2,691 research outputs found
On the properties of the transition matrix in bouncing cosmologies
We elaborate further on the evolution properties of cosmological fluctuations
through a bounce. We show this evolution to be describable either by
``transmission'' and ``reflection'' coefficients or by an effective unitary
S-matrix. We also show that they behave in a time reversal invariant way.
Therefore, earlier results are now interpreted in a different perspective and
put on a firmer basis.Comment: 4 pages, 1 figure, to appear in PR
Revisiting the calculations of inflationary perturbations
We present a new approximation scheme that allows us to increase the accuracy
of analytical predictions of the power spectra of inflationary perturbations
for two specific classes of inflationary models. Among these models are chaotic
inflation with a monomial potential, power-law inflation and natural inflation
(inflation at a maximum). After reviewing the established first order results
we calculate the amplitudes and spectral indices for these classes of models at
higher orders in the slow-roll parameters for scalar and tensorial
perturbations.Comment: Extended version of the talk to be published in the proceedings of
the Mexican Meeting on Exact Solutions and Scalar Fields in Gravity. Mexico,
1-6 October, 200
Induced currents in the quantum Hall regime: energy storage, persistence, and I-V characteristics
Copyright © 2012 American Physical SocietyInduced currents associated with the quantum Hall effect are studied in the temperature range 39 mK to 1.6 K, and at Landau-level filling factors ν=1,2,3,4, and 6, using torsion-balance magnetometry. A quantitative link is demonstrated between (nonlinear induced current) vs (inducing electromotive force) curves, and the subexponential decay of the induced current in a static magnetic field. The energy storage in the induced currents is reexamined with the conclusion that the predominant mechanism for storage is inductive, through the mutual inductance between the sample and the magnet, not capacitive as previous reports have assumed. The temperature dependencies of the currents are consistent with previous models, except for a low-temperature saturation at filling factors ν=1 and ν=2, which we attribute to electron heating
Non-Commutative Inflation
We show how a radiation dominated universe subject to space-time quantization
may give rise to inflation as the radiation temperature exceeds the Planck
temperature. We consider dispersion relations with a maximal momentum (i.e. a
mimimum Compton wavelength, or quantum of space), noting that some of these
lead to a trans-Planckian branch where energy increases with decreasing
momenta. This feature translates into negative radiation pressure and, in
well-defined circumstances, into an inflationary equation of state. We thus
realize the inflationary scenario without the aid of an inflaton field. As the
radiation cools down below the Planck temperature, inflation gracefully exits
into a standard Big Bang universe, dispensing with a period of reheating.
Thermal fluctuations in the radiation bath will in this case generate curvature
fluctuations on cosmological scales whose amplitude and spectrum can be tuned
to agree with observations.Comment: 4 pages, 3 figure
Fictitious fluxes in doped antiferromagnets
In a tight binding model of charged spin-1/2 electrons on a square lattice, a
fully polarized ferromagnetic spin configuration generates an apparent U(1)
flux given by times the skyrmion charge density of the ferromagnetic
order parameter. We show here that for an antiferromagnet, there are two
``fictitious'' magnetic fields, one staggered and one unstaggered. The
staggered topological flux per unit cell can be varied between
with a negligible change in the value of the effective
nearest neighbor coupling constant whereas the magnitude of the unstaggered
flux is strongly coupled to the magnitude of the second neighbor effective
coupling.Comment: RevTeX, 5 pages including 4 figure
Multiple field inflation
Inflation offers a simple model for very early evolution of our Universe and
the origin of primordial perturbations on large scales. Over the last 25 years
we have become familiar with the predictions of single-field models, but
inflation with more than one light scalar field can alter preconceptions about
the inflationary dynamics and our predictions for the primordial perturbations.
