Perturbations on a moving D3-brane and mirage cosmology

We study the evolution of perturbations on a moving probe D3-brane coupled to a 4-form field in an AdS$_5$-Schwarzschild bulk. The unperturbed dynamics are parametrised by a conserved energy $E$ and lead to Friedmann-Robertson-Walker `mirage' cosmology on the brane with scale factor $a(\tau)$. The fluctuations about the unperturbed worldsheet are then described by a scalar field $\phi(\tau,\vec{x})$. We derive an equation of motion for $\phi$, and find that in certain regimes of $a$ the effective mass squared is negative. On an expanding BPS brane with E=0 superhorizon modes grow as $a^4$ whilst subhorizon modes are stable. When the brane contracts, all modes grow. We also briefly discuss the case when $E>0$, BPS anti-branes as well as non-BPS branes. Finally, the perturbed brane embedding gives rise to scalar perturbations in the FRW universe. We show that $\phi$ is proportional to the gauge invariant Bardeen potentials on the brane.Comment: 26 pages, 5 figures, to appear in Phys.Rev.D, comments and minor corrections adde

Anatomy of a Bounce

Holographic considerations are used in the scrutiny of a special class of brane-world cosmologies. Inherently to this class, the brane typically bounces, at a finite size, as a consequence of a charged black hole in the bulk. Whereas a prior treatment [hep-th/0301010] emphasized a brane that is void of standard-model matter, the analysis is now extended to include an intrinsic (radiation-dominated) matter source. An interesting feature of this generalized model is that a bounce is no longer guaranteed but, rather, depends on the initial conditions. Ultimately, we demonstrate that compliance with an appropriate holographic bound is a sufficient prerequisite for a bounce to occur.Comment: 14 pages, Revtex; (v2) minor revisions; (v3) reference adde

Axisymmetric metrics in arbitrary dimensions

We consider axially symmetric static metrics in arbitrary dimension, both with and without a cosmological constant. The most obvious such solutions have an SO(n) group of Killing vectors representing the axial symmetry, although one can also consider abelian groups which represent a flat `internal space'. We relate such metrics to lower dimensional dilatonic cosmological metrics with a Liouville potential. We also develop a duality relation between vacuum solutions with internal curvature and those with zero internal curvature but a cosmological constant. This duality relation gives a solution generating technique permitting the mapping of different spacetimes. We give a large class of solutions to the vacuum or cosmological constant spacetimes. We comment on the extension of the C-metric to higher dimensions and provide a novel solution for a braneworld black hole.Comment: 36 pages, LaTeX (JHEP), 4 figures, section added (published version

The Cosmological Slingshot Scenario: A Stringy Early Times Universe

A cosmological model for the early time Universe is proposed. In this model, the Universe is a wandering brane moving in a warped throat of a Calabi-Yau space. A non-zero angular momentum induces a turning point in the brane trajectory, and leads to a bouncing cosmology as experienced by an observer living on the brane. The Universe undergoes a decelerated contraction followed by an accelerating expansion and no big-bang singularity. Although the number of e-folds of accelerated motion is low (less than 2), standard cosmological problems are not present in our model thanks to the absence of an initial singularity and the violation of energy conditions of mirage matter at high energies. Density perturbations are also calculated in our model and we find a slightly red spectral index with negligible tensorial perturbations in compatibility with WMAP data.Comment: v5: clarifications and references added, results unchanged, version accepted in Class. Quant. Grav. (2008), 34 pages, 5 figure

The fundamental constants and their variation: observational status and theoretical motivations

This article describes the various experimental bounds on the variation of the fundamental constants of nature. After a discussion on the role of fundamental constants, of their definition and link with metrology, the various constraints on the variation of the fine structure constant, the gravitational, weak and strong interactions couplings and the electron to proton mass ratio are reviewed. This review aims (1) to provide the basics of each measurement, (2) to show as clearly as possible why it constrains a given constant and (3) to point out the underlying hypotheses. Such an investigation is of importance to compare the different results, particularly in view of understanding the recent claims of the detections of a variation of the fine structure constant and of the electron to proton mass ratio in quasar absorption spectra. The theoretical models leading to the prediction of such variation are also reviewed, including Kaluza-Klein theories, string theories and other alternative theories and cosmological implications of these results are discussed. The links with the tests of general relativity are emphasized.Comment: 56 pages, l7 figures, submitted to Rev. Mod. Phy

The Cosmological Slingshot Scenario: Myths and Facts

We generalize the Cosmological Slingshot Scenario for a Slingshot brane moving in a Klebanov-Strassler throat. We show that the horizon and isotropy problems of standard cosmology are avoided, while the flatness problem is acceptably alleviated. Regarding the primordial perturbations, we identify their vacuum state and elucidate the evolution from the quantum to the classical regimes. Also, we calculate their exact power spectrum showing its compatibility with current data. We discuss the bouncing solution from a four dimensional point of view. In this framework the radial and angular motion of the Slingshot brane are described by two scalar fields. We show that the bouncing solution for the scale factor in String frame is mapped into a monotonically increasing (in conformal time) solution in the Einstein frame. We finally discuss about the regularity of the geometry in Einstein frame.Comment: 16 pages, 2 figs. Major clarifications and references added, version accepted in Gen. Rel. Grav. (2009