89 research outputs found

### Exact cosmological solutions for MOG

We find some new exact cosmological solutions for the covariant
scalar-tensor-vector gravity theory, the so-called MOdified Gravity (MOG). The
exact solution of the vacuum field equations has been derived. Also, for non
vacuum cases we have found some exact solutions with the aid of the Noether
symmetry approach. More specifically, the symmetry vector and also the Noether
conserved quantity associated to the point-like Lagrangian of the theory have
been found. Also we find the exact form of the generic vector field potential
of this theory by considering the behavior of the relevant point-like
Lagrangian under the infinitesimal generator of the Noether symmetry. Finally,
we discuss the cosmological implications of the solutions.Comment: 8 pages, to appear in European Physical Journal

### Jeans analysis in modified gravity

MOdified Gravity (MOG) is a covariant modification of Einstein's general
relativity. This theory is one of the current alternatives to dark matter
models. We describe dynamics of collisionless self-gravitating systems in the
context of MOG. By studying the weak field approximation of this theory, we
derive the equations governing the dynamics of the self-gravitating systems.
More specifically, we consider the Jeans instability for self-gravitating fluid
and stellar systems, and derive new Jeans mass limit $\tilde{M}_J$ and
wave-number $\tilde{k}_J$. Furthermore, considering the gravitational
instability in star forming regions, we show that MOG has not a significant
difference with general relativity on this astrophysical scale. However, at
larger scales such as intergalactic space MOG may lead to different galaxy and
structure formation processes

### The phase space analysis of modified gravity (MOG)

We investigate the cosmological consequences of a scalar-vector-tensor theory
of gravity known as MOG. In MOG, in addition to metric tensor, there are two
scalar fields $G(x)$ and $\mu(x)$, and one vector field $\phi_{\alpha}(x)$.
Using the phase space analysis, we explore the cosmological consequences of a
model of MOG and find some new interesting features which are absent in
$\Lambda$CDM model. More specifically we study the possibility that if the
extra fields of this theory behave like dark energy to explain the cosmic
speedup. More interestingly, with or without cosmological constant, strongly
phantom crossing happens. Also we find that this theory in its original form
($\Lambda\neq 0$), possesses a true sequence of cosmological epochs. Albeit we
show that, surprisingly, there are two radiation dominated epochs $f_5$ and
$f_6$, two matter dominated phases $f_3$ and $f_4$, and two late time
accelerated eras $f_{12}$ and $f_{7}$. Depending on the initial conditions the
universe will realize only three of these six eras. However, the matter
dominated phases are dramatically different from the standard matter dominated
epoch. In these phases the cosmic scale factor grows as $a(t)\sim t^{0.46}$ and
$t^{0.52}$, respectively, which are slower than the standard case, i.e.
$a(t)\sim t^{2/3}$. Considering these results we discuss the cosmological
viability of MOG.Comment: To appear in EPJ

### Exact cosmological solutions from Hojman conservation quantities

We present a new approach to find exact solutions for cosmological models. By
requiring the existence of a symmetry transformation vector for the equations
of motion of the given cosmological model (without using either Lagrangian or
Hamiltonian), one can find corresponding Hojman conserved quantities. With the
help of these conserved quantities, the analysis of the cosmological model can
be simplified. In the case of quintessence scalar-tensor models, we show that
the Hojman conserved quantities exist for a wide range of V(\phi)-potentials
and allow to find exact solutions for the cosmic scale factor and the scalar
field. Finally, we investigate the general cosmological behavior of solutions
by adopting a phase-space view.Comment: 11 pages, 13 figures, accepted for publication in Phys. Lett.

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