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We argue that the wellknown beta functions of quadratic gravity do not correspond to the physical dependence of scattering amplitudes on external momenta, and derive the correct physical beta functions. Asymptotic freedom turns out to be compatible with the absence of tachyons.more » « lessFree, publiclyaccessible full text available July 1, 2025

The scalar and tensor fluctuations produced during inflation can be correlated, if arising from the same underlying mechanism. In this paper we investigate such correlation in the model of axion inflation, where the rolling inflaton produces quanta of a U(1) gauge field which, in turn, source scalar and tensor fluctuations. We compute the primordial correlator of the curvature perturbation, ζ, with the amplitude of the gravitational waves squared, hijhij, at frequencies probed by gravitational wave detectors. This twopoint function receives two contributions: one arising from the correlation of gravitational waves with the scalar perturbations generated by the standard mechanism of amplification of vacuum fluctuations, and the other coming from the correlation of gravitational waves with the scalar perturbations sourced by the gauge field. Our analysis shows that the latter effect is generally dominant. The correlator, normalized by the amplitude of ζ and of hijhij, turns out to be of the order of 10−2×(fequilNL)1/3, where fequilNL measures the scalar bispectrum sourced by the gauge modes.more » « lessFree, publiclyaccessible full text available March 5, 2025

We explore the properties of a simple renormalizable shiftsymmetric model with a higherderivative kinetic energy and quarticderivative coupling that can serve as a toy model for higherderivative theories of gravity. The scattering amplitude behaves as in a normal effective field theory below the threshold for the production of ghosts, but has an unexpectedly soft behavior above the threshold. The physical running of the parameters is extracted from the twopoint and fourpoint amplitudes. The results are compared to those obtained by other methods and are found to agree only in limiting cases. We draw several lessons that may also apply to gravity.more » « lessFree, publiclyaccessible full text available February 1, 2025

Standard axion electrodynamics has two closely related features. First, the coupling of a massless axion field to photons is quantized, in units proportional to the electric gauge coupling squared. