arXiv subjects

Gabriele Veneziano

Publications and source records attributed to Gabriele Veneziano.

47 records · Page 3Linked to original sources

arXiv2012

The second-order luminosity-redshift relation in a generic inhomogeneous cosmology

After recalling a general non-perturbative expression for the luminosity-redshift relation holding in a recently proposed "geodesic light-cone" gauge, we show how it can be transformed to phenomenologically more convenient gauges in which cosmological perturbation theory is better understood. We present, in particular, the complete result on the luminosity-redshift relation in the Poisson gauge up to second order for a fairly generic perturbed cosmology, assuming that appreciable vector and tensor perturbations are only generated at second order. This relation provides a basic ingredient for the computation of the effects of stochastic inhomogeneities on precision dark-energy cosmology whose results we have anticipated in a recent letter. More generally, it can be used in connection with any physical information carried by light-like signals traveling along our past light-cone.

astro-ph.CO↗

arXiv2012

Backreaction on the luminosity-redshift relation from gauge invariant light-cone averaging

Using a recently proposed gauge invariant formulation of light-cone averaging, together with adapted "geodesic light-cone" coordinates, we show how an "induced backreaction" effect emerges, in general, from correlated fluctuations in the luminosity distance and covariant integration measure. Considering a realistic stochastic spectrum of inhomogeneities of primordial (inflationary) origin we find that both the induced backreaction on the luminosity-redshift relation and the dispersion are larger than naively expected. On the other hand the former, at least to leading order and in the linear perturbative regime, cannot account by itself for the observed effects of dark energy at large-redshifts. A full second-order calculation, or even better a reliable estimate of contributions from the non-linear regime, appears to be necessary before firm conclusions on the correct interpretation of the data can be drawn.

astro-ph.CO↗

arXiv2011

k-String Tension from Eguchi-Kawai Reduction

We consider three-dimensional SU(N) gauge theories with massless Dirac or Majorana fermions in the adjoint representation. We use the Eguchi-Kawai volume reduction to two-dimensions to calculate the tension sigma_k of the k-string in such theories. Under some assumptions whose validity we discuss, we derive the previously conjectured sine formula, sigma_k ~ N sin pi k/N.

hep-th↗

arXiv2010

Dimensionally reduced SYM$_4$ at large-$N$: an intriguing Coulomb approximation

We consider the light-cone (LC) gauge and LC quantization of the dimensional reduction of super Yang Mills theory from four to two dimensions. After integrating out all unphysical degrees of freedom, the non-local LC Hamiltonian exhibits an explicit ${\cal N}=(2,2)$ supersymmetry. A further SUSY-preserving compactification of LC-space on a torus of radius $R$, allows for a large-$N$ numerical study where the smooth large-$R$ limit of physical quantities can be checked. As a first step, we consider a simple, yet quite rich, "Coulomb approximation" that maintains an ${\cal N}=(1,1)$ subgroup of the original supersymmetry and leads to a non-trivial generalization of 't Hooft's model with an arbitrary --but conserved-- number of partons. We compute numerically the eigenvalues and eigenvectors both in momentum and in position space. Our results, so far limited to the sectors with 2, 3 and 4 partons, directly and quantitatively confirm a simple physical picture in terms of a string-like interaction with the expected tension among pairs of nearest-neighbours along the single-trace characterizing the large-$N$ limit. Although broken by our approximation, traces of the full ${\cal N}=(2,2)$ supersymmetry are still visible in the low-lying spectrum.

hep-th↗

arXiv2010

High-energy string-brane scattering: leading eikonal and beyond

We extend previous techniques for calculations of transplanckian-energy string-string collisions to the high-energy scattering of massless closed strings from a stack of N Dp-branes in Minkowski spacetime. We show that an effective non-trivial metric emerges from the string scattering amplitudes by comparing them against the semiclassical dynamics of high-energy strings in the extremal p-brane background. By changing the energy, impact parameter and effective open string coupling, we are able to explore various interesting regimes and to reproduce classical expectations, including tidal-force excitations, even beyond the leading-eikonal approximation.