I will discuss how future observational data could distinguish between
inflation driven by one field, or many fields. As an example, I briefly review
the curvaton as an alternative to the inflaton scenario for the origin of
structure.Comment: 27 pages, no figures. To appear in proceedings of 22nd IAP
Colloquium, Inflation +25, Paris, June 200
Kasner and Mixmaster behavior in universes with equation of state w \ge 1
We consider cosmological models with a scalar field with equation of state
that contract towards a big crunch singularity, as in recent cyclic
and ekpyrotic scenarios. We show that chaotic mixmaster oscillations due to
anisotropy and curvature are suppressed, and the contraction is described by a
homogeneous and isotropic Friedmann equation if . We generalize the
results to theories where the scalar field couples to p-forms and show that
there exists a finite value of , depending on the p-forms, such that chaotic
oscillations are suppressed. We show that orbifold compactification also
contributes to suppressing chaotic behavior. In particular, chaos is avoided in
contracting heterotic M-theory models if at the crunch.Comment: 25 pages, 2 figures, minor changes, references adde
Issues on Generating Primordial Anisotropies at the End of Inflation
We revisit the idea of generating primordial anisotropies at the end of
inflation in models of inflation with gauge fields. To be specific we consider
the charged hybrid inflation model where the waterfall field is charged under a
U(1) gauge field so the surface of end of inflation is controlled both by
inflaton and the gauge fields. Using delta N formalism properly we find that
the anisotropies generated at the end of inflation from the gauge field
fluctuations are exponentially suppressed on cosmological scales. This is
because the gauge field evolves exponentially during inflation while in order
to generate appreciable anisotropies at the end of inflation the spectator
gauge field has to be frozen and scale invariant. We argue that this is a
generic feature, that is, one can not generate observable anisotropies at the
end of inflation within an FRW background.Comment: V3: new references added, JCAP published versio
Parametric amplification of metric fluctuations through a bouncing phase
We clarify the properties of the behavior of classical cosmological
perturbations when the Universe experiences a bounce. This is done in the
simplest possible case for which gravity is described by general relativity and
the matter content has a single component, namely a scalar field in a closed
geometry. We show in particular that the spectrum of scalar perturbations can
be affected by the bounce in a way that may depend on the wave number, even in
the large scale limit. This may have important implications for string
motivated models of the early Universe.Comment: 17 pages, 12 figures, LaTeX-ReVTeX format, version to match Phys.
Rev.
Global Incidence and mortality of oesophageal cancer and their correlation with socioeconomic indicators temporal patterns and trends in 41 countries
Oesophageal cancers (adenocarcinomas [AC] and squamous cell carcinomas [SCC]) are characterized by high incidence/mortality in many countries. We aimed to delineate its global incidence and mortality, and studied whether socioeconomic development and its incidence rate were correlated. The age-standardized rates (ASRs) of incidence and mortality of this medical condition in 2012 for 184 nations from the GLOBOCAN database; national databases capturing incidence rates, and the WHO mortality database were examined. Their correlations with two indicators of socioeconomic development were evaluated. Joinpoint regression analysis was used to generate trends. The ratio between the ASR of AC and SCC was strongly correlated with HDI (r = 0.535 [men]; r = 0.661 [women]) and GDP (r = 0.594 [men]; r = 0.550 [women], both p < 0.001). Countries that reported the largest reduction in incidence in male included Poland (Average Annual Percent Change [AAPC] = −7.1, 95%C.I. = −12,−1.9) and Singapore (AAPC = −5.8, 95%C.I. = −9.5,−1.9), whereas for women the greatest decline was seen in Singapore (AAPC = −12.3, 95%C.I. = −17.3,−6.9) and China (AAPC = −5.6, 95%C.I. = −7.6,−3.4). The Philippines (AAPC = 4.3, 95%C.I. = 2,6.6) and Bulgaria (AAPC = 2.8, 95%C.I. = 0.5,5.1) had a significant mortality increase in men; whilst Columbia (AAPC = −6.1, 95%C.I. = −7.5,−4.6) and Slovenia (AAPC = −4.6, 95%C.I. = −7.9,−1.3) reported mortality decline in women. These findings inform individuals at increased risk for primary prevention
- …