arXiv subjects

T. Rostami

Publications and source records attributed to T. Rostami.

5 recordsLinked to original sources

arXiv2015

Why the measured cosmological constant is small

In a quest to explain the small value of the today's cosmological constant, following the approach introduced in [1], we show that the theoretical value of cosmological constant is consistent with its observational value. In more detail, we study the Freidmann-Lama\^ıtre-Robertson-Walker cosmology embedded isometrically in an $11$-dimensional ambient space. The field equations determines $Λ$ in terms of other measurable fundamental constants. Specifically, it predicts that the cosmological constant measured today be $ΛL^2_{\text{Pl}}=2.56\times10^{-122}$, as observed.

gr-qc↗

arXiv2015

Quantum cosmological intertwining: Factor ordering and boundary conditions from hidden symmetries

We explore the implications of hidden symmetries present in a particular quantum cosmological setting, extending the results reported in \cite{10,11}. In more detail, our case study is constituted by a spatially closed Friedmann-Lema\^ıtre-Robertson-Walker universe, in the presence of a conformally coupled scalar field. The $su(1,1)$ hidden symmetries of this model, together with the Hamiltonian constraint, lead to the gauge invariance of its corresponding Bargmann indices. We subsequently show that some factor-ordering choices can be related to the allowed spectrum of Bargmann indices and hence, to the hidden symmetries. Moreover, the presence of those hidden symmetries also implies a set of appropriate boundary conditions to choose from. In summary, our results suggest that factor ordering and boundary conditions can be intertwined when a quantum cosmological model has hidden symmetries.

gr-qc↗

arXiv2015

Covariant extrinsic gravity and the geometric origin of dark energy

We construct the covariant or model independent induced Einstein-Yang-Mills field equations on a 4-dimensional brane embedded isometrically in an D-dimensional bulk space, assuming the matter fields are confined to the brane. Applying this formalism to cosmology, we derive the generalized Friedmann equations. We derive the density parameter of dark energy in terms of width of the brane, normal curvature radii and the number of extra large dimensions. We show that dark energy could actually be the manifestation of the local extrinsic shape of the brane. It is shown that the predictions of this model are in good agreement with observation if we consider an 11-dimensional bulk space.

gr-qc↗

arXiv2014

Five dimensional cosmological traversable wormhole

In this paper, a traversable wormhole in the Friedmann-Lema\^ıtre-Robertson-Walker (FLRW) model with one extra spacelike compact dimension is studied. We have chosen dynamical compactification as the evolution of the fifth dimension. In this respect, we study how the existence of the extra dimension, affect the behavior of the energy density, the shape function and the scale factor. It is shown that the total matter can be non-exotic and the violation of the weak energy condition can be avoided.

gr-qc↗

arXiv2014

On the relation between boundary proposals and hidden symmetries of the extended pre-big bang quantum cosmology

A framework associating quantum cosmological boundary conditions to minisuperspace hidden symmetries has been introduced in \cite{7}. The scope of the application was, notwithstanding the novelty, restrictive because it lacked a discussion involving realistic matter fields. Therefore, in the herein letter, we extend the framework scope to encompass elements from a scalar-tensor theory in the presence of a cosmological constant. More precisely, it is shown that hidden minisuperspace symmetries present in a pre-big bang model suggest a process from which boundary conditions can be selected.

gr-qc↗