120 research outputs found
Late time cosmological approach in mimetic gravity
In this paper, we investigate the late-time cosmic acceleration in mimetic
gravity with Lagrange multiplier and potential in a Universe
containing, besides radiation and dark energy, a self-interacting (collisional)
matter. We obtain through the modified Friedmann equations, the main equation
that can describe the cosmological evolution and with several models from
and the well known particular model , we perform an analysis of
the late-time evolution. We examine the behavior of the Hubble parameter, the
dark energy equation of state and the total effective equation of state and we
compare in each case the resulting picture with the non-collisional matter
(assumed as dust) and also with the collisional matter in mimetic
gravity. The results obtained are in good agreement with the observational data
and show that in presence of the collisional matter the dark energy
oscillations in mimetic f(R, T) gravity can be damped.Comment: 18 pages, 2 figure
Late time cosmological approach in mimetic gravity
In this paper, we investigate the late-time cosmic acceleration in mimetic
gravity with Lagrange multiplier and potential in a Universe
containing, besides radiation and dark energy, a self-interacting (collisional)
matter. We obtain through the modified Friedmann equations, the main equation
that can describe the cosmological evolution and with several models from
and the well known particular model , we perform an analysis of
the late-time evolution. We examine the behavior of the Hubble parameter, the
dark energy equation of state and the total effective equation of state and we
compare in each case the resulting picture with the non-collisional matter
(assumed as dust) and also with the collisional matter in mimetic
gravity. The results obtained are in good agreement with the observational data
and show that in presence of the collisional matter the dark energy
oscillations in mimetic f(R, T) gravity can be damped.Comment: 18 pages, 2 figure
Rastall Cosmology and the \Lambda CDM Model
Rastall's theory is based on the non-conservation of the energy-momentum
tensor. We show that, in this theory, if we introduce a two-fluid model, one
component representing vacuum energy whereas the other pressureless matter
(e.g. baryons plus cold dark matter), the cosmological scenario is the same as
for the \Lambda CDM model, both at background and linear perturbative levels,
except for one aspect: now dark energy may cluster. We speculate that this can
lead to a possibility of distinguishing the models at the non-linear
perturbative level.Comment: 9 pages, 1 figure. Accepted for publication in Physical Review
The Brans-Dicke-Rastall theory
We formulate a theory combining the principles of a scalar-tensor gravity and
Rastall's proposal of a violation of the usual conservation laws. We obtain a
scalar-tensor theory with two parameters and , the latter
quantifying the violation of the usual conservation laws. The only exact
spherically symmetric solution is that of Robinson-Bertotti besides
Schwarzschild solution. A PPN analysis reveals that General Relativity results
are reproduced when . The cosmological case displays a possibility
of deceleration/acceleration or acceleration/deceleration transitions during
the matter dominated phase depending on the values of the free parameters.Comment: 17 pages, 3 figure
G\"odel-type universes in f(T) gravity
The issue of causality in gravity is investigated by examining the
possibility of existence of the closed timelike curves in the G\"{o}del-type
metric. By assuming a perfect fluid as the matter source, we find that the
fluid must have an equation of state parameter greater than minus one in order
to allow the G\"{o}del solutions to exist, and furthermore the critical radius
, beyond which the causality is broken down, is finite and it depends on
both matter and gravity. Remarkably, for certain models, the perfect
fluid that allows the G\"{o}del-type solutions can even be normal matter, such
as pressureless matter or radiation. However, if the matter source is a special
scalar field rather than a perfect fluid, then and the
causality violation is thus avoided.Comment: 18 pages, introduction revised, reference adde
Birkhoff's Theorem in f(T) Gravity up to the Perturbative Order
f(T) gravity, a generally modified teleparallel gravity, has become very
popular in recent times as it is able to reproduce the unification of inflation
and late-time acceleration without the need of a dark energy component or an
inflation field. In this present work, we investigate specifically the range of
validity of Birkhoff's theorem with the general tetrad field via perturbative
approach. At zero order, Birkhoff's theorem is valid and the solution is the
well known Schwarzschild-(A)dS metric. Then considering the special case of the
diagonal tetrad field, we present a new spherically symmetric solution in the
frame of f(T) gravity up to the perturbative order. The results with the
diagonal tetrad field satisfy the physical equivalence between the Jordan and
the so-called Einstein frames, which are realized via conformal transformation,
at least up to the first perturbative order.Comment: 8 pages, no figure. Final version, accepted for publication in EPJ
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