124 research outputs found

### Reconstruction from scalar-tensor theory and the inhomogeneous equation of state in f(T) Gravity

General relativity (GR) characterizes gravity as a geometric properly
exhibited as curvature on spacetime. Teleprallelism describes gravity through
torsional properties, and can reproduce GR at the level of equations. Similar
to f(R) gravity, on taking a generalization, f(T) gravity can produce various
modifications its gravitational mechanism. The resulting field equations are
inherently distinct to f(R) gravity in that they are second order. In the
present work, f(T) gravity is examined in the cosmological context with a
number of solutions reconstructed by means of an auxiliary scalar field. To do
this, various forms of the Hubble parameter are considered with an f(T)
lagrangian emerging for each instance. In addition, the inhomogeneous equation
of state (EoS) is investigated with a particular Hubble parameter model used to
show how this can be used to reconstruct the f(T) lagrangian. Observationally,
both the auxiliary scalar field or exotic terms in the FRW field equations give
the same results, meaning that the variation in the Hubble parameter may be
interpreted as the need to reformulate gravity in some way as is done in f(T)
gravity.Comment: 9 page

### A Perturbative Approach to Neutron Stars in $f(T, \mathcal{T})-$Gravity

We derive a Tolman-Oppenheimer-Volkoff equation in neutron star systems
within the modified $f(T, \mathcal{T})$-gravity class of models using a
perturbative approach. In our approach $f(T, \mathcal{T})$-gravity is
considered to be a static spherically symmetric space-time. In this instance
the metric is built from a more fundamental tetrad vierbein which can be used
to relate inertial and global coordinates. A linear function $f = T(r) +
\mathcal{T}(r) + \chi h(T, \mathcal{T}) + \mathcal{O}(\chi^{2})$ is taken as
the Lagrangian density for the gravitational action. Finally we impose the
polytropic equation of state of neutron star upon the derived equations in
order to derive the mass profile and mass-central density relations of the
neutron star in $f(T, \mathcal{T})$-gravity.Comment: arXiv admin note: text overlap with arXiv:1701.0476

### Quark Stars in $f(T, \mathcal{T})-$Gravity

We derive a working model for the Tolman-Oppenheimer-Volkoff equation for
quark star systems within the modified $f(T, \mathcal{T})$-gravity class of
models. We consider $f(T, \mathcal{T})$-gravity for a static spherically
symmetric space-time. In this instance the metric is built from a more
fundamental tetrad vierbein from which the metric tensor can be derived. We
impose a linear $f(T)$ parameter parameter, namely taking $f=\alpha T(r) +
\beta \mathcal{T}(r) + \varphi$ and investigate the behavior of a linear
energy-momentum tensor trace, $\mathcal{T}$. We also outline the restrictions
which modified $f(T, \mathcal{T})$-gravity imposes upon the coupling
parameters. Finally we incorporate the MIT bag model in order to derive the
mass-radius and mass-central density relations of the quark star within $f(T,
\mathcal{T})$-gravity

### The EPR correlation in Kerr-Newman spacetime

The EPR correlation has become an integral part of quantum communications as
has general relativity in classical communication theory, however when combined
an apparent deterioration is observed for spin states. We consider appropriate
changes in directions of measurement to exploit full EPR entanglement for a
pair of particles and show that it can be deduced only up to the outer even
horizon of a Kerr-Newman black hole, even in the case of freely falling
observer.Comment: 8 pages, 3 figure

### Some aspects of reconstruction using a scalar field in f (T ) Gravity

General relativity characterizes gravity as a geometric property exhibited on
spacetime by massive objects while teleparallel gravity achieves the same
results, at the level of equations, by taking a torsional perspective of
gravity. Similar to f (R) theory, teleparallel gravity can also be generalized
to f (T ), with the resulting field equations being inherently distinct from f
(R) gravity in that they are second order, while in the former case they turn
out to be fourth order. In the present case, a minimally coupled scalar field
is investigated in the f (T ) gravity context for several forms of the scalar
field potential. A number of new f (T ) solutions are found for these
potentials, with their respective state parameters also being examined.Comment: 22 pages, 19 figures, to appear in EPJ

### Generalized Tachyonic Teleparallel cosmology

In this paper we propose a new dark energy model in the teleparallel
alternative of general relativity, by considering a generalized non--minimal
coupling of a tachyonic scalar field with the teleparallel boundary term.
Within the framework of teleparallel gravity, the boundary coupling term is
associated with the divergence of the torsion vector. Considering the linear
stability technique for various potentials and couplings, we have analyzed the
dynamical properties of the present tachyonic dark energy model in the phase
space, uncovering the corresponding essential dynamical features. Our study of
the phase space structure revealed that for a specific class of potential
energy, this model exhibits various critical points which are related to
different cosmological behaviors, such as accelerated expansion and scaling
solutions, determining the existence conditions and the corresponding physical
features.Comment: Matches published version in EPJ

### Can Horndeski Theory be recast using Teleparallel Gravity?

Horndeski gravity is the most general scalar tensor theory, with a single
scalar field, leading to second order field equations and after the GW170817 it
has been severely constrained. In this paper, we study the analogue of
Horndeski's theory in the teleparallel gravity framework were gravity is
mediated through torsion instead of curvature. We show that, even though, many
terms are the same as in the curvature case, we have much richer phenomenology
in the teleparallel setting because of the nature of the torsion tensor.
Moreover, Teleparallel Horndenski contains the standard Horndenski gravity as a
subcase and also contains many modified Teleparallel theories considered in the
past, such as $f(T)$ gravity or Teleparallel Dark energy. Thus, due to the
appearing of a new term in the Lagrangian, this theory can explain dark energy
without a cosmological constant, may describe a crossing of the phantom
barrier, explain inflation and also solve the tension for $H_0$, making it a
good candidate for a correct modified theory of gravity.Comment: 18 pages, 1 figur

### Charged Cylindrical Black Holes in Conformal Gravity

Considering cylindrical topology we present the static solution for a charged
black hole in conformal gravity. We show that unlike the general relativistic
case there are two different solutions, both including a factor that when set
to zero recovers the familiar static charged black string solution in
Einstein's theory. This factor gives rise to a linear term in the potential
that also features in the neutral case and may have significant ramifications
for particle trajectories.Comment: 8 page

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