520 research outputs found
Володимир Антонович і Дмитро Яворницький: до історії наукових і особистих взаємин
Невід’ємною складовою інтелектуальної історії є дослідження міжособистісних стосунків науковців. Особливо, якщо кожен із них є знаковою постаттю, уособленням поєднання високого професіоналізму і патріотичної громадянської позиції. Дослідження останнього часу, присвячені В.Б.Антоновичу визначають його безперечно новаторську місію у складанні першої науково доказової національно-демократичної концепції минулого України та розробленні періодизації вітчизняного історичного процесу
Challenges for creating magnetic fields by cosmic defects
We analyse the possibility that topological defects can act as a source of
magnetic fields through the Harrison mechanism in the radiation era. We give a
detailed relativistic derivation of the Harrison mechanism at first order in
cosmological perturbations, and show that it is only efficient for temperatures
above T ~ 0.2 keV. Our main result is that the vector metric perturbations
generated by the defects cannot induce vorticity in the matter fluids at linear
order, thereby excluding the production of currents and magnetic fields. We
show that anisotropic stress in the matter fluids is required to source
vorticity and magnetic fields. Our analysis is relevant for any mechanism
whereby vorticity is meant to be transferred purely by gravitational
interactions, and thus would also apply to dark matter or neutrinos.Comment: 9 pages, 1 figure; minor corrections and additions; accepted for
publication in Physical Review
Semi-analytic galaxy formation in early dark energy cosmologies
We study the impact of early dark energy (EDE) cosmologies on galaxy
properties by coupling high-resolution numerical simulations with semi-analytic
modeling (SAM) of galaxy formation and evolution. EDE models are characterized
by a non-vanishing high-redshift contribution of dark energy, producing an
earlier growth of structures and a modification of large-scale structure
evolution. They can be viewed as typical representatives of non-standard dark
energy models in which only the expansion history is modified, and hence the
impact on galaxy formation is indirect. We show that in EDE cosmologies the
predicted space density of galaxies is enhanced at all scales with respect to
the standard LCDM scenario, and the corresponding cosmic star formation history
and stellar mass density is increased at high-redshift. We compare these
results with a set of theoretical predictions obtained with alternative SAMs
applied to our reference LCDM simulation, yielding a rough measure of the
systematic uncertainty of the models. We find that the modifications in galaxy
properties induced by EDE cosmologies are of the same order of magnitude as
intra-SAM variations for a standard LCDM realization (unless rather extreme EDE
models are considered), suggesting that is difficult to use such predictions
alone to disentangle between different cosmological scenarios. However, when
independent information on the underlying properties of host dark matter haloes
is included, the SAM predictions on galaxy bias may provide important clues on
the expansion history and the equation-of-state evolution.Comment: 7 pages; 4 figures, MNRAS submitte
Can slow roll inflation induce relevant helical magnetic fields?
We study the generation of helical magnetic fields during single field
inflation induced by an axial coupling of the electromagnetic field to the
inflaton. During slow roll inflation, we find that such a coupling always leads
to a blue spectrum with , as long as the theory is treated
perturbatively. The magnetic energy density at the end of inflation is found to
be typically too small to backreact on the background dynamics of the inflaton.
We also show that a short deviation from slow roll does not result in strong
modifications to the shape of the spectrum. We calculate the evolution of the
correlation length and the field amplitude during the inverse cascade and
viscous damping of the helical magnetic field in the radiation era after
inflation. We conclude that except for low scale inflation with very strong
coupling, the magnetic fields generated by such an axial coupling in single
field slow roll inflation with perturbative coupling to the inflaton are too
weak to provide the seeds for the observed fields in galaxies and clusters.Comment: 33 pages 6 figures; v4 to match the accepted version to appear in
JCA
Imperfect Dark Energy from Kinetic Gravity Braiding
We introduce a large class of scalar-tensor models with interactions
containing the second derivatives of the scalar field but not leading to
additional degrees of freedom. These models exhibit peculiar features, such as
an essential mixing of scalar and tensor kinetic terms, which we have named
kinetic braiding. This braiding causes the scalar stress tensor to deviate from
the perfect-fluid form. Cosmology in these models possesses a rich
phenomenology, even in the limit where the scalar is an exact Goldstone boson.
Generically, there are attractor solutions where the scalar monitors the
behaviour of external matter. Because of the kinetic braiding, the position of
the attractor depends both on the form of the Lagrangian and on the external
energy density. The late-time asymptotic of these cosmologies is a de Sitter
state. The scalar can exhibit phantom behaviour and is able to cross the
phantom divide with neither ghosts nor gradient instabilities. These features
provide a new class of models for Dark Energy. As an example, we study in
detail a simple one-parameter model. The possible observational signatures of
this model include a sizeable Early Dark Energy and a specific equation of
state evolving into the final de-Sitter state from a healthy phantom regime.Comment: 41 pages, 7 figures. References and some clarifying language added.
