We analyze the spectrum of density perturbations generated in models of the recently discovered ''D-cceleration'' mechanism of inflation. In this scenario, strong coupling quantum field theoretic effects sum to provide a DBI-like action for the inflaton. We show that the model has a strict lower bound on the non-Gaussianity of the CMBR power spectrum at an observable level, and is thus falsifiable. This in particular observationally distinguishes this mechanism from traditional slow roll inflation generated by weakly interacting scalar fields. The model also favors a large observable tensor component to the CMBR spectrum.

DBI in the sky

Causality on the gravity side of the AdS/CFT correspondence restricts motion on the moduli space of the N = 4 super Yang Mills theory by imposing a speed limit on how fast the scalar field may roll. This effect can be traced to higher derivative operators arising from integrating out light degrees of freedom near the origin. In the strong coupling limit of the theory, the dynamics is well approximated by the Dirac-Born-Infeld Lagrangian for a probe D3-brane moving toward the horizon of the AdS Poincare patch, combined with an estimate of the (ultimately suppressed) rate of particle and string production in the system. We analyze the motion of a rolling scalar field explicitly in the strong coupling regime of the field theory, and extend the analysis to cosmological systems obtained by coupling this type of field theory to four dimensional gravity. This leads to a mechanism for slow roll inflation for a massive scalar at subPlanckian VEV without need for a flat potential (realizing a version of k-inflation in a microphysical framework). It also leads to a variety of novel FRW cosmologies, some of which are related to those obtained with tachyon matter.

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Scalar speed limits and cosmology: Acceleration from D-cceleration

In this review we describe our current understanding of the properties of open string tachyons on an unstable D-brane or brane–antibrane system in string theory. The various string theoretic methods used for this study include techniques of two-dimensional conformal field theory, open string field theory, boundary string field theory, noncommutative solitons, etc. We also describe various attempts to understand these results using field theoretic methods. These field theory models include toy models like singular potential models and p-adic string theory, as well as more realistic version of the tachyon effective action based on Dirac–Born–Infeld type action. Finally we study closed string background produced by the "decaying" unstable D-branes, both in the critical string theory and in the two-dimensional string theory, and describe the open string completeness conjecture that emerges out of this study. According to this conjecture the quantum dynamics of an unstable D-brane system is described by an int...

Tachyon Dynamics in Open String Theory

We discuss the emergence of $ \mathcal{W} \mbox{-algebras}$
 as asymptotic symmetries of higher-spin gauge theories coupled to three-dimensional Einstein gravity with a negative cosmological constant. We focus on models involving a finite number of bosonic higher-spin fields, and especially on the example provided by the coupling of a spin-3 field to gravity. It is described by a SL(3) × SL(3) Chern-Simons theory and its asymptotic symmetry algebra is given by two copies of the classical $ {\mathcal{W}_3}\mbox{-algebra} $
 with central charge the one computed by Brown and Henneaux in pure gravity with negative cosmological constant.

/pdf/asymptotic-symmetries-of-three-dimensional-gravity-coupled-4x4zwo0492.pdf

Asymptotic symmetries of three-dimensional gravity coupled to higher-spin fields

Various holographic approaches to QCD in five dimensions are explored using input both from the putative five-dimensional non-critical string theory as well as QCD. It is argued that a gravity theory in five dimensions coupled to a dilaton and an axion may capture the important qualitative features of pure YM theory. A part of the effects of higher α'-corrections is resummed into a dilaton potential. The potential is shown to be in one-to-one correspondence with the exact β-function of QCD, and its knowledge determines the full structure of the vacuum solution. The geometry near the UV boundary is that of AdS5 with logarithmic corrections reflecting the asymptotic freedom of QCD. We find that all relevant confining backgrounds have an IR singularity of the ``good" kind that allows unambiguous spectrum computations. Near the singularity the 't Hooft coupling is driven to infinity. Asymptotically linear glueball masses can also be achieved. The classification of all confining asymptotics, the associated glueball spectra and meson dynamics are addressed in a companion paper arXiv:0707.1349

https://iopscience.iop.org/article/10.1088/1126-6708/2008/02/032/pdf

Exploring improved holographic theories for QCD: part I

We give a detailed review of the construction of gauge invariant Lagrangians for free and interacting higher spin fields using the BRST approach developed over the past few years.

