Four-dimensional Einstein gravity in the Palatini first order formalism is shown to possess a vector supersymmetry of the same type as found in the topological theories for Yang-Mills fields. A peculiar feature of the gravitationel theory, characterized by diffeomorphism invariance, is a direct link of vector supersymmetry with the field equation of motion for the Faddeev-Popov ghost of diffeom...

We argue that Einstein gravity coupled to a Born-Infeld theory provides an attractive candidate to represent dark matter and dark energy. For cosmological models, the BornInfeld field has an equation of state which interpolates between matter, p = 0 (small times), and a cosmological constant p = −ρ (large times). On galactic scales, the BornInfeld theory predicts asymptotically flat rotation cu...

By assuming a particular mass function we find new exact solutions to the Einstein field equations with an anisotropic matter distribution. The solutions are shown to be relevant for the description of compact stars. A distinguishing feature of this class of solutions is that they admit a linear equation of state which can be applied to strange stars with quark matter.

We examine the Einstein equation with the energy-momentum tensors which correspond to an infinite line string of finite radius plus an outgoing radiation field. It is used to see the effect of the radiation field on the spacetime of a cosmic string. We make some assumptions about the metric coefficients and find a class of exact solution. The result can be applied to study the back reaction on ...

We derive the ‘exact’ Newtonian limit of general relativity with a positive cosmological constant Λ. We point out that in contrast to the case with Λ ≤ 0, the presence of a positive Λ in Einsteins’s equations enforces, via the condition |Φ| 1, on the potential Φ, a range Rmax(Λ) r Rmin(Λ), within which the Newtonian limit is valid. It also leads to the existence of a maximum mass, Mmax(Λ). As a...

The vacuum solution ds 2 = dx 2 + x 2 dy 2 + 2 dz dt + ln x dt 2 of the Einstein gravitational field equation follows from the general ansatz ds 2 = dx 2 + g αβ (x) dx α dx β but fails to follow from it if the symmetric matrix g αβ (x) is assumed to be in diagonal form.

Consider a Riemannian spin manifold of dimension n ≥ 3 and denote by D the Dirac operator acting on spinor fields. A solution of the Einstein-Dirac equation is a spinor field ψ solving the equations Ric − 1 2 S · g = ± 1 4 T ψ , D(ψ) = λψ. Here S denotes the scalar curvature of the space, λ is a real constant and T ψ is the energy-momentum tensor of the spinor field ψ defined by the formula

Assuming that the mechanism proposed by Gell-Mann and Hartle works as a mechanism for decoherence and classicalization of the metric field, we formally derive the form of an effective theory for the gravitational field in a semiclassical regime. This effective theory takes the form of the usual semiclassical theory of gravity, based on the semiclassical Einstein equation, plus a stochastic corr...

Assuming that the mechanism proposed by Gell-Mann and Hartle works as a mechanism for decoherence and classicalization of the metric field, we formally derive the form of an effective theory for the gravitational field in a semiclassical regime. This effective theory takes the form of the usual semiclassical theory of gravity, based on the semiclassical Einstein equation, plus a stochastic corr...

A four-dimensional static black hole solution of Einstein equation conformally coupled to a massive and self interacting scalar field is obtained. A nontrivial scalar solution proposes a weak scalar hair. A dressed black hole shows a trace of scalar charge in the metric signalling the presence of scalar hair. A number of metrics with regular horizons and temperatures are also proposed. PACS num...