Minas Tsoukalas

Black hole solutions in 5D Horava-Lifshitz gravity

We study the full spectrum of spherically symmetric solutions in the five-dimensional nonprojectable Horava-Lifshitz type gravity theories. For appropriate ranges of the coupling parameters, we have found several classes of solutions which are characterized by an AdS{sub 5}, dS{sub 5}, or flat large distance asymptotic behavior, plus the standard 1/r{sup 2} tail of the usual five-dimensional Schwarzschild black holes. In addition we have found solutions with an unconventional short or large distance behavior, and, for a special range of the coupling parameters, solutions which coincide with black hole solutions of conventional relativistic five-dimensional Gauss-Bonnet gravity.

Generalised Scale Invariant Theories

We present the most general actions of a single scalar field and two scalar fields coupled to gravity, consistent with second order field equations in four dimensions, possessing local scale invariance. We apply two different methods to arrive at our results. One method, Ricci gauging, was known to the literature and we find this to produce the same result for the case of one scalar field as a more efficient method presented here. However, we also find our more efficient method to be much more general when we consider two scalar fields. Locally scale invariant actions are also presented for theories with more than two scalar fields coupled to gravity and we explain how one could construct the most general actions for any number of scalar fields. Our generalised scale invariant actions have obvious applications to early universe cosmology, and include, for example, the Bezrukov-Shaposhnikov action as a subset.

Cosmology in new gravitational scalar-tensor theories

We investigate the cosmological applications of new gravitational scalar-tensor theories, which are novel modifications of gravity possessing 2+2 propagating degrees of freedom, arising from a Lagrangian that includes the Ricci scalar and its first and second derivatives. Extracting the field equations we obtain an effective dark energy sector that consists of both extra scalar degrees of freedom, and we determine various observables. We analyze two specific models and we obtain a cosmological behavior in agreement with observations, i.e. transition from matter to dark energy era, with the onset of cosmic acceleration. Additionally, for a particular range of the model parameters, the equation-of-state parameter of the effective dark energy sector can exhibit the phantom-divide crossing. These features reveal the capabilities of these theories, since they arise solely from the novel, higher-derivative terms.

Black holes on thin 3-branes of codimension-2 and their extension into the bulk

Abstract We discuss black hole solutions in six-dimensional gravity with a Gauss–Bonnet term in the bulk and an induced gravity term on a thin 3-brane of codimension-2. We show that these black holes can be localized on the brane, and they can further be extended into the bulk by a warp function. These solutions have regular horizons and no other curvature singularities appear apart from the string-like ones. The projection of the Gauss–Bonnet term on the brane imposes a constraint relation which requires the presence of matter in the extra dimensions.

Bañados-Teitelboim-Zanelli-like string on codimension-2 braneworlds in the thin brane limit.

We consider five-dimensional gravity with a Gauss-Bonnet term in the bulk and an induced gravity term on a 2-brane of codimension 2. We show that this system admits Bañados-Teitelboim-Zanelli black holes on the 2-brane which are extended into the bulk with regular horizons.

String-Like BTZ on Codimension-2 Braneworlds in the Thin Brane Limit

We consider five-dimensional gravity with a Gauss-Bonnet term in the bulk and an induced gravity term on a 2-brane of codimension 2. We show that this system admits Bañados-Teitelboim-Zanelli black holes on the 2-brane which are extended into the bulk with regular horizons.

Lovelock Galileons and black holes

We study a scalar-tensor version of Lovelock theory with a non trivial higher order galileon term involving the coupling of the Lovelock two tensor with derivatives of the scalar galileon field. For a static and spherically symmetric spacetime we extend the Boulware-Deser solution to the presence of a Galileon field. The hairy solution has a regular scalar field on the black hole event horizon and presents certain self tuning properties for the bulk cosmological constant and the Gauss-Bonnet coupling. The combined time and radial dependence of the galileon field permits its horizon regularity. Furthermore in order to investigate the effects of linear time dependence we find spherically symmetric solutions in 4 and 5 spacetime dimensions. They are shown to have singular horizons. Afar from the Schwarzschild radius and for weak higher dimensional couplings the solutions are perturbratively close to GR representing GR like star solutions for scalar tensor theories.

Massive Gravity with Anisotropic Scaling

We study a massive gravity theory which is Lorentz violating all the way from ultraviolet to infrared energy scales. At short distances the theory breaks diffeomorphism invariance and time and space scale differently. Dynamical metric fields are introduced which upon linearization over a Minkowski background correspond to Lorentz violating mass terms at large distances. We perform a scalar perturbation analysis and we show that with an appropriate choice of parameters the theory is healthy without ghosts, tachyons, strong coupling problems and instabilities

Perturbations of Gauss-Bonnet black strings in codimension-2 braneworlds

We derive the Lichnerowicz equation in the presence of the Gauss-Bonnet term. Using the modified Lichnerowicz equation we study the metric perturbations of Gauss-Bonnet black strings in Codimension-2 Braneworlds.

Gravitational collapse of a homogeneous scalar field coupled kinematically to Einstein tensor

We study the gravitational collapse of a homogeneous time-dependent scalar field that, besides its coupling to curvature, it is also kinematically coupled to the Einstein tensor. This coupling is a part of the Horndeski theory and we investigate its effect on the collapsing process. We find that the time required for the scalar field to collapse depends on the value of the derivative coupling and the singularity is protected by a horizon. Matching the internal solution with an external Schwarzschild- AdS metric we show that a black hole is formed, while the weak energy condition is satisfied during the collapsing process. The scalar field takes on a finite value at the singularity.