arXiv subjects

Andrew R. Frey

Publications and source records attributed to Andrew R. Frey.

At least 19 recordsLinked to original sources

arXiv2026

Quadratic Axion Couplings in String Theory

Axions and axion-like particles are a compelling candidate for physics beyond the standard model. While many axion searches are focused on the linear coupling to photons $θF \tilde{F}$, the possibility of a quadratic coupling to the electromagnetic kinetic term, $θ^2 F^2$, leads to novel phenomenology and new opportunities for testing axion-like particles. In this work we propose mechanisms for generating this coupling in string theory, which can be broadly classified as classical, perturbative, and non-perturbative. In benchmark examples, we find that both perturbative and non-perturbative quantum contributions such as instantons lead to couplings that are suppressed, $g \ll 1$ in units of $1/f^2$ where $f$ is axion decay constant, though easily larger than analogous coupling of the QCD axion that is generated through loops of charged pions. These analyses suggest that quadratic axion couplings to gauge fields are ubiquitous in string theory, and should be taken seriously as a probe of the string theory axiverse, both of string theory candidates for dynamical axions, such as dark matter or dark energy, and for spectroscopy of the string theory axiverse.

hep-th↗

arXiv2026

Holographic complexity of the Klebanov-Strassler background

We study the complexity of the gravity dual to the confining $SU(N)\times SU(N+M)$ Klebanov-Strassler gauge theory, which is an important test case for holographic complexity in higher-dimensional and nonconformal gauge/gravity dualities. We emphasize the dependence of the complexity on parameters of the gauge theory, finding a common behavior with confinement scale for several complexity functionals. We also analyze how the complexity diverges with the UV cut off, which is more complicated than in AdS backgrounds because the theory is nonconformal. Our results may provide new perspectives on questions in the holographic complexity program as well as a starting point for further studies of complexity in general gauge/gravity dualities.

hep-th↗

arXiv2026

Toward an Effective Theory of the Volume Modulus

We investigate the 4-dimensional effective theory of the warped volume modulus in the presence of stabilizing effects from gaugino condensation by analyzing the linearized 10-dimensional supergravity equations of motion. Warping is generally expected to scale down the masses of bulk modes to the IR scale at the tip of a throat. We find that the mass of the warped volume modulus evades expectations and is largely insensitive to the effects of warping, even in strongly warped backgrounds. Instead, the mass is parametrically tied to the 4-dimensional AdS curvature scale $m^2 \sim {\mathcal O}(1) |\hat R_{\rm AdS}|$, presenting a challenge for scale separation in these backgrounds. We trace this effect to a universal contribution arising from the 10-dimensional equations of motion, and comment on the importance of a 10-dimensional treatment of the warped volume modulus for effective field theories and model building.

hep-th↗

arXiv2025

Dimensional Reduction and Kähler Metric for Metric Moduli in Imaginary Self-Dual Flux

Understanding which effective field theories are consistent with an ultraviolet completion in quantum gravity is an important theoretical question. Therefore, it is important to know the structure of the 4D effective theory associated with a given compactification of string theory. We present a first-principles derivation of the low-energy 4D effective theory of geometric moduli in a warped Calabi-Yau compactification of type IIB string theory with imaginary self-dual 3-form flux. This completes the derivation of the metric on Kähler moduli space from the 10D equations of motion. We also give the first derivation of an effective action for flat directions in the complex structure moduli space of the Calabi-Yau (which generically mix with the axiodilaton) and work out explicit examples of complex structure flat directions in toroidal compactifications. Finally, we outline applications to a variety of settings, including precision string phenomenology and the tadpole conjecture.

hep-th↗

arXiv2025

Holographic Complexity in String and M Theory

Thus far, the literature regarding holographic complexity almost entirely focuses on the context of $(d+1)$-dimensional anti-de Sitter spacetime rather than the full higher-dimensional gauge/gravity duality in string or M theory. We provide a framework to study holographic complexity in the full duality, explaining the relation of complexity functionals in the higher-dimensional theory to those in anti-de Sitter spacetime and when complexity functionals can apply universally to gauge/gravity dualities rather than a specific dual pair. We also show that gauge invariance constrains boundary terms for complexity functionals, using the ten- and eleven-dimensional supergravity actions as key examples. Finally, we propose new universal complexity functionals following these considerations, including a revised gauge-invariant action complexity.

hep-th↗

arXiv2024

Gravitational Waves from High Temperature Strings

We study finite temperature effects in string cosmology and their potential gravitational wave signature. Expanding on our recent work arXiv:2310.11494 , we consider a general configuration of highly excited open and closed strings at high enough temperature to be in the Hagedorn phase in 3+1 dimensions, in order to explore its cosmological implications. We find conditions, which can be satisfied in compactifications with moduli stabilization, that allow the long strings to remain in equilibrium in a controlled effective field theory, with equilibration driven by the joining and splitting of the dominant open string population. We calculate the emission rate of gravitons by long open strings, which we show is determined by ten dimensional flat space transition amplitudes available in the literature, and then find the total gravitational wave spectrum generated by the gas of long strings. The gravitational wave spectrum has robust characteristics. It peaks at frequencies of order 50-100 GHz, the same as for gravitational waves from the reheating epoch of the Standard Model. But the amplitude of the string signal is significantly larger than predicted by the Standard Model and its field theoretic extensions. The amplitude and other physical observables (such as the contribution to $ΔN_{\text eff}$) are directly proportional to the string scale $M_s$; indicating that a potential signal may also determine the string scale. Our calculations provide one of the few examples of a signal of stringy origin that dominates over the field theory predictions. We give a physical explanation of our results and discuss further implications.

