ICTP Lectures on Large Extra Dimensions

I give a brief and elementary introduction to braneworld models with large extra dimensions. Three conceptually distinct scenarios are outlined: (i) Large compact extra dimensions; (ii) Warped extra dimensions; (iii) Infinite-volume extra dimensions. As an example I discuss in detail an application of (iii) to late-time cosmology and the acceleration problem of the Universe.Comment: 39 LaTex pgs; 6 ps figures; Based on lectures given at Summer School on Astroparticle Physics and Cosmology Triese, Italy, June 17 -- July 5, 200

Looking At The Cosmological Constant From Infinite--Volume Bulk

I briefly review the arguments why the braneworld models with infinite-volume extra dimensions could solve the cosmological constant problem, evading Weinberg's no-go theorem. Then I discuss in detail the established properties of these models, as well as the features which should be studied further in order to conclude whether these models can truly solve the problem. This article is dedicated to the memory of Ian Kogan.Comment: 64 pages, 4 figures; To appear in Ian Kogan Memorial Volume, ``From Fields to Strings: Circumnavigating Theoretical Physics'', M. Shifman, A. Vainshtein, and J. Wheater, eds. (World Scientific, 2004

The Big Constant Out, The Small Constant In

Some time ago, Tseytlin has made an original and unusual proposal for an action that eliminates an arbitrary cosmological constant. The form of the proposed action, however, is strongly modified by gravity loop effects, ruining its benefit. Here I discuss an embedding of Tseytlin's action into a broader context, that enables to control the loop effects. The broader context is another universe, with its own metric and dynamics, but only globally connected to ours. One possible Lagrangian for the other universe is that of unbroken AdS supergravity. A vacuum energy in our universe does not produce any curvature for us, but instead increases or decreases the AdS curvature in the other universe. I comment on how to introduce the accelerated expansion in this framework in a technically natural way, and consider the case where this is done by the self-accelerated solutions of massive gravity and its extensions.Comment: 14 pages; a brief paragraph unfolded; 3 refs added; minor improvement

(De)coupling Limit of DGP

We investigate the decoupling limit in the DGP model of gravity by studying its nonlinear equations of motion. We show that, unlike 4D massive gravity, the limiting theory does not reduce to a sigma model of a single scalar field: Non-linear mixing terms of the scalar with a tensor also survive. Because of these terms physics of DGP is different from that of the scalar sigma model. We show that the static spherically-symmetric solution of the scalar model found in hep-th/0404159, is not a solution of the full set of nonlinear equations. As a consequence of this, the interesting result on hidden superluminality uncovered recently in the scalar model in hep-th/0602178, is not applicable to the DGP model of gravity. While the sigma model violates positivity constraints imposed by analyticity and the Froissart bound, the latter cannot be applied here because of the long-range tensor interactions that survive in the decoupling limit. We discuss further the properties of the Schwarzschild solution that exhibits the gravitational mass-screening phenomenon.Comment: 14 pages; v2: typos corrected, footnote added, to apppear in PL

Holographic CBK Relation

The Crewther-Broadhurst-Kataev (CBK) relation connects the Bjorken function for deep-inelastic sum rules (or the Gross - Llewellyn Smith function) with the Adler function for electron-positron annihilation in QCD; it has been checked to hold up to four loops in perturbation theory. Here we study non-perturbative terms in the CBK relation using a holographic dual theory that is believed to capture properties of QCD. We show that for the large invariant momenta the perturbative CBK relation is exactly satisfied. For the small momenta non-perturbative corrections enter the relation and we calculate their significant effects. We also give an exact holographic expression for the Bjorken function, as well as for the entire three-point axial-vector-vector correlation function, and check their consistency in the conformal limit.Comment: 16 latex pages, 4 figures; v2: comments and references added; a remark about Schwinger's paper corrected; to appear in PL