8,086 research outputs found
Dark Energy and the Return of the Phoenix Universe
In cyclic universe models based on a single scalar field (e.g., the radion
determining the distance between branes in M-theory), virtually the entire
universe makes it through the ekpyrotic smoothing and flattening phase,
bounces, and enters a new epoch of expansion and cooling. This stable evolution
cannot occur, however, if scale-invariant curvature perturbations are produced
by the entropic mechanism because it requires two scalar fields (e.g., the
radion and the Calabi-Yau dilaton) evolving along an unstable classical
trajectory. In fact, we show here that an overwhelming fraction of the universe
fails to make it through the ekpyrotic phase; nevertheless, a sufficient volume
survives and cycling continues forever provided the dark energy phase of the
cycle lasts long enough, of order a trillion years. Two consequences are a new
role for dark energy and a global structure of the universe radically different
from that of eternal inflation.Comment: 5 pages, 3 figure
The spherical symmetry Black hole collapse in expanding universe
The spherical symmetry Black holes are considered in expanding background.
The singularity line and the marginally trapped tube surface behavior are
discussed. In particular, we address the conditions whether dynamical horizon
forms for these cosmological black holes. We also discuss about the
cosmological constant effect on these black hole and the redshift of the light
which comes from the marginally trapped tube surface.Comment: 7 pages, 3 figures. Accepted for publication in International Journal
of Modern Physics D (IJMPD). arXiv admin note: text overlap with
arXiv:gr-qc/0308033 and arXiv:gr-qc/030611
Dark Matter Prediction from Canonical Quantum Gravity with Frame Fixing
We show how, in canonical quantum cosmology, the frame fixing induces a new
energy density contribution having features compatible with the (actual) cold
dark matter component of the Universe. First we quantize the closed
Friedmann-Robertson-Walker (FRW) model in a sinchronous reference and determine
the spectrum of the super-Hamiltonian in the presence of ultra-relativistic
matter and a perfect gas contribution. Then we include in this model small
inhomogeneous (spherical) perturbations in the spirit of the Lemaitre-Tolman
cosmology. The main issue of our analysis consists in outlining that, in the
classical limit, the non-zero eigenvalue of the super-Hamiltonian can make
account for the present value of the dark matter critical parameter.
Furthermore we obtain a direct correlation between the inhomogeneities in our
dark matter candidate and those one appearing in the ultra-relativistic matter.Comment: 5 pages, to appear on Modern Physics Letters
Statistical mechanics of damage phenomena
This paper applies the formalism of classical, Gibbs-Boltzmann statistical
mechanics to the phenomenon of non-thermal damage. As an example, a non-thermal
fiber-bundle model with the global uniform (meanfield) load sharing is
considered. Stochastic topological behavior in the system is described in terms
of an effective temperature parameter thermalizing the system. An equation of
state and a topological analog of the energy-balance equation are obtained. The
formalism of the free energy potential is developed, and the nature of the
first order phase transition and spinodal is demonstrated.Comment: Critical point appeared to be a spinodal poin
Big Bang Nucleosynthesis Constraints on the Self-Gravity of Pressure
Using big bang nucleosynthesis and present, high-precision measurements of
light element abundances, we constrain the self-gravity of radiation pressure
in the early universe. The self-gravity of pressure is strictly non-Newtonian,
and thus the constraints we set provide a direct test of this prediction of
general relativity and of the standard, Robertson-Walker-Friedmann cosmology.Comment: 5 pages, 1 figure. This paper was developed from an earlier version
which was posted as arXiv:0707.358
Interpretations of the Accelerating Universe
It is generally argued that the present cosmological observations support the
accelerating models of the universe, as driven by the cosmological constant or
`dark energy'. We argue here that an alternative model of the universe is
possible which explains the current observations of the universe. We
demonstrate this with a reinterpretation of the magnitude-redshift relation for
Type Ia supernovae, since this was the test that gave a spurt to the current
trend in favour of the cosmological constant.Comment: 12 pages including 2 figures, minor revision, references added, a
paragraph on the interpretation of the CMB anisotropy in the QSSC added in
conclusion, general results unchanged. To appear in the October 2002 issue of
the "Publications of the Astronmical Society of the Pacific
Inhomogeneous Dust Collapse in 5D Einstein-Gauss-Bonnet Gravity
We consider a Lemaitre - Tolman - Bondi type space-time in Einstein gravity
with the Gauss-Bonnet combination of quadratic curvature terms, and present
exact solution in closed form. It turns out that the presence of the coupling
constant of the Gauss-Bonnet terms alpha > 0 completely changes the causal
structure of the singularities from the analogous general relativistic case.
The gravitational collapse of inhomogeneous dust in the five-dimensional
Gauss-Bonnet extended Einstein equations leads to formation of a massive, but
weak, timelike singularity which is forbidden in general relativity.
Interestingly, this is a counterexample to three conjecture viz. cosmic
censorship conjecture, hoop conjecture and Seifert's conjecture.Comment: 8 Latex Pages, 2 EPS figure
Longitudinal spin transport in diluted magnetic semiconductor superlattices: the effect of the giant Zeeman splitting
Longitudinal spin transport in diluted magnetic semiconductor superlattices
is investigated theoretically. The longitudinal magnetoconductivity (MC) in
such systems exhibits an oscillating behavior as function of an external
magnetic field. In the weak magnetic field region the giant Zeeman splitting
plays a dominant role which leads to a large negative magnetoconductivity. In
the strong magnetic field region the MC exhibits deep dips with increasing
magnetic field. The oscillating behavior is attributed to the interplay between
the discrete Landau levels and the Fermi surface. The decrease of the MC at low
magnetic field is caused by the exchange interaction between the electron
in the conduction band and the magnetic ions.Comment: 6 pages, 9 figures, submitted to Phys. Rev.
Applicability and non-applicability of equilibrium statistical mechanics to non-thermal damage phenomena: II. Spinodal behavior
This paper investigates the spinodal behavior of non-thermal damage
phenomena. As an example, a non-thermal fiber-bundle model with the global
uniform (meanfield) load sharing is considered. In the vicinity of the spinodal
point the power-law scaling behavior is found. For the meanfield fiber-bundle
model the spinodal exponents are found to have typical meanfield values.Comment: Version related: More careful explanation for the critical
slowing-down. General: The topological properties of non-thermal damage are
described by the formalism of statistical mechanics. This is the continuation
of arXiv:0805.0346. Comments, especially negative, are very welcom
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