Noncompact gaugings, chiral reduction and dual sigma models in supergravity

We show that the half-maximal SU(2) gauged supergravity with topological mass term admits coupling of an arbitrary number of n vector multiplets. The chiral circle reduction of the ungauged theory in the dual 2-form formulation gives N=(1,0) supergravity in 6D coupled to 3p scalars that parametrize the coset SO(p,3)/SO(p)x SO(3), a dilaton and (p+3) axions with p < n+1. Demanding that R-symmetry gauging survives in 6D is shown to put severe restrictions on the 7D model, in particular requiring noncompact gaugings. We find that the SO(2,2) and SO(3,1) gauged 7D supergravities give a U(1)_R, and the SO(2,1) gauged 7D supergravity gives an Sp(1)_R gauged chiral 6D supergravities coupled to certain matter multiplets. In the 6D models obtained, with or without gauging, we show that the scalar fields of the matter sector parametrize the coset SO(p+1,4)/SO(p+1)x SO(4), with the (p+3) axions corresponding to its abelian isometries. In the ungauged 6D models, upon dualizing the axions to 4-form potentials, we obtain coupling of p linear multiplets and one special linear multiplet to chiral 6D supergravity.Comment: 41 pages, late

Pseudo-3-Branes in a Curved 6D Bulk

We consider a model involving a 4-brane in a 6D bulk which carries sigma model fields. An axion field on the 4-brane cancels the pressure along one direction leading to an effective codimension-2 3-brane. For a range of parameters of the theory, we get a transverse space which is non-compact, providing a possible solution to the cosmological constant problem. A setup with two branes in a compact space is also treated. In this case, a mild fine-tuning between the radii of the two 4-branes is necessary. Finally, we explore the 4-brane problem in the Gauss-Codazzi formulation and we discuss general aspects of gravity in the presence of additional brane sources.Comment: 13 page

The Stability of Branonium

We analyse the orbital motion of a light anti D6-brane in the presence of a stack of heavy, distant D6-branes in ten dimensions, taking account of possible time-variations in the background moduli fields. The Coulomb-like central potential arising through brane-antibrane interactions is then modified to include time-dependent prefactors, which generally preclude the existence of stable elliptical orbits.Comment: 13 pages, Latex, 3 eps figure

The Ineludible non-Gaussianity of the Primordial Black Hole Abundance

We study the formation of primordial black holes when they are generated by the collapse of large overdensities in the early universe. Since the density contrast is related to the comoving curvature perturbation by a nonlinear relation, the overdensity statistics is unavoidably non-Gaussian. We show that the abundance of primordial black holes at formation may not be captured by a perturbative approach which retains the first few cumulants of the non-Gaussian probability distribution. We provide two techniques to calculate the non-Gaussian abundance of primordial black holes at formation, one based on peak theory and the other on threshold statistics. Our results show that the unavoidable non-Gaussian nature of the inhomogeneities in the energy density makes it harder to generate PBHs. We provide simple (semi-)analytical expressions to calculate the non-Gaussian abundances of the primordial black holes and show that for both narrow and broad power spectra the gaussian case from threshold statistics is reproduced by increasing the amplitude of the power spectrum by a factor ${\cal O}(2\div 3)$.Comment: 26 pages, 10 figures, matching published versio

Testing extra dimensions with boundaries using Newton's law modifications

Extra dimensions with boundaries are often used in the literature, to provide phenomenological models that mimic the standard model. In this context, we explore possible modifications to Newton's law due to the existence of an extra-dimensional space, at the boundary of which the gravitational field obeys Dirichlet, Neumann or mixed boundary conditions. We focus on two types of extra space, namely, the disk and the interval. As we prove, in order to have a consistent Newton's law modification (i.e., of the Yukawa-type), some of the extra-dimensional spaces that have been used in the literature, must be ruled out.Comment: Published version, title changed, 6 figure

Bouncing Cosmology in Three Dimensions

We consider a dynamical two-brane in a four dimensional black hole background with scalar hair. At high temperature this black hole goes through a phase transition by radiating away the scalar. The end phase is a topological adS-Schwarzschild black hole. We argue here that for a sufficiently low temperature, the brane motion in this geometry is non-singular. This results in a universe which passes over from a contracting phase to an expanding one without reaching a singularity.Comment: 7 pages, LaTex, 3 figures, Journal versio

Brane Gas Inflation

We consider the brane gas picture of the early universe. At later stages, when there are no winding modes and the background is free to expand, we show that a moving 3-brane, which we identify with our universe, can inflate even though it is radiation-dominated. The crucial ingredients for successful inflation are the coupling to the dilaton and the equation of state of the bulk. If we suppose the brane initially forms in a collision of higher-dimensional branes, then the spectrum of primordial density fluctuations naturally has a thermal origin.Comment: 4 pages, 1 figur

Using Muonic Hydrogen in Optical Spectroscopy Experiment to Detect Extra Dimensions

Considering that gravitational force might deviate from Newton's inverse-square law (ISL) and become much stronger in small scale, we propose a kind of optical spectroscopy experiment to detect this possible deviation and take electronic, muonic and tauonic hydrogen atoms as examples. This experiment might be used to indirectly detect the deviation of ISL down to nanometer scale and to explore the possibility of three extra dimensions in ADD's model, while current direct gravity tests cannot break through micron scale and go beyond two extra dimensions scenario.Comment: 9 pages, 2 figures. To appear in IJT

Braneworld inflation

We discuss various realizations of the four dimensional braneworld inflation in warped geometries of string theory. In all models the inflaton field is represented by a Dp probe brane scalar specifying its position in the warped throat of the compactification manifold. We study existing inflationary throat local geometries, and construct a new example. The inflationary brane is either a D3- or a D5-brane of type IIB string theory. In the latter case the inflationary brane is wrapping a two-cycle of the compactification manifold. We discuss some phenomenological aspects of the model where slow-roll conditions are under computational control.Comment: 31 pages + 6 figures, v2: published PRD versio