26,664 research outputs found
Revisiting the confrontation of the energy conditions with supernovae data
In the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) approach to model
the Universe the violation of the so-called energy conditions is related to
some important properties of the Universe as, for example, the current and the
inflationary accelerating expansion phases. The energy conditions are also
necessary in the formulation and proofs of Hawking-Penrose singularity
theorems. In two recent articles we have derived bounds from energy conditions
and made confrontations of these bounds with supernovae data. Here, we extend
these results in following way: first, by using our most recent statistical
procedure for calculating new q(z) estimates from the \emph{gold} and
\emph{combined} type Ia supernovae samples; second, we use these estimates to
obtain a new picture of the energy conditions fulfillment and violation for the
recent past () in the context of the standard cosmology.Comment: 5 pages. To appear in Int. J. Mod. Phys. D. Talk presented at the 3rd
International Workshop on Astronomy and Relativistic Astrophysics. V2: typos
correcte
Critical Behavior of a Three-State Potts Model on a Voronoi Lattice
We use the single-histogram technique to study the critical behavior of the
three-state Potts model on a (random) Voronoi-Delaunay lattice with size
ranging from 250 to 8000 sites. We consider the effect of an exponential decay
of the interactions with the distance,, with , and
observe that this system seems to have critical exponents and
which are different from the respective exponents of the three-state Potts
model on a regular square lattice. However, the ratio remains
essentially the same. We find numerical evidences (although not conclusive, due
to the small range of system size) that the specific heat on this random system
behaves as a power-law for and as a logarithmic divergence for
and Comment: 3 pages, 5 figure
Energy conditions bounds and their confrontation with supernovae data
The energy conditions play an important role in the understanding of several
properties of the Universe, including the current accelerating expansion phase
and the possible existence of the so-called phantom fields. We show that the
integrated bounds provided by the energy conditions on cosmological observables
such as the distance modulus and the lookback time are not
sufficient (nor necessary) to ensure the local fulfillment of the energy
conditions, making explicit the limitation of these bounds in the confrontation
with observational data. We recast the energy conditions as bounds on the
deceleration and normalized Hubble parameters, obtaining new bounds which are
necessary and sufficient for the local fulfillment of the energy conditions. A
statistical confrontation, with confidence levels, between
our bounds and supernovae data from the gold and combined samples is made for
the recent past. Our analyses indicate, with confidence levels, the
fulfillment of both the weak energy condition (WEC) and dominant energy
condition (DEC) for and , respectively. In addition,
they suggest a possible recent violation of the null energy condition (NEC)
with , i.e. a very recent phase of super-acceleration. Our analyses
also show the possibility of violation of the strong energy condition
(\textbf{SEC}) with in the recent past (), but
interestingly the -best-fit curve crosses the SEC-fulfillment divider at
, which is a value very close to the beginning of the epoch of
cosmic acceleration predicted by the standard concordance flat CDM
scenario.Comment: 7 pages, 3 figures. V2: Version to appear in Phys.Rev.D, analyses
extended to 1sigma, 2sigma and 3sigma confidence levels, references added,
minors change
Clustering, Angular Size and Dark Energy
The influence of dark matter inhomogeneities on the angular size-redshift
test is investigated for a large class of flat cosmological models driven by
dark energy plus a cold dark matter component (XCDM model). The results are
presented in two steps. First, the mass inhomogeneities are modeled by a
generalized Zeldovich-Kantowski-Dyer-Roeder (ZKDR) distance which is
characterized by a smoothness parameter and a power index ,
and, second, we provide a statistical analysis to angular size data for a large
sample of milliarcsecond compact radio sources. As a general result, we have
found that the parameter is totally unconstrained by this sample of
angular diameter data.Comment: 9 pages, 7 figures, accepted in Physical Review
Motion of falling object
A simple setup was assembled to study the motion of an object while it falls.