This version was accepted for publication in JCA
Zero-point quantum fluctuations in cosmology
We re-examine the classic problem of the renormalization of zero-point
quantum fluctuations in a Friedmann-Robertson-Walker background. We discuss a
number of issues that arise when regularizing the theory with a momentum-space
cutoff, and show explicitly how introducing non-covariant counter-terms allows
to obtain covariant results for the renormalized vacuum energy-momentum tensor.
We clarify some confusion in the literature concerning the equation of state of
vacuum fluctuations. Further, we point out that the general structure of the
effective action becomes richer if the theory contains a scalar field phi with
mass m smaller than the Hubble parameter H(t). Such an ultra-light particle
cannot be integrated out completely to get the effective action. Apart from the
volume term and the Einstein-Hilbert term, that are reabsorbed into
renormalizations of the cosmological constant and Newton's constant, the
effective action in general also has a term proportional to F(phi)R, for some
function F(phi). As a result, vacuum fluctuations of ultra-light scalar fields
naturally lead to models where the dark energy density has the form
rho_{DE}(t)=rho_X(t)+rho_Z(t), where rho_X is the component that accelerates
the Hubble expansion at late times and rho_Z(t) is an extra contribution
proportional to H^2(t). We perform a detailed comparison of such models with
CMB, SNIa and BAO data.Comment: 23 pages, 9 figures. v3: refs added. To appear in Phys. Rev.
Probe measurements of plasma potential nonuniformity due to edge asymmetry in large-area radio-frequency reactors: the telegraph effect
In large-area radio-frequency (rf) capacitive reactors, the redistribution of rf current to maintain current continuity near asymmetric sidewalls causes a perturbation in rf plasma potential to propagate along the resistive plasma between capacitive sheaths. The damping length of the perturbation can be determined by a telegraph equation. Experiments are described using a surface array of unbiased electrostatic probes in the ground electrode to verify the theoretical model of the telegraph effect in Howling [J. Appl. Phys. 96, 5429 (2004)]. The measured spatial dependence of the plasma potential rf amplitude and circulating nonambipolar current agree well with two-dimensional numerical solutions of the telegraph equation. The rf plasma potential can be made uniform by using symmetric reactor sidewalls
Self-esteem as a complex dynamic system:intrinsic and extrinsic microlevel dynamics
The variability of self-esteem is an important characteristic of self-esteem. However, little is known about the mechanisms that underlie it. The goal of the current study was to empirically explore these underlying mechanisms. It is commonly assumed that state self-esteem (the fleeting experience of the self) is a response to the immediate social context. Drawing from a complex dynamic systems perspective, the self-organizing self-esteem model asserts that this responsivity is not passive or stimulus-response like, but that the impact of the social context on state self-esteem is intimately connected to the intrinsic dynamics of self-esteem. The model suggests that intrinsic dynamics are the result of higher-order self-esteem attractors that can constrain state self-esteem variability. The current study tests this model, and more specifically, the prediction that state self-esteem variability is less influenced by changes in the immediate context if relatively strong, as opposed to weak, self-esteem attractors underlie intrinsic dynamics of self-esteem. To test this, parent-adolescent dyads (N=13, Mage=13.6) were filmed during seminaturalistic discussions. Observable components of adolescent state self-esteem were coded in real time, as well as real-time parental autonomy-support and relatedness. Kohonen’s self-organizing maps were used to derive attractor-like patterns: repeated higher-order patterns of adolescents’ self-esteem components. State space grids were used to assess how much adolescents’ self-esteem attractors constrained their state self-esteem variability. We found varying levels of attractor strength in our sample. In accordance with our prediction, we found that state self-esteem was less sensitive to changes in parental support and relatedness for adolescents with stronger self-esteem attractors. Discussion revolves around the implications of our findings for the ontology of self-esteem
Future of the universe in modified gravitational theories: Approaching to the finite-time future singularity
We investigate the future evolution of the dark energy universe in modified
gravities including gravity, string-inspired scalar-Gauss-Bonnet and
modified Gauss-Bonnet ones, and ideal fluid with the inhomogeneous equation of
state (EoS). Modified Friedmann-Robertson-Walker (FRW) dynamics for all these
theories may be presented in universal form by using the effective ideal fluid
with an inhomogeneous EoS without specifying its explicit form. We construct
several examples of the modified gravity which produces accelerating
cosmologies ending at the finite-time future singularity of all four known
types by applying the reconstruction program. Some scenarios to resolve the
finite-time future singularity are presented. Among these scenarios, the most
natural one is related with additional modification of the gravitational action
in the early universe. In addition, late-time cosmology in the non-minimal
Maxwell-Einstein theory is considered. We investigate the forms of the
non-minimal gravitational coupling which generates the finite-time future
singularities and the general conditions for this coupling in order that the
finite-time future singularities cannot emerge. Furthermore, it is shown that
the non-minimal gravitational coupling can remove the finite-time future
singularities or make the singularity stronger (or weaker) in modified gravity.Comment: 25 pages, no figure, title changed, accepted in JCA
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