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Improved dynamics and gravitational collapse of tachyon field coupled with a barotropic fluid
Publication . Marto, João; Tavakoli, Yaser; Moniz, Paulo
We consider a spherically symmetric gravitational collapse of a tachyon field
with an inverse square potential, which is coupled with a barotropic fluid. By
employing an holonomy correction imported from loop quantum cosmology, we
analyse the dynamics of the collapse within a semiclassical description. Using
a dynamical system approach, we find that the stable fixed points given by the
standard general relativistic setting turn into saddle points in the present
context. This provides a new dynamics in contrast to the black hole and naked
singularities solutions appearing in the classical model. Our results suggest
that classical singularities can be avoided by quantum gravity effects and are
replaced by a bounce. By a thorough numerical studies we show that, depending
on the barotropic parameter $\gamma$, there exists a class of solutions
corresponding to either a fluid or a tachyon dominated regimes. Furthermore,
for the case $\gamma \sim 1$, we find an interesting tracking behaviour between
the tachyon and the fluid leading to a dust-like collapse. In addition, we show
that, there exists a threshold scale which determines when an outward energy
flux emerges, as a non-singular black hole is forming, at the corresponding
collapse final stages.
Inflation in a two 3-form fields scenario
Publication . Kumar, K. Sravan; Marto, João; Nunes, Nelson J.; Moniz, Paulo
A setting constituted by N 3-form fields, without any direct
interaction between them, minimally coupled to gravity, is introduced in this
paper as a framework to study the early evolution of the universe. We focus
particularly on the two 3-forms case. An inflationary scenario is found,
emerging from the coupling to gravity. More concretely, the fields coupled in
this manner exhibit a complex interaction, mediated by the time derivative of
the Hubble parameter. Our investigation is supported by means of a suitable
choice of potentials, employing numerical methods and analytical
approximations. In more detail, the oscillations on the small field limit
become correlated, and one field is intertwined with the other. In this type of
solution, a varying sound speed is present, together with the generation of
isocurvature perturbations. The mentioned features allow to consider an
interesting model, to test against observation. It is subsequently shown how
our results are consistent with current CMB data (viz.Planck and BICEP2).
Gravitational Collapse of a Homogeneous Scalar Field in Deformed Phase Space
Publication . Rasouli, Seyed Meraj Mousavi; Ziaie, Amir Hadi; Marto, João; Moniz, Paulo
We study the gravitational collapse of a homogeneous scalar field, minimally
coupled to gravity, in the presence of a particular type of dynamical
deformation between the canonical momenta of the scale factor and of the scalar
field. In the absence of such a deformation, a class of solutions can be found
in the literature [R. Goswami and P. S. Joshi, arXiv:gr-qc/0410144],
%\cite{JG04}, whereby a curvature singularity occurs at the collapse end state,
which can be either hidden behind a horizon or be visible to external
observers. However, when the phase-space is deformed, as implemented herein
this paper, we find that the singularity may be either removed or instead,
attained faster. More precisely, for negative values of the deformation
parameter, we identify the emergence of a negative pressure term, which slows
down the collapse so that the singularity is replaced with a bounce. In this
respect, the formation of a dynamical horizon can be avoided depending on the
suitable choice of the boundary surface of the star. Whereas for positive
values, the pressure that originates from the deformation effects assists the
collapse toward the singularity formation. In this case, since the collapse
speed is unbounded, the condition on the horizon formation is always satisfied
and furthermore the dynamical horizon develops earlier than when the
phase-space deformations are absent. These results are obtained by means of a
thoroughly numerical discussion.
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Funding agency
Fundação para a Ciência e a Tecnologia
Funding programme
3599-PPCDTI
Funding Award Number
CERN/FP/123618/2011