causal propagation. We also briefly discuss
metric fluctuations and derive evolution equation for matter perturbations in
galileon gravity.Comment: 11 pages, no figure, minor clarifications and few refs added, to
appear in pr
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Nanodielectics (A "Universal" Panacea for Solving All Electrical Insualation Problems?)
This text summarizes the keynote presentation that is based on the full-length paper of the same title. Dr. FreÌchette's oral presentation should not be seen as a summary of the âBrainstorm paperâ but a glance at some major accomplishments, hinrances and still remaining questions relative to nanodielectrics. Are nanodielectrics a âuniversalâ panacea? The answer to that question is no - but they've got a lot of potential
Cosmological Aspects of Rolling Tachyon
We examine the possibility of rolling tachyon to play the dual roll of
inflaton at early epochs and dark matter at late times. We argue that enough
inflation can be generated with the rolling tachyon either by invoking the
large number of branes or brane world assisted inflation. However, reheating is
problematic in this model.Comment: RevTeX 4 pages, Talk delivered in PASCOS held at TIFR (Mumbai) from
3rd Jan to 9th Jan. To appear in the proceedings of PASCOS to be published in
a special issue of Praman
- âŠ