A Non - Singular Cosmological Model with Shear and Rotation

We have investigated a non-static and rotating model of the universe with an imperfect fluid distribution. It is found that the model is free from singularity and represents an ever expanding universe with shear and rotation vanishing for large value of time.Comment: 10 pages, late

Bianchi type II,III and V diagonal Einstein metrics re-visited

We present, for both minkowskian and euclidean signatures, short derivations of the diagonal Einstein metrics for Bianchi type II, III and V. For the first two cases we show the integrability of the geodesic flow while for the third case a somewhat unusual bifurcation phenomenon takes place: for minkowskian signature elliptic functions are essential in the metric while for euclidean signature only elementary functions appear

Supersymmetric Barotropic FRW Model and Dark Energy

Using the superfield approach we construct the $n=2$ supersymmetric lagrangian for the FRW Universe with barotropic perfect fluid as matter field. The obtained supersymmetric algebra allowed us to take the square root of the Wheeler-DeWitt equation and solve the corresponding quantum constraint. This model leads to the relation between the vacuum energy density and the energy density of the dust matter.Comment: 11 pages, minor corrections, published versio

Phytosociological study of Hirschfeldia incana (L.) Lagraze-Fossat (Cruciferae) communities in mainland Greece

Using numerical analysis, the phytosociological study of Hirschfeldia incana communities in mainland Greece allowed their classification into the Rapistro rugosi-Hirschfeldietum incanae ass. nov., a new subnitrophilous association of the Hordeion leporini alliance. Three subassociations were distinguished (anthemidetosum incrassatae, hedypnoidetosum creticae and cardarietosum drabae), the distribution of which seems to depend on latitudinal alteration of rainfall. The new association has its optimum growth in habitats with moderate human influence, specifically in abandoned cultivations and wastelands. With respect to its floristic composition, the Rapistro rugosi-Hirschfeldietum incanae is close to anthropogenic vegetation with a high degree of naturalness, particularly to the therophytic, subnitrophilous vegetation of the Thero-Brometalia (Stellarietea mediae) and the perennial, subnitrophilous vegetation of Carthametalia lanati (Artemisietea vulgaris)

Towards Canonical Quantum Gravity for G1 Geometries in 2+1 Dimensions with a Lambda--Term

The canonical analysis and subsequent quantization of the (2+1)-dimensional action of pure gravity plus a cosmological constant term is considered, under the assumption of the existence of one spacelike Killing vector field. The proper imposition of the quantum analogues of the two linear (momentum) constraints reduces an initial collection of state vectors, consisting of all smooth functionals of the components (and/or their derivatives) of the spatial metric, to particular scalar smooth functionals. The demand that the midi-superspace metric (inferred from the kinetic part of the quadratic (Hamiltonian) constraint) must define on the space of these states an induced metric whose components are given in terms of the same states, which is made possible through an appropriate re-normalization assumption, severely reduces the possible state vectors to three unique (up to general coordinate transformations) smooth scalar functionals. The quantum analogue of the Hamiltonian constraint produces a Wheeler-DeWitt equation based on this reduced manifold of states, which is completely integrated.Comment: Latex 2e source file, 25 pages, no figures, final version (accepted in CQG

Anisotropic Solutions in Symmetric Teleparallel f(Q)f\left( Q\right)-theory: Kantowski-Sachs and Bianchi III LRS Cosmologies

We investigate the existence of anisotropic self-similar exact solutions in symmetric teleparallel $f\left( Q\right)$-theory. For the background geometry we consider the Kantowski-Sachs and the Locally Rotationally Symmetric Bianchi type III geometries. These two anisotropic spacetimes are of special interest because in the limit of isotropy they are related to the closed and open Friedmann--Lema\^{\i}tre--Robertson--Walker cosmologies respectively. For each spacetime there exist two distinct families of flat, symmetric connections, which share the symmetries of the spacetime. We present the field equations, and from them, we determine the functional form of the $f\left( Q\right)$ Lagrangian which yields self-similar solutions. We initially consider the vacuum case and subsequently we introduce a matter source in terms of a perfect fluid. Last but not least, we report some self-similar solutions corresponding to static spherically symmetric spacetimes.Comment: 31 pages, no figures, Latex2e source fil