Gauge Invariant Lagrangians for Free and Interacting Higher Spin Fields. A Review of the BRST formulation

We construct an off-shell extension of cubic interaction vertices between massless bosonic Higher Spin fields on a flat background which can be obtained from perturbative bosonic string theory. We demonstrate how to construct higher quartic interaction vertices using a simple particular example. We examine whether BCFW recursion relations for interacting Higher Spin theories are applicable. We argue that for several interesting examples such relations should exist, but consistency of the theories might require that we supplement Higher Spin field theories with extended and possibly non-local objects.

On the tensionless limit of string theory, off-shell higher spin interaction vertices and BCFW recursion relations

We propose a method of construction of a cubic interaction in massless higher spin gauge theory both in flat and in AdS space-times of arbitrary dimensions. We consider a triplet formulation of the higher spin gauge theory and generalize the higher spin symmetry algebra of the free model to the corresponding algebra for the case of cubic interaction. The generators of this new algebra carry indexes which label the three higher spin fields involved into the cubic interaction. The method is based on the use of oscillator formalism and on the Becchi-Rouet-Stora-Tyutin (BRST) technique. We derive general conditions on the form of cubic interaction vertex and discuss the ambiguities of the vertex which result from field redefinitions. This method can in principle be applied for constructing the higher spin interaction vertex at any order. Our results are a first step towards the construction of a Lagrangian for interacting higher spin gauge fields that can be holographically studied.

Constructing the Cubic Interaction Vertex of Higher Spin Gauge Fields

We elaborate on the proposal of flat holography in which four-dimensional physics is encoded in two-dimensional celestial conformal field theory (CCFT). The symmetry underlying CCFT is the extended BMS symmetry of (asymptotically) flat space­ time. We use soft and collinear theorems of Einstein-Yang-Mills theory to derive the OPEs of BMS field operators generating superrotations and supertranslations. The energy­ momentum tensor, given by a shadow transform of a soft graviton operator, implements superrotations in the Virasoro subalgebra of 𝔟𝔪𝔰4. Supertranslations can be obtained from a single translation generator along the light-cone direction by commuting it with the energy-momentum tensor. This operator also originates from a soft graviton and generates a flow of conformal dimensions. All supertranslations can be assembled into a single primary conformal field operator on celestial sphere.

/pdf/extended-bms-algebra-of-celestial-cft-4jno5dahjw.pdf

Extended BMS algebra of celestial CFT

We study tree-level celestial amplitudes in Yang-Mills theory — Mellin transforms of multi-gluon scattering amplitudes that convert them into the correlators of conformal primary fields on two-dimensional celestial sphere. By using purely field-theoretical methods, we show that the soft conformal limit of celestial amplitudes, in which one of the primary field operators associated to gauge bosons becomes a dimension one current, is dominated by the contributions of low-energy soft particles. This result confirms conclusions reached by using Yang-Mills theory formulated in curvilinear coordinates, as pioneered by Strominger. By using well-known collinear limits of Yang-Mills amplitudes, we derive the OPE rules for the primary fields and the holomorphic currents arising in the conformally soft limit. The Ward identities following from OPE have the same form as the identities derived by using soft theorems.

/pdf/soft-limits-of-yang-mills-amplitudes-and-conformal-4m95esnhah.pdf

Angelos Fotopoulos

Papers

Gauge Invariant Lagrangians for Free and Interacting Higher Spin Fields. A Review of the BRST formulation

On the tensionless limit of string theory, off-shell higher spin interaction vertices and BCFW recursion relations

Constructing the Cubic Interaction Vertex of Higher Spin Gauge Fields

Extended BMS algebra of celestial CFT

Soft Limits of Yang-Mills Amplitudes and Conformal Correlators