hep-th↗

arXiv2024

String Thermodynamics In and Out of Equilibrium: Boltzmann Equations and Random Walks

We revisit the study of string theory close to the Hagedorn temperature with the aim towards cosmological applications. We consider interactions of open and closed strings in a gas of D$p-$branes, and/or one isolated D$p$-brane, in an arbitrary number $d$ of flat non-compact dimensions and general compact dimensions. Leading order string perturbation theory is used to obtain the basic interaction rates in a flat background, which are shown to be consistent with the random walk picture of highly excited strings that should apply in more general backgrounds. Using the random walk interpretation we infer the structure of more general semi-inclusive string scattering rates and then write down the corresponding Boltzmann equations describing ensembles of highly excited closed and open strings. We organise the interaction terms in Boltzmann equations so that detailed balance becomes manifest. We obtain the equilibrium solutions and show that they reduce to previously computed solutions for $d=0$. We further study the behaviour of non-equlibrium fluctuations and find explicit analytic expressions for the equilibration rates (and for the number of open strings in $d=0$). Potential implications for an early universe with strings at high temperatures are outlined.

hep-th↗

arXiv2024

Complexity, scaling, and a phase transition

We investigate the holographic complexity of CFTs compactified on a circle with a Wilson line, dual to magnetized solitons in AdS$_4$ and AdS$_5$. These theories have a confinement-deconfinement phase transition as a function of the Wilson line, and the complexity of formation acts as an order parameter for this transition. Through explicit calculation, we show that proposed complexity functionals based on volume and action obey a scaling relation with radius of the circle and further prove that a broad family of potential complexity functionals obeys this scaling behavior. As a result, we conjecture that the scaling law applies to the complexity of conformal field theories on a circle in more general circumstances.

hep-th↗

arXiv2022

Dark Radiation and the Hagedorn Phase

We point out that if the sector associated with the Standard Model degrees of freedom entered an open string Hagedorn phase in the early universe while the dark radiation sector was not part of this plasma, then this can lead to low values of the observable $ΔN_{\text{eff}}$ (number of additional neutrinolike species) from the dark radiation. For explicit analysis, we focus on warped string compactifications with the Standard Model degrees of freedom at the bottom of a warped throat. If the Hubble scale during inflation is above the warped string scale associated with the throat, then the Standard Model sector will enter the Hagedorn phase. In this scenario, bulk axions are no longer dangerous from the point of view of dark radiation. While this article focuses on warped compactifications, the basic idea can be relevant to any scenario where the early universe entered a Hagedorn phase.

hep-th↗

arXiv2022

Complexity of Scalar Collapse in Anti-de Sitter Spacetime

We calculate the volume and action forms of holographic complexity for the gravitational collapse of scalar field matter in asymptotically anti-de Sitter spacetime, using numerical methods to reproduce the geometry responding to the oscillating field over multiple crossing times. Like the scalar field pulse, the volume complexity oscillates quasiperiodically before horizon formation. It also shows a scaling symmetry with the amplitude of the scalar field. The action complexity is also quasiperiodic with spikes of increasing amplitude.

hep-th↗

arXiv2020

Dirac branes for Dirichlet branes: Supergravity actions

Nontrivial Bianchi identities with local magnetic sources are solved by recognizing that gauge potentials are sections rather than globally defined functions, but properly accounting for the source degrees of freedom requires a modification of the field strength. Following work by Teitelboim and by Cariglia and Lechner, we extend Dirac's string formalism for monopoles to D-branes in type IIA and IIB string theory. We give novel derivations of brane-induced Chern-Simons terms in the supergravity actions, including a prescription for integrating over potentials in the presence of magnetic sources. We give a noncovariant formulation of the IIB theory, keeping only the independent degrees of freedom of the self-dual 4-form potential. Finally, it is well-known that D8-branes source the mass parameter of IIA supergravity; we show that the additional couplings of the massive IIA supergravity, including on other D-brane worldvolumes, are a consequence of the corresponding Dirac branes.

hep-th↗

arXiv2020

Phase Diagram of Stability for Massive Scalars in Anti-de Sitter Spacetime

We diagram the behavior of 5-dimensional anti-de Sitter spacetime against horizon formation in the gravitational collapse of a scalar field, treating the scalar field mass and width of initial data as free parameters, which we call the stability phase diagram. We find that the class of stable initial data becomes larger and shifts to smaller widths as the field mass increases. In addition to classifying initial data as stable or unstable, we identify two other classes based on nonperturbative behavior. The class of metastable initial data forms a horizon over longer time scales than suggested by the lowest order perturbation theory at computationally accessible amplitudes, and irregular initial data can exhibit non-monotonic and possibly chaotic behavior in the horizon formation times. Our results include evidence for chaotic behavior even in the collapse of a massless scalar field.

