arXiv subjects

A. Kehagias

Publications and source records attributed to A. Kehagias.

At least 55 records · Page 3Linked to original sources

arXiv2012

The Four-point Correlator in Multifield Inflation, the Operator Product Expansion and the Symmetries of de Sitter

We study the multifield inflationary models where the cosmological perturbation is sourced by light scalar fields other than the inflaton. We exploit the operator product expansion and partly the symmetries present during the de Sitter epoch to characterize the non-Gaussian four-point correlator in the squeezed limit. We point out that the contribution to it from the intrinsic non-Gaussianity of the light fields at horizon crossing can be larger than the usually studied contribution arising on superhorizon scales and it comes with a different shape. Our findings indicate that particular attention needs to be taken when studying the effects of the primordial NG on real observables, such as the clustering of dark matter halos.

hep-th↗

arXiv2012

Operator Product Expansion of Inflationary Correlators and Conformal Symmetry of de Sitter

We study the multifield inflationary models where the cosmological perturbation is sourced by light scalar fields other than the inflaton. The corresponding perturbations are both scale invariant and special conformally invariant. We exploit the operator product expansion technique of conformal field theories to study the inflationary correlators enjoying the symmetries present during the de Sitter epoch. The operator product expansion is particularly powerful in characterizing inflationary correlation functions in two observationally interesting limits, the squeezed limit of the three-point correlator and the collapsed limit of the four-point correlator. Despite the fact that the shape of the four-point correlators is not fixed by the symmetries of de Sitter, its exact shape can be found in the collapsed limit making use of the operator product expansion. By employing the fact that conformal invariance imposes the two-point cross-correlations of the light fields to vanish unless the fields have the same conformal weights, we are able to show that the Suyama-Yamaguchi inequality relating the coefficients $f_{\rm NL}$ of the bispectrum in the squeezed limit and $τ_{\rm NL}$ of the trispectrum in the collapsed limit also holds when the light fields are intrinsically non-Gaussian. In fact, we show that the inequality is valid irrespectively of the conformal symmetry, being just a consequence of fundamental physical principles, such as the short-distance expansion of operator products. The observation of a strong violation of the inequality will then have profound implications for inflationary models as it will imply either that multifield inflation cannot be responsible for generating the observed fluctuations independently of the details of the model or that some new non-trivial degrees of freedom play a role during inflation.

hep-th↗

arXiv2007

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.

hep-th↗

arXiv2005

Mirage Cosmology

A brane universe moving in a curved higher dimensional bulk space is considered. The motion induces a cosmological evolution on the universe brane that is indistiguishable from a similar one induced by matter density on the brane. The phenomenological implications of such an idea are discussed. Various mirage energy densities are found, corresponding to dilute matter driving the cosmological expansion, many having superluminal properties $|w|>1$ or violating the positive energy condition. It is shown that energy density due to the world-volume fields is nicely incorporated into the picture. It is also pointed out that the initial singularity problem is naturally resolved in this context.

hep-th↗

arXiv2005

Box Compactification and Supersymmetry Breaking

We discuss all possible compactifications on flat three-dimensional smooth spaces. In particular, various fields are studied on a box with opposite sides identified, after two of them are rotated by $π$, and their spectra are obtained. The compactification of a general 7D supersymmetric theory in such a box is considered and the corresponding four-dimensional theory is studied, in relation to the boundary conditions chosen. The resulting spectrum, according to the allowed field boundary conditions, corresponds to partially or completely broken supersymmmetry. We briefly discuss also the breaking of gauge symmetries under the proposed box compactification.

hep-th↗

arXiv2004

Cosmology with exponential potentials

We examine in the context of general relativity the dynamics of a spatially flat Robertson-Walker universe filled with a classical minimally coupled scalar field ϕof exponential potential ~ e^{-μϕ} plus pressureless baryonic matter. This system is reduced to a first-order ordinary differential equation, providing direct evidence on the acceleration/deceleration properties of the system. As a consequence, for positive potentials, passage into acceleration not at late times is generically a feature of the system, even when the late-times attractors are decelerating. Furthermore, the structure formation bound, together with the constraints on the present values of Ω_{m}, w_ϕ provide, independently of initial conditions and other parameters, necessary conditions on μ. Special solutions are found to possess intervals of acceleration. For the almost cosmological constant case w_ϕ ~ -1, as well as, for the generic late-times evolution, the general relation Ω_ϕ(w_ϕ) is obtained.