hep-th↗

arXiv2007

A note on C-Parity Conservation and the Validity of Orientifold Planar Equivalence

We analyze the possibility of a spontaneous breaking of C-invariance in gauge theories with fermions in vector-like - but otherwise generic - representations of the gauge group. QCD, supersymmetric Yang-Mills theory, and orientifold field theories, all belong to this class. We argue that charge conjugation is not spontaneously broken as long as Lorentz invariance is maintained. Uniqueness of the vacuum state in pure Yang-Mills theory (without fermions) and convergence of the expansion in fermion loops are key ingredients. The fact that C-invariance is conserved has an interesting application to our proof of planar equivalence between supersymmetric Yang-Mills theory and orientifold field theory on R4, since it allows the use of charge conjugation to connect the large-N limit of Wilson loops in different representations.

hep-th↗

arXiv2005

Quark Condensate in Massless QCD from Planar Equivalence

A previous estimate of the quark condensate in one-flavour massless QCD from the known value of the gluino condensate in super Yang-Mills (SYM) theory is extended to Nf >1 by considering the large-N limit of an SU(N) gauge theory with one flavour in the antisymmetric representation and nf = Nf -1 extra flavours in the fundamental representation. We argue that, even at nf> 0, suitably chosen correlators in this theory can be mapped to those of SYM as N--> infinity. We give arguments why this correspondence should be particularly good for Nf =3 and compare our prediction with available (real and Monte Carlo) data.

hep-ph↗

arXiv2002

Runaway dilaton and equivalence principle violations

In a recently proposed scenario, where the dilaton decouples while cosmologically attracted towards infinite bare string coupling, its residual interactions can be related to the amplitude of density fluctuations generated during inflation, and are large enough to be detectable through a modest improvement on present tests of free-fall universality. Provided it has significant couplings to either dark matter or dark energy, a runaway dilaton can also induce time-variations of the natural "constants" within the reach of near-future experiments.

gr-qc↗

arXiv2002

Black holes from high-energy beam--beam collisions

Using a recent technique, proposed by Eardley and Giddings, we extend their results to the high-energy collision of two beams of massless particles, i.e. of two finite-front shock waves. Closed (marginally) trapped surfaces can be determined analytically in several cases even for collisions at non-vanishing impact parameter in D\ge 4 space-time dimensions. We are able to confirm and extend earlier conjectures by Yurtsever, and to deal with arbitrary axisymmetric profiles, including an amusing case of ``fractal'' beams. We finally discuss some implications of our results in high-energy experiments and in cosmology.

gr-qc↗

arXiv2000

O(d,d)-invariant collapse/inflation from colliding superstring waves

Generalizing previous work, we study the collision of massless superstring plane waves in D space-time dimensions within an explicitly O(D-2,D-2)-invariant set of field equations. We discuss some general properties of the solutions, showing in particular that they always lead to the formation of a singularity in the future. Using the above symmetry, we obtain entire classes of new analytic solutions with non-trivial metric, dilaton and antisymmetric field, and discuss some of their properties of specific relevance to string cosmology.

hep-th↗

arXiv1999

Self-gravitating fundamental strings and black-holes

The configuration of typical highly excited (M >> M_s ~ (alpha')^{-1/2}) string states is considered as the string coupling g is adiabatically increased. The size distribution of very massive single string states is studied and the mass shift, due to long-range gravitational, dilatonic and axionic attraction, is estimated. By combining the two effects, in any number of spatial dimensions d, the most probable size of a string state becomes of order l_s = sqrt{2 alpha'} when g^2 M / M_s ~ 1. Depending on the dimension d, the transition between a random-walk-size string state (for low g) and a compact (~ l_s) string state (when g^2 M / M_s ~ 1) can be very gradual (d=3), fast but continuous (d=4), or discontinuous (d > 4). Those compact string states look like nuggets of an ultradense state of string matter, with energy density rho ~ g^{-2} M_s^{d+1}. Our results extend and clarify previous work by Susskind, and by Horowitz and Polchinski, on the correspondence between self-gravitating string states and black holes.

hep-th↗