1,242 research outputs found
Exotic-singularity-driven dark energy
We discuss various types of exotic (non-standard) singularities in the
Universe: a Big-Rip (BR or type I), a Sudden Future Singularity (SFS or type
II), a Generalized Sudden Future Singularity, a Finite Scale Factor singularity
(FSF or type III), a Big-Separation (BS or type IV) and a -singularity. They
are characterized by violation of all or some of the energy conditions which
results in a blow-up of all or some of the physical quantities: the scale
factor, the energy density, the pressure, and the barotropic index. We relate
the emergence of these singularities with physical theories (superstring,
brane, higher-order gravity, loop quantum cosmology). We show how the models
involving exotic singularities may serve as dark energy by applying the
observational data. In particular, we show that some of these exotic
singularities (though being of a weak type according to relativistic
definitions) may occur in the near future of the universe.Comment: 10 pages, 9 figures, contribution to "Invisible Universe", paris
29.06-3.07.09, to appear in AIP Conference Proceeding
Non-commutative connections of the second kind
A connection-like objects, termed {\em hom-connections} are defined in the
realm of non-commutative geometry. The definition is based on the use of
homomorphisms rather than tensor products. It is shown that hom-connections
arise naturally from (strong) connections in non-commutative principal bundles.
The induction procedure of hom-connections via a map of differential graded
algebras or a differentiable bimodule is described. The curvature for a
hom-connection is defined, and it is shown that flat hom-connections give rise
to a chain complex.Comment: 13 pages, LaTe
A critical assessment of some inhomogeneous pressure Stephani models
We consider spherically symmetric inhomogeneous pressure Stephani universes,
the center of symmetry being our location. The main feature of these models is
that comoving observers do not follow geodesics. In particular, comoving
perfect fluids have necessarily a radially dependent pressure. We consider a
subclass of these models characterized by some inhomogeneity parameter .
We show that also the velocity of sound, like the (effective) equation of state
parameter, of comoving perfect fluids acquire away from the origin a time and
radial dependent change proportional to . In order to produce a
realistic universe accelerating at late times without dark energy component one
must take . The redshift gets a modified dependence on the scale
factor with a relative modification of peaking at and
vanishing at the big-bang and today on our past lightcone. The equation of
state parameter and the speed of sound of dustlike matter (corresponding to a
vanishing pressure at the center of symmetry ) behave in a similar way and
away from the center of symmetry they become negative -- a property usually
encountered for the dark energy component only. In order to mimic the observed
late-time accelerated expansion, the matter component must significantly depart
from standard dust, presumably ruling this subclass of Stephani models out as a
realistic cosmology. The only way to accept these models is to keep all
standard matter components of the universe including dark energy and take an
inhomogeneity parameter small enough.Comment: REVTEX4-1, 12 pages, 6 figures, explanatory material added, version
to appear in PRD, conclusions and results unchange
Sudden Future Singularity models as an alternative to Dark Energy?
Current observational evidence does not yet exclude the possibility that dark
energy could be in the form of phantom energy. A universe consisting of a
phantom constituent will be driven toward a drastic end known as the `Big Rip'
singularity where all the matter in the universe will be destroyed. Motivated
by this possibility, other evolutionary scenarios have been explored by Barrow,
including the phenomena which he called Sudden Future Singularities (SFS). In
such a model it is possible to have a blow up of the pressure occurring at
sometime in the future evolution of the universe while the energy density would
remain unaffected. The particular evolution of the scale factor of the universe
in this model that results in a singular behaviour of the pressure also admits
acceleration in the current era. In this paper we will present the results of
our confrontation of one example class of SFS models with the available
cosmological data from high redshift supernovae, baryon acoustic oscillations
(BAO) and the cosmic microwave background (CMB). We then discuss the viability
of the model in question as an alternative to dark energy.Comment: 9 pages, 7 figures. Accepted for publication in MNRA
Cosmological tests of sudden future singularities
We discuss combined constraints, coming from the cosmic microwave background
shift parameter , baryon acoustic oscillations (BAO) distance
parameter , and from the latest type Ia supernovae data, imposed
on cosmological models which allow sudden future singularities of pressure. We
show that due to their weakness such sudden singularities may happen in the
very near future and that at present they can mimic standard dark energy
models.Comment: 8 pages, 2 references adde
Active shooters — how close are they?
Recent terrorist attacks in the United States, Canada and Western Europe have shown an increase in the incidence of “Active Shooters” [1]. These ruthless and desperate assassins usually attack urban and poorly protected areas (lack of armed protection) that are densely populated [2]. Utilizing their strength, they realize that their plan is to maximize the number of casualties, without counting on the consequences of their actions. The basis of their action may be based on extremely radical views. Frequent outcomes for active shooters include suicide during an attack (90%) or the resolution of the threat by the authorities [3]. In response to the ever-increasing number of assassinations and the risk of such incidents in one’s immediate surroundings, comprehensive education should be widely spread. Thus, it is important to promote appropriate behaviour, rules of reaction during an attack by an armed assailant, as well as cooperation with incoming service personnel. Such actions will not only help one prevent, but also allow one to prepare for such incidences
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