gr-qc↗

arXiv2002

Graviton localization and Newton law for a dS_4 brane in 5D bulk

We consider an dS_4 brane embedded in a five-dimensional bulk with a positive, vanishing or negative bulk cosmological constant and derive the localized graviton spectrum that consists of a normalizable zero-mode separated by a gap from a continuum of massive states. We estimate the massive sector contribution to the static potential at short distances and find that only in the case of a negative bulk cosmological constant there is a range, determined by the effective four-dimensional and the bulk cosmological constants, where the conventional Newton law is valid.

hep-th↗

arXiv2002

Penrose Limits of Orbifolds and Orientifolds

We study the Penrose limit of various AdS_p X S^q orbifolds. The limiting spaces are waves with parallel rays and singular wave fronts. In particular, we consider the orbifolds AdS_3 X S^3/Γ, AdS_5 X S^5/Γand AdS_{4,7} X S^{7,4}/Γwhere Γacts on the sphere and/or the AdS factor. In the pp-wave limit, the wave fronts are the orbifolds C^2/Γ, C^4/Γand R XC^4/Γ, respectively. When desingularization is possible, we get asymptotically locally pp-wave backgrounds (ALpp). The Penrose limit of orientifolds are also discussed. In the AdS_5 X RP^5 case, the limiting singularity can be resolved by an Eguchi-Hanson gravitational instanton. The pp-wave limit of D3-branes near singularities in F-theory is also presented. Finally, we give the embedding of D-dimensional pp-waves in flat M^{2,D} space.

hep-th↗

arXiv2002

Geometric Transitions, Brane Dynamics and Gauge Theories

We consider the interplay between brane constructions and type IIA, IIB or M-theory geometries on Calabi-Yau (CY) and G_2 holonomy manifolds. This is related to N=1 (and N=2) gauge theories in four dimensions. We first discuss simple geometric transitions corresponding to brane set ups involving orthogonal (or parallel) Neveu-Schwarz branes that approach each other. This is related to confinement and Seiberg duality in SQCD. In particular, we argue that in type IIA, a CP^1 of singularities and one unit of Ramond-Ramond (RR) flux is dual to a D6 brane wrapped on a Lens space, describing the UV and IR of N=1 Super-Yang-Mills (SYM), respectively. Also, in the large N_c duality that relates D6 branes on S^3 to an S^2 with RR flux, we implement the presence of $N_f$ flavors of quarks. We then compactify M-theory on T^*(S^3) X S^1)/Z_2 and observe that one phase describes SO(4) SYM in the UV and two others describe confinement. Moreover, we consider compact 7-spaces (CY X S^1)/Z_2. We describe transitions where disconnected S^3's approach and connect each other before they vanish. These effects correspond to non-Abelian Higgs mechanism and confinement. The similar transitions involving S^2 X S^1's are also considered. Finally, we present transitions at finite distance in moduli space, where the first Betti number b_1 of 3-cycles of singularities changes.

hep-th↗

arXiv2001

Phases of supersymmetric gauge theories from M-theory on G_2 manifolds

We consider M-theory on compact spaces of G_2 holonomy constructed as orbifolds of the form (CY x S^1)/Z_2 with fixed point set Σon the CY. This describes N=1 SU(2) gauge theories with b_1(Σ) chiral multiplets in the adjoint. For b_1=0, it generalizes to compact manifolds the study of the phase transition from the non-Abelian to the confining phase through geometrical S^3 flops. For b_1=1, the non-Abelian and Coulomb phases are realized, where the latter arises by desingularization of the fixed point set, while an S^2 x S^1 flop occurs. In addition, an extremal transition between G_2 spaces can take place at conifold points of the CY moduli space where unoriented membranes wrapped on CP^1 and RP^2 become massless.

hep-th↗

arXiv2001

A note on brane cosmology

We derive a new class of time-dependent solutions for the Randall-Sundrum model by patching together isometries broken by the brane. Solutions generated by generalized boosts along the fifth dimension are associated with localized gravity and lead to an effective Friedman equation on the brane with a scale factor exhibiting power law or exponential behaviour. The effective energy-density on the brane depends linearly on the brane tension.

hep-ph↗

arXiv2000

A Self-Tuning Solution of the Cosmological Constant Problem

We discuss the four-dimensional cosmological constant problem in a five-dimensional setting. A scalar field coupled to the SM forms dynamically a smooth brane with four-dimensional Poincare invariance, independently of SM physics. In this respect, our solution may be regarded as a self-tuning solution, free of any singularities and fine-tuning problems.

