654 research outputs found

### The matter Lagrangian and the energy-momentum tensor in modified gravity with non-minimal coupling between matter and geometry

We show that in modified $f(R)$ type gravity models with non-minimal coupling
between matter and geometry, both the matter Lagrangian, and the
energy-momentum tensor, are completely and uniquely determined by the form of
the coupling. This result is obtained by using the variational formulation for
the derivation of the equations of motion in the modified gravity models with
geometry-matter coupling, and the Newtonian limit for a fluid obeying a
barotropic equation of state. The corresponding energy-momentum tensor of the
matter in modified gravity models with non-minimal coupling is more general
than the usual general-relativistic energy-momentum tensor for perfect fluids,
and it contains a supplementary, equation of state dependent term, which could
be related to the elastic stresses in the body, or to other forms of internal
energy. Therefore, the extra-force induced by the coupling between matter and
geometry never vanishes as a consequence of the thermodynamic properties of the
system, or for a specific choice of the matter Lagrangian, and it is non-zero
in the case of a fluid of dust particles.Comment: 6 pages, accepted for publication in PRD; references adde

### On Einstein clusters as galactic dark matter halos

We consider global and gravitational lensing properties of the recently
suggested Einstein clusters of WIMPs as galactic dark matter halos. Being
tangential pressure dominated, Einstein clusters are strongly anisotropic
systems which can describe any galactic rotation curve by specifying the
anisotropy. Due to this property, Einstein clusters may be considered as dark
matter candidates. We analyse the stability of the Einstein clusters against
both radial and non-radial pulsations, and we show that the Einstein clusters
are dynamically stable. With the use of the Buchdahl type inequalities for
anisotropic bodies, we derive upper limits on the velocity of the particles
defining the cluster. These limits are consistent with those obtained from
stability considerations. The study of light deflection shows that the
gravitational lensing effect is slightly smaller for the Einstein clusters, as
compared to the singular isothermal density sphere model for dark matter.
Therefore lensing observations may discriminate, at least in principle, between
Einstein cluster and other dark matter models.Comment: MNRAS LaTeX, 7 pages, accepted by MNRAS; reference adde

### Constraints on extra-dimensions and variable constants from cosmological gamma ray bursts

The observation of the time delay between the soft emission and the
high-energy radiation from cosmological gamma ray bursts can be used as an
important observational test of multi-dimensional physical theories. The main
source of the time delay is the variation of the electromagnetic coupling, due
to dimensional reduction, which induces an energy dependence of the speed of
light. For photons with energies around 1 TeV, the time delay could range from
a few seconds in the case of Kaluza-Klein models to a few days for models with
large extra-dimensions. Based on these results we suggest that the detection of
the 18-GeV photon $\sim$4500 s after the keV/MeV burst in GRB 940217 provides
a strong evidence for the existence of extra-dimensions. The time delay of
photons, if observed by the next generation of high energy detectors, like, for
example, the SWIFT and GLAST satellite based detectors, or the VERITAS
ground-based TeV gamma-ray instrument, could differentiate between the
different models with extra-dimensions.Comment: 8 pages, 4 figures, contribution to the proceedings of the II
Workshop on Unidentified Gamma-Ray Sources, Hong Kong, June 1-4, 200

### Exact Dissipative Cosmologies with Stiff Fluid

The general solution of the gravitational field equations in the flat
Friedmann-Robertson-Walker geometry is obtained in the framework of the full
Israel-Stewart-Hiscock theory for a bulk viscous stiff cosmological fluid, with
bulk viscosity coefficient proportional to the energy density.Comment: 7 pages, 6 figure

### Isotropic stars in general relativity

We present a general solution of the Einstein gravitational field equations
for the static spherically symmetric gravitational interior spacetime of an
isotropic fluid sphere. The solution is obtained by transforming the pressure
isotropy condition, a second order ordinary differential equation, into a
Riccati type first order differential equation, and using a general
integrability condition for the Riccati equation. This allows us to obtain an
exact non-singular solution of the interior field equations for a fluid sphere,
expressed in the form of infinite power series. The physical features of the
solution are studied in detail numerically by cutting the infinite series
expansions, and restricting our numerical analysis by taking into account only
$n=21$ terms in the power series representations of the relevant astrophysical
parameters. In the present model all physical quantities (density, pressure,
speed of sound etc.) are finite at the center of the sphere. The physical
behavior of the solution essentially depends on the equation of state of the
dense matter at the center of the star. The stability properties of the model
are also analyzed in detail for a number of central equations of state, and it
is shown that it is stable with respect to the radial adiabatic perturbations.
The astrophysical analysis indicates that this solution can be used as a
realistic model for static general relativistic high density objects, like
neutron stars.Comment: 12 pages, 10 figures, accepted for publication in EPJC; references
adde

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