10 research outputs found
Late time cosmic acceleration from vacuum Brans-Dicke theory in 5D
We show that the scalar-vacuum Brans-Dicke equations in 5D are equivalent to
Brans-Dicke theory in 4D with a self interacting potential and an effective
matter field. The cosmological implication, in the context of FRW models, is
that the observed accelerated expansion of the universe comes naturally from
the condition that the scalar field is not a ghost, i.e., . We
find an effective matter-dominated 4D universe which shows accelerated
expansion if . We study the question of whether
accelerated expansion can be made compatible with large values of ,
within the framework of a 5D scalar-vacuum Brans-Dicke theory with variable,
instead of constant, parameter . In this framework, and based on a
general class of solutions of the field equations, we demonstrate that
accelerated expansion is incompatible with large values of .Comment: In V2 the summary section is expanded. To be published in Classical
and Quantum Gravity
Inertial mechanism: dynamical mass as a source of particle creation
A kinetic theory of vacuum particle creation under the action of an inertial
mechanism is constructed within a nonpertrubative dynamical approach. At the
semi-phenomenological level, the inertial mechanism corresponds to quantum
field theory with a time-dependent mass. At the microscopic level, such a
dependence may be caused by different reasons: The non-stationary Higgs
mechanism, the influence of a mean field or condensate, the presence of the
conformal multiplier in the scalar-tensor gravitation theory etc. In what
follows, a kinetic theory in the collisionless approximation is developed for
scalar, spinor and massive vector fields in the framework of the oscillator
representation, which is an effective tool for transition to the quasiparticle
description and for derivation of non-Markovian kinetic equations. Properties
of these equations and relevant observables (particle number and energy
densities, pressure) are studied. The developed theory is applied here to
describe the vacuum matter creation in conformal cosmological models and
discuss the problem of the observed number density of photons in the cosmic
microwave background radiation. As other example, the self-consistent evolution
of scalar fields with non-monotonic self-interaction potentials (the
W-potential and Witten - Di Vecchia - Veneziano model) is considered. In
particular, conditions for appearance of tachyonic modes and a problem of the
relevant definition of a vacuum state are considered.Comment: 51 pages, 18 figures, submitted to PEPAN (JINR, Dubna); v2: added
reference
General Relativity and Weyl Geometry
We show that the general theory of relativity can be formulated in the
language of Weyl geometry. We develop the concept of Weyl frames and point out
that the new mathematical formalism may lead to different pictures of the same
gravitational phenomena. We show that in an arbitrary Weyl frame general
relativity, which takes the form of a scalar-tensor gravitational theory, is
invariant with respect to Weyl tranformations. A kew point in the development
of the formalism is to build an action that is manifestly invariant with
respect to Weyl transformations. When this action is expressed in terms of
Riemannian geometry we find that the theory has some similarities with
Brans-Dicke gravitational theory. In this scenario, the gravitational field is
not described by the metric tensor only, but by a combination of both the
metric and a geometrical scalar field. We illustrate this point by, firstly,
discussing the Newtonian limit in an arbitrary frame, and, secondly, by
examining how distinct geometrical and physical pictures of the same phenomena
may arise in different frames. To give an example, we discuss the gravitational
spectral shift as viewed in a general Weyl frame. We further explore the
analogy of general relativity with scalar-tensor theories and show how a known
Brans-Dicke vacuum solution may appear as a solution of general relativity
theory when reinterpreted in a particular Weyl frame. Finally, we show that the
so-called WIST gravity theories are mathematically equivalent to Brans-Dicke
theory when viewed in a particular frame.Comment: LATEX, 22 page
General classification of charged test particle circular orbits in ReissnerâNordström spacetime
Abstract We investigate charged particlesâ circular motion in the gravitational field of a charged mass distribution described by the ReissnerâNordström spacetime. We introduce a set of independent parameters completely characterizing the different spatial regions in which circular motion is allowed. We provide a most complete classification of circular orbits for different sets of particle and source charge-to-mass ratios. We study both black holes and naked singularities and show that the behavior of charged particles depend drastically on the type of source. Our analysis shows in an alternative manner that the behavior of circular orbits can in principle be used to distinguish between black holes and naked singularities. From this analysis, special limiting values for the dimensionless charge of black hole and naked singularity emerge, namely, Q/M = 1/2, Q / M = 13 / 5 and Q / M = 2 / 3 for the black hole case and Q/M = 1, Q / M = 5 / ( 2 6 ) , Q / M = 3 6 / 7 , and finally Q / M = 9 / 8 for the naked singularity case. Similarly and surprisingly, analogous limits emerge for the orbiting particles charge-to-mass ratio Ï” , for positive charges Ï” = 1 , Ï” = 2 and Ï” = M / Q . These limits play an important role in the study of the coupled electromagnetic and gravitational interactions, and the investigation of the role of the charge in the gravitational collapse of compact objects
Unperturbed inverse kinematics nucleon knockout measurements with a carbon beam
From superconductors to atomic nuclei, strongly-interacting many-body systems
are ubiquitous in nature. Measuring the microscopic structure of such systems
is a formidable challenge, often met by particle knockout scattering
experiments. While such measurements are fundamental for mapping the structure
of atomic nuclei, their interpretation is often challenged by quantum
mechanical initial- and final-state interactions (ISI/FSI) of the incoming and
scattered particles. Here we overcome this fundamental limitation by measuring
the quasi-free scattering of 48 GeV/c 12C ions from hydrogen. The distribution
of single protons is studied by detecting two protons at large angles in
coincidence with an intact 11B nucleus. The 11B detection is shown to select
the transparent part of the reaction and exclude the otherwise large ISI/FSI
that would break the 11B apart. By further detecting residual 10B and 10Be
nuclei, we also identified short-range correlated (SRC) nucleon-nucleon pairs,
and provide direct experimental evidence for the separation of the pair
wave-function from that of the residual many-body nuclear system. All measured
reactions are well described by theoretical calculations that do not contain
ISI/FSI distortions. Our results thus showcase a new ability to study the
short-distance structure of short-lived radioactive atomic nuclei at the
forthcoming FAIR and FRIB facilities. These studies will be pivotal for
developing a ground-breaking microscopic understanding of the structure and
properties of nuclei far from stability and the formation of visible matter in
the universe.Comment: Accepted for publication in Nature Physics. 28 pages, 19 figures, and
1 table including main text, Methods, and Supplementary material