hep-th↗

arXiv2000

Localized Gravitons, Gauge Bosons and Chiral Fermions in Smooth Spaces Generated by a Bounce

We study five-dimensional solutions to Einstein equations coupled to a scalar field. Bounce-type solutions for the scalar field are associated with AdS_5 spaces with smooth warp functions. Gravitons are dynamically localized in this framework in analogy to the Randall-Sundrum solution whereas, a bulk fermion gives rise to a single chiral zero mode localized at the bounce. Additional bulk scalar fields are incorporated in this picture. The dilaton, as a bulk scalar leads, through its coupling, to localized gauge boson fields, something that holds also in the case that the bounce system is replaced by a brane.

hep-th↗

arXiv2000

Lattice Evidence for Gauge Field Localization on a Brane

We examine the problem of gauge-field localization in higher-dimensional gauge theories. In particular, we study a five-dimensional U(1) by lattice techniques and we find that gauge fields can indeed be localized. Two models are considered. The first one has anisotropic couplings independent of each other and of the coordinates. It can be realized on a homogeneous but anisotropic flat Euclidean space. The second model has couplings depending on the extra transverse fifth direction. This model can be realized by a U(1) gauge theory on the Randall-Sundrum background. We find that in both models a new phase exists, the layer phase, in which a massless photon is localized on the four-dimensional layers. We find that this phase is separated from the strong coupling phase by a second order phase transition.

hep-th↗

arXiv2000

Hyperbolic Spaces in String and M-Theory

We describe string-theory and $d=11$ supergravity solutions involving symmetric spaces of constant negative curvature. Many examples of non-supersymmetric string compactifications on hyperbolic spaces $H_r$ of finite volume are given in terms of suitable cosets of the form $H_r/Γ$, where $Γ$ is a discrete group. We describe in some detail the cases of the non-compact hyperbolic spaces $F_2$ and $F_3$, representing the fundamental regions of $H_2$ and $H_3$ under $SL(2,Z)$ and the Picard group, respectively. By writing $AdS$ as a U(1) fibration, we obtain new solutions where $AdS_{2p+1}$ gets untwisted by T-duality to ${\bf R}\times SU(p,1)/(SU(p)\times U(1))$. Solutions with time-dependent dilaton field are also constructed by starting with a solution with NS5-brane flux over $H_3$. A new class of non-supersymmetric conformal field theories can be defined via holography.

hep-th↗

arXiv1999

Deviations from the $1/r^2$ Newton law due to extra dimensions

We systematically examine corrections to the gravitational inverse square law, which are due to compactified extra dimensions. We find the induced Yukawa-type potentials for which we calculate the strength αand range. In general the range of the Yukawa correction is given by the wavelength of the lightest Kaluza-Klein state and its strength, relative to the standard gravitational potential, by the corresponding degeneracy. In particular, when n extra dimensions are compactified on an n-torus, we find that the strength of the potential is α=2n, whereas the compactification on an n-sphere gives α= n+1. For Calabi-Yau compactifications the strength can be at most α=20.

hep-ph↗

arXiv1999

Non-Compact Compactifications with Brane Worlds

The possibility of neutral, brane-like solutions in a higher dimensional setting is discussed. In particular, we describe a supersymmetric solution in six dimensions which can be interpreted as a "three-brane" with a non-compact transverse space of finite volume. The construction can be generalized to n+4-dimensions and the result is a n+3-brane compactified on a n-1-dimensional Einstein manifold with a non-compact extra dimension. We find that there always exists a massless graviton trapped in four-dimensions while a bulk abelian gauge field gives rise to a unique four-dimensional massless photon. Moreover, all massless modes are accompanied by massive KK states and we show that it is possible in such a scenario the masses of the KK states to be at the TeV scale without hierarchically large extra dimensions.

hep-th↗

arXiv1999

Exponential and power-law hierarchies from supergravity

We examine how a d-dimensional mass hierarchy can be generated from a d+1-dimensional set up. We consider a d+1--dimensional scalar, the hierarchon, which has a potential as in gauged supergravities. We find that when it is in its minimum, there exist solutions of Horava-Witten topology R^d X S^1/Z^2 with domain walls at the fixed points and anti-de Sitter geometry in the bulk. We show that while standard Poincare supergravity leads to power-law hierarchies, (e.g. a power law dependence of masses on the compactification scale), gauged supergravity produce an exponential hierarchy as recently proposed by Randall and Sundrum.

hep-th↗