12,277 research outputs found
Dilaton-derived quintessence scenario leading naturally to the late-time acceleration of the Universe
Quintessence scenarios provide a simple explanation for the observed acceleration of the Universe. Yet, explaining why acceleration did not start a long time ago remains a challenge. The idea that the transition from radiation to matter domination played a dynamical role in triggering acceleration has been put forward in various guises. We propose a simple dilaton-derived quintessence model in which temporary vacuum domination is naturally triggered by the radiation to matter transition. In this model Einstein's gravity is preserved but quintessence couples non-minimally to the cold dark matter, but not to ``visible'' matter. Such couplings have been attributed to the dilaton in the low-energy limit of string theory beyond tree level. We also show how a cosmological constant in the string frame translates into a quintessence-type of potential in the atomic frame
Constraining Isocurvature Initial Conditions with WMAP 3-year data
We present constraints on the presence of isocurvature modes from the
temperature and polarization CMB spectrum data from the WMAP satellite alone,
and in combination with other datasets including SDSS galaxy survey and SNLS
supernovae. We find that the inclusion of polarization data allows the WMAP
data alone, as well as in combination with complementary observations, to place
improved limits on the contribution of CDM and neutrino density isocurvature
components individually. With general correlations, the upper limits on these
sub-dominant isocurvature components are reduced to ~60% of the first year WMAP
results, with specific limits depending on the type of fluctuations. If
multiple isocurvature components are allowed, however, we find that the data
still allow a majority of the initial power to come from isocurvature modes. As
well as providing general constraints we also consider their interpretation in
light of specific theoretical models like the curvaton and double inflation.Comment: 8 pages, 7 figures. Revised Sec 4 and Figs 3-4 post-publication to
correct an error for models with varying isocurvature spectral inde
A Modified Gravity and its Consequences for the Solar System, Astrophysics and Cosmology
A relativistic modified gravity (MOG) theory leads to a self-consistent,
stable gravity theory that can describe the solar system, galaxy and clusters
of galaxies data and cosmology.Comment: 16 pages. Latex file. Talk given at the International Workshop "From
Quantum to Cosmos: Fundamental Physics in Space", 22-24 May, 2006, Warrenton,
Virginia, USA. To be published in Int. J. Mod. Phys D. Equation correcte
Field Induced Nodal Order Parameter in the Tunneling Spectrum of YBaCuO Superconductor
We report planar tunneling measurements on thin films of
YBaCuO at various doping levels under magnetic fields. By
choosing a special setup configuration, we have probed a field induced energy
scale that dominates in the vicinity of a node of the d-wave superconducting
order parameter. We found a high doping sensitivity for this energy scale. At
Optimum doping this energy scale is in agreement with an induced
order parameter. We found that it can be followed down to low fields at optimum
doping, but not away from it.Comment: 9 pages, 8 figures, accepted for publication in Phys. Rev.
New Analysis Indicates No Thermal Inversion in the Atmosphere of HD 209458b
An important focus of exoplanet research is the determination of the
atmospheric temperature structure of strongly irradiated gas giant planets, or
hot Jupiters. HD 209458b is the prototypical exoplanet for atmospheric thermal
inversions, but this assertion does not take into account recently obtained
data or newer data reduction techniques. We re-examine this claim by
investigating all publicly available Spitzer Space Telescope secondary-eclipse
photometric data of HD 209458b and performing a self-consistent analysis. We
employ data reduction techniques that minimize stellar centroid variations,
apply sophisticated models to known Spitzer systematics, and account for
time-correlated noise in the data. We derive new secondary-eclipse depths of
0.119 +/- 0.007%, 0.123 +/- 0.006%, 0.134 +/- 0.035%, and 0.215 +/- 0.008% in
the 3.6, 4.5, 5.8, and 8.0 micron bandpasses, respectively. We feed these
results into a Bayesian atmospheric retrieval analysis and determine that it is
unnecessary to invoke a thermal inversion to explain our secondary-eclipse
depths. The data are well-fitted by a temperature model that decreases
monotonically between pressure levels of 1 and 0.01 bars. We conclude that
there is no evidence for a thermal inversion in the atmosphere of HD 209458b.Comment: 8 pages, 5 figures; accepted for publication in Ap
Vector magnetic hysteresis of hard superconductors
Critical state problems which incorporate more than one component for the
magnetization vector of hard superconductors are investigated. The theory is
based on the minimization of a cost functional
which weighs the changes of the magnetic field vector within the sample. We
show that Bean's simplest prescription of choosing the correct sign for the
critical current density in one dimensional problems is just a particular
case of finding the components of the vector . is
determined by minimizing under the constraint , with a bounded set. Upon the selection of
different sets we discuss existing crossed field measurements and
predict new observable features. It is shown that a complex behavior in the
magnetization curves may be controlled by a single external parameter, i.e.:
the maximum value of the applied magnetic field .Comment: 10 pages, 9 figures, accepted in Phys. Rev.
Cosmological quintessence accretion onto primordial black holes : conditions for their growth to the supermassive scale
In this work we revisit the growth of small primordial black holes (PBHs)
immersed in a quintessential field and/or radiation to the supermassive black
hole (SMBHs) scale. We show the difficulties of scenarios in which such huge
growth is possible. For that purpose we evaluated analytical solutions of the
differential equations (describing mass evolution) and point out the strong
fine tuning for that conclusions. The timescale for growth in a model with a
constant quintessence flux is calculated and we show that it is much bigger
than the Hubble time.The fractional gain of the mass is further evaluated in
other forms, including quintessence and/or radiation. We calculate the
cosmological density due to quintessence necessary to grow BHs to the
supermassive range and show it to be much bigger than one. We also describe the
set of complete equations analyzing the evolution of the BH+quintessence
universe, showing some interesting effects such the quenching of the BH mass
growth due to the evolution of the background energy. Additional constraints
obtained by using the Holographic Bound are also described. The general
equilibrium conditions for evaporating/accreting black holes evolving in a
quintessence/radiation universe are discussed in the Appendix.Comment: 21 pp., 2 Figures, To appear in IJMP
Autonomous clustering using rough set theory
This paper proposes a clustering technique that minimises the need for subjective
human intervention and is based on elements of rough set theory. The proposed algorithm is
unified in its approach to clustering and makes use of both local and global data properties to
obtain clustering solutions. It handles single-type and mixed attribute data sets with ease and
results from three data sets of single and mixed attribute types are used to illustrate the
technique and establish its efficiency
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