Cosmology in Six Dimensions

We discuss cosmological models in six-dimensional spacetime. For codimension-1 branes, we consider a (4+1) braneworld model and discuss its cosmological evolution. For codimension-2 branes, we consider an infinitely thin conical braneword model in the presence of an induced gravity term on the brane and a Gauss-Bonnet term in the bulk. We discuss the cosmological evolution of isotropic and anisotropic matter on the brane. We also briefly discuss cosmological models in six-dimensional supergravity.Comment: 18 pages, 1 figure, plenary talk at 100 Years of Relativity: International Conference on Classical and Quantum Aspects of Gravity and Cosmology, Sao Paulo, Brazil, 22-24 August 200

Induced Curvature in Brane Worlds by Surface Terms in String Effective Actions with Higher-Curvature Corrections

In string-inspired effective actions, representing the low-energy bulk dynamics of brane/string theories, the higher-curvature ghost-free Gauss-Bonnet combination is obtained by local field redefinitions which leave the (perturbative) string amplitudes invariant. We show that such redefinitions lead to surface terms which induce curvature on the brane world boundary of the bulk spacetime.Comment: 15 pages, no figures, comments added, typos corrected, to appear in PR

Black Holes and Black String-like Solutions in Codimension-2 Braneworlds

We discuss black hole solutions with a Gauss-Bonnet term in the bulk and an induced gravity term on a thin brane of codimension-2. We show that these black holes can be localized on the brane, and they can be extended further 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 dictates the form of matter on the brane and in the bulk.Comment: 9 pages, no figures, plenary talk given at the 7th Friedmann International Seminar on Gravitation and Cosmology, 29 June-5 July 2008, Joao Pessoa, Brazil, to appear in the proceeding

Constraints on scalar-tensor theory of gravity by the recent observational results on gravitational waves

The speed of gravitational waves provides us a new tool to test alternative theories of gravity. The constraint on the speed of gravitational waves from GW170817 and GRB170817A is used to test some classes of Horndeski theory. In particular, we consider the coupling of a scalar field to Einstein tensor and the coupling of the Gauss-Bonnet term to a scalar field. The coupling strength of the Gauss-Bonnet coupling is constrained to be in the order of $10^{-15}$. In the Horndeski theory we show that in order for this theory to satisfy the stringent constraint on the speed of GWs the mass scale $M$ introduced in the non-minimally derivative coupling is constrained to be in the range $10^{15}\text{GeV}\gg M \gtrsim 2\times 10^{-35}$GeV taking also under consideration the early times upper bound for the mass scale $M$. The large mass ranges require no fine-tuning because the effect of non-minimally derivative coupling is negligible at late times.Comment: 7 pages, 1 figure, typo corrected, references added and updated, Eur. Phys. J. C in pres

Geometrical Tachyon Dynamics in the Background of a Bulk Tachyon Field

We study the dynamics of a D3-brane moving in the background of a bulk tachyon field of a D3-brane solution of Type-0 string theory. We show that the dynamics on the probe D3-brane can be described by a geometrical tachyon field rolling down its potential which is modified by a function of the bulk tachyon and inflation occurs at weak string coupling, where the bulk tachyon condenses, near the top of the geometrical tachyon potential. We also find a late accelerating phase when the bulk tachyon asymptotes to zero which in the geometrical tachyon picture corresponds to the minimum of the geometrical potential.Comment: 27 pages, 2 figures, accepted for publication in JHE

A Novel Mechanism to Generate FFLO States in Holographic Superconductors

We discuss a novel mechanism to set up a gravity dual of FFLO states in strongly coupled superconductors. The gravitational theory utilizes two U(1) gauge fields and a scalar field coupled to a charged AdS black hole. The first gauge field couples with the scalar sourcing a charge condensate below a critical temperature, and the second gauge field provides a coupling to spin in the boundary theory. The scalar is neutral under the second gauge field. By turning on an interaction between the Einstein tensor and the scalar, it is shown that, in the low temperature limit, an inhomogeneous solution possesses a higher critical temperature than the homogeneous case, giving rise to FFLO states.Comment: v2: 7 pages, 5 figures, improved discussion on critical temperature, to appear in PLB. arXiv admin note: text overlap with arXiv:1208.458