hep-th↗

arXiv2018

Variations on the Dirac string

Dirac's original solution of the nontrivial Bianchi identity for magnetic monopoles [Dirac 1948], which redefines the fieldstrength along the Dirac string, diagonalizes the gauge and monopole degrees of freedom. We provide a variant of the Dirac string, which we motivate through a formal expansion of the Bianchi identity. We show how to use our variant prescription to study monopole electrodynamics without reference to a dual potential and provide some applications.

hep-th↗

arXiv2016

Dimensional Reduction for D3-brane Moduli

Warped string compactifications are central to many attempts to stabilize moduli and connect string theory with cosmology and particle phenomenology. We present a first-principles derivation of the low-energy 4D effective theory from dimensional reduction of a D3-brane in a warped Calabi-Yau compactification of type IIB string theory with imaginary self-dual 3-form flux, including effects of D3-brane motion beyond the probe approximation, and find the metric on the moduli space of brane positions, the universal volume modulus, and axions descending from the 4-form potential. As D3-branes may be considered as carrying either electric or magnetic charges for the self-dual 5-form field strength, we present calculations in both duality frames. Our results are consistent with, but extend significantly, earlier results on the low-energy effective theory arising from D3-branes in string compactifications.

hep-th↗

arXiv2016

Black Hole Formation in AdS Einstein-Gauss-Bonnet Gravity

AdS spacetime has been shown numerically to be unstable against a large class of arbitrarily small perturbations. In arXiv:1410.1869, the authors presented a preliminary study of the effects on stability of changing the local dynamics by adding a Gauss-Bonnet term to the Einstein action. Here we provide further details as well as new results with improved numerical methods. In particular, we elucidate new structure in Choptuik scaling plots. We also provide evidence of chaotic behavior at the transition between immediate horizon formation and horizon formation after the matter pulse reflects from the AdS conformal boundary. Finally, we present data suggesting the formation of naked singularities in spacetimes with ADM mass below the algebraic bound for black hole formation.

hep-th↗

arXiv2016

Classes of Stable Initial Data for Massless and Massive Scalars in Anti-de Sitter Spacetime

Since horizon formation in global anti-de Sitter spacetime is dual to thermalization of a conformal field theory on a compact space, whether generic initial data is stable or unstable against gravitational collapse is of great interest. We argue that all the known stable initial data for massless scalars are dominated by single scalar eigenmodes, specifically providing strong numerical evidence consistent with the interpretation that initial data with equal energies in two modes collapse on time scales of order the inverse square of the amplitude. We further scan the parameter space for massive scalar field initial data and present evidence for a novel class of stable or quasi-stable solutions for massive scalars with energy spread through several eigenmodes.

hep-th↗

arXiv2015

Consistency of dark matter interpretations of the 3.5 keV X-ray line

Tentative evidence of a 3.5 keV X-ray line has been found in the stacked spectra of galaxy clusters, individual clusters, the Andromeda galaxy and the galactic center, leading to speculation that it could be due to decays of metastable dark matter such as sterile neutrinos. However searches for the line in other systems such as dwarf satellites of the Milky Way have given negative or ambiguous results. We reanalyze both the positive and negative searches from the point of view that the line is due to inelastic scattering of dark matter to an excited state that subsequently decays---the mechanism of excited dark matter (XDM). Unlike the metastable dark matter scenario, XDM gives a stronger signal in systems with higher velocity dispersions, such as galaxy clusters. We show that the predictions of XDM can be consistent with null searches from dwarf satellites, while the signal from the closest individual galaxies can be detectable having a flux consistent with that from clusters. We discuss the impact of our new fits to the data for two specific realizations of XDM.

astro-ph.CO↗

arXiv2014

Nonabelian dark matter models for 3.5 keV X-rays

A recent analysis of XXM-Newton data reveals the possible presence of an X-ray line at approximately 3.55 keV, which is not readily explained by known atomic transitions. Numerous models of eV-scale decaying dark matter have been proposed to explain this signal. Here we explore models of multicomponent nonabelian dark matter with typical mass ~ 1-10 GeV (higher values being allowed in some models) and eV-scale splittings that arise naturally from the breaking of the nonabelian gauge symmetry. Kinetic mixing between the photon and the hidden sector gauge bosons can occur through a dimension-5 or 6 operator. Radiative decays of the excited states proceed through transition magnetic moments that appear at one loop. The decaying excited states can either be primordial or else produced by upscattering of the lighter dark matter states. These models are significantly constrained by direct dark matter searches or cosmic microwave background distortions, and are potentially testable in fixed target experiments that search for hidden photons. We note that the upscattering mechanism could be distinguished from decays in future observations if sources with different dark matter velocity dispersions seem to require different values of the scattering cross section to match the observed line strengths.

hep-ph↗