12,975 research outputs found
Cosmologies with variable parameters and dynamical cosmon: implications on the cosmic coincidence problem
Dynamical dark energy (DE) has been proposed to explain various aspects of
the cosmological constant (CC) problem(s). For example, it is very difficult to
accept that a strictly constant Lambda-term constitutes the ultimate
explanation for the DE in our Universe. It is also hard to acquiesce in the
idea that we accidentally happen to live in an epoch where the CC contributes
an energy density value right in the ballpark of the rapidly diluting matter
density. It should perhaps be more plausible to conceive that the vacuum
energy, is actually a dynamical quantity as the Universe itself. More
generally, we could even entertain the possibility that the total DE is in fact
a mixture of vacuum energy and other dynamical components (e.g. fields, higher
order terms in the effective action etc) which can be represented collectively
by an effective entity X (dubbed the ``cosmon''). The ``cosmon'', therefore,
acts as a dynamical DE component different from the vacuum energy. While it can
actually behave phantom-like by itself, the overall DE fluid may effectively
appear as standard quintessence, or even mimic at present an almost exact CC
behavior. Thanks to the versatility of such cosmic fluid we can show that a
composite DE system of this sort (``LXCDM'') may have a key to resolving the
mysterious coincidence problem.Comment: LaTeX, 13 pages, 5 figure
Dark energy: a quantum fossil from the inflationary Universe?
The discovery of dark energy (DE) as the physical cause for the accelerated
expansion of the Universe is the most remarkable experimental finding of modern
cosmology. However, it leads to insurmountable theoretical difficulties from
the point of view of fundamental physics. Inflation, on the other hand,
constitutes another crucial ingredient, which seems necessary to solve other
cosmological conundrums and provides the primeval quantum seeds for structure
formation. One may wonder if there is any deep relationship between these two
paradigms. In this work, we suggest that the existence of the DE in the present
Universe could be linked to the quantum field theoretical mechanism that may
have triggered primordial inflation in the early Universe. This mechanism,
based on quantum conformal symmetry, induces a logarithmic,
asymptotically-free, running of the gravitational coupling. If this evolution
persists in the present Universe, and if matter is conserved, the general
covariance of Einstein's equations demands the existence of dynamical DE in the
form of a running cosmological term whose variation follows a power law of the
redshift.Comment: LaTeX, 14 pages, extended discussion. References added. Accepted in
J. Phys. A: Mathematical and Theoretica
Phase-conjugate optical coherence tomography
Quantum optical coherence tomography (Q-OCT) offers a factor-of-two
improvement in axial resolution and the advantage of even-order dispersion
cancellation when it is compared to conventional OCT (C-OCT). These features
have been ascribed to the non-classical nature of the biphoton state employed
in the former, as opposed to the classical state used in the latter.
Phase-conjugate OCT (PC-OCT), introduced here, shows that non-classical light
is not necessary to reap Q-OCT's advantages. PC-OCT uses classical-state signal
and reference beams, which have a phase-sensitive cross-correlation, together
with phase conjugation to achieve the axial resolution and even-order
dispersion cancellation of Q-OCT with a signal-to-noise ratio that can be
comparable to that of C-OCT.Comment: 4 pages, 3 figure
Nonlinear dynamics of beta induced Alfv\'en eigenmode driven by energetic particles
Nonlinear saturation of beta induced Alfv\'en eigenmode, driven by slowing
down energetic particles via transit resonance, is investigated by the
nonlinear hybrid magnetohyrodynamic gyro-kinetic code (XHMGC). Saturation is
characterized by frequency chirping and symmetry breaking between co- and
counter-passing particles, which can be understood as the the evidence of
resonance-detuning. The scaling of the saturation amplitude with the growth
rate is also demonstrated to be consistent with radial resonance detuning due
to the radial non-uniformity and mode structure
From Solar to Stellar Brightness Variations: The Effect of Metallicity
Context. Comparison studies of Sun-like stars with the Sun suggest an
anomalously low photometric variability of the Sun compared to Sun-like stars
with similar magnetic activity. Comprehensive understanding of stellar
variability is needed, to find a physical reasoning for this observation. Aims.
We investigate the effect of metallicity and effective temperature on the
photometric brightness change of Sun-like stars seen at different inclinations.
The considered range of fundamental stellar parameters is sufficiently small so
the stars, investigated here, still count as Sun-like or even as solar twins.
Methods. To model the brightness change of stars with solar magnetic activity,
we extend a well established model of solar brightness variations, SATIRE
(which stands for Spectral And Total Irradiance Reconstruction), which is based
on solar spectra, to stars with different fundamental parameters. For that we
calculate stellar spectra for different metallicities and effective temperature
using the radiative transfer code ATLAS9. Results. We show that even a small
change (e.g. within the observational error range) of metallicity or effective
temperature significantly affects the photometric brightness change compared to
the Sun. We find that for Sun-like stars, the amplitude of the brightness
variations obtained for Str\"omgren (b + y)/2 reaches a local minimum for
fundamental stellar parameters close to the solar metallicity and effective
temperature. Moreover, our results show that the effect of inclination
decreases for metallicity values greater than the solar metallicity. Overall,
we find that an exact determination of fundamental stellar parameters is
crucially important for understanding stellar brightness changes.Comment: 12 pages, 12 figures, accepted in A&
Complete transfer of populations from a single state to a pre-selected superposition of states using Piecewise Adiabatic Passage
We develop a method for executing robust and selective transfer of
populations between a single level and pre-selected superpositions of energy
eigenstates. Viewed in the frequency domain, our method amounts to executing a
series of simultaneous adiabatic passages into each component of the target
superposition state. Viewed in {the} time domain, the method works by
accumulating the wavefunction of the target wave packet as it revisits the
Franck Condon region, in what amounts to an extension of the Piecewise
Adiabatic Passage technique [ Shapiro et.al., Phys. Rev. Lett. 99, 033002
(2007)] to the multi-state regime. The viability of the method is verified by
performing numerical tests for the Na_2 molecule.Comment: 8 pages, 4 figure
The role of the Fraunhofer lines in solar brightness variability
The solar brightness varies on timescales from minutes to decades. A clear
identification of the physical processes behind such variations is needed for
developing and improving physics-based models of solar brightness variability
and reconstructing solar brightness in the past. This is, in turn, important
for better understanding the solar-terrestrial and solar-stellar connections.
We estimate the relative contributions of the continuum, molecular, and
atomic lines to the solar brightness variations on different timescales.
Our approach is based on the assumption that variability of the solar
brightness on timescales greater than a day is driven by the evolution of the
solar surface magnetic field. We calculated the solar brightness variations
employing the solar disc area coverage of magnetic features deduced from the
MDI/SOHO observations. The brightness contrasts of magnetic features relative
to the quiet Sun were calculated with a non-LTE radiative transfer code as
functions of disc position and wavelength. By consecutive elimination of
molecular and atomic lines from the radiative transfer calculations, we
assessed the role of these lines in producing solar brightness variability.
We show that the variations in Fraunhofer lines define the amplitude of the
solar brightness variability on timescales greater than a day and even the
phase of the total solar irradiance variability over the 11-year cycle. We also
demonstrate that molecular lines make substantial contribution to solar
brightness variability on the 11-year activity cycle and centennial timescales.
In particular, our model indicates that roughly a quarter of the total solar
irradiance variability over the 11-year cycle originates in molecular lines.
The maximum of the absolute spectral brightness variability on timescales
greater than a day is associated with the CN violet system between 380 and 390
nm.Comment: 9 pages, 4 figures, accepted for publication in
Astronomy&Astrophysic
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