The setup was used to determine the instantaneous velocity, terminal velocity
and acceleration due to gravity. Also, since the whole project was done within
$20 it can easily be popularized.Comment: 11 pages, 4 figur
Black Hole Formation with an Interacting Vacuum Energy Density
We discuss the gravitational collapse of a spherically symmetric massive core
of a star in which the fluid component is interacting with a growing vacuum
energy density. The influence of the variable vacuum in the collapsing core is
quantified by a phenomenological \beta-parameter as predicted by dimensional
arguments and the renormalization group approach. For all reasonable values of
this free parameter, we find that the vacuum energy density increases the
collapsing time but it cannot prevent the formation of a singular point.
However, the nature of the singularity depends on the values of \beta. In the
radiation case, a trapped surface is formed for \beta<1/2 whereas for
\beta>1/2, a naked singularity is developed. In general, the critical value is
\beta=1-2/3(1+\omega), where the \omega-parameter describes the equation of
state of the fluid component.Comment: 9 pages, 8 figure
An accurate formula for the period of a simple pendulum oscillating beyond the small-angle regime
A simple approximation formula is derived here for the dependence of the
period of a simple pendulum on amplitude that only requires a pocket calculator
and furnishes an error of less than 0.25% with respect to the exact period. It
is shown that this formula describes the increase of the pendulum period with
amplitude better than other simple formulas found in literature. A good
agreement with experimental data for a low air-resistance pendulum is also
verified and it suggests, together with the current availability/precision of
timers and detectors, that the proposed formula is useful for extending the
pendulum experiment beyond the usual small-angle oscillations.Comment: 15 pages and 4 figures. to appear in American Journal of Physic
Thermodynamics of Decaying Vacuum Cosmologies
The thermodynamic behavior of vacuum decaying cosmologies is investigated
within a manifestly covariant formulation. Such a process corresponds to a
continuous irreversible energy flow from the vacuum component to the created
matter constituents. It is shown that if the specific entropy per particle
remains constant during the process, the equilibrium relations are preserved.
In particular, if the vacuum decays into photons, the energy density and
average number density of photons scale with the temperature as and . The temperature law is determined and a generalized
Planckian type form of the spectrum, which is preserved in the course of the
evolution, is also proposed. Some consequences of these results for decaying
vacuum FRW type cosmologies as well as for models with ``adiabatic'' photon
creation are discussed.Comment: 21 pages, uses LATE
New Cosmic Accelerating Scenario without Dark Energy
We propose an alternative, nonsingular, cosmic scenario based on
gravitationally induced particle production. The model is an attempt to evade
the coincidence and cosmological constant problems of the standard model
(CDM) and also to connect the early and late time accelerating stages
of the Universe. Our space-time emerges from a pure initial de Sitter stage
thereby providing a natural solution to the horizon problem. Subsequently, due
to an instability provoked by the production of massless particles, the
Universe evolves smoothly to the standard radiation dominated era thereby
ending the production of radiation as required by the conformal invariance.
Next, the radiation becomes sub-dominant with the Universe entering in the cold
dark matter dominated era. Finally, the negative pressure associated with the
creation of cold dark matter (CCDM model) particles accelerates the expansion
and drives the Universe to a final de Sitter stage. The late time cosmic
expansion history of the CCDM model is exactly like in the standard
CDM model, however, there is no dark energy. This complete scenario is
fully determined by two extreme energy densities, or equivalently, the
associated de Sitter Hubble scales connected by , a result that has no correlation with the cosmological constant
problem. We also study the linear growth of matter perturbations at the final
accelerating stage. It is found that the CCDM growth index can be written as a
function of the growth index, . In this
framework, we also compare the observed growth rate of clustering with that
predicted by the current CCDM model. Performing a statistical test
we show that the CCDM model provides growth rates that match sufficiently well
with the observed growth rate of structure.Comment: 12 pages, 3 figures, accepted for publication by Phys. Rev. D. (final
version, some references have corrected). arXiv admin note: substantial text
overlap with arXiv:1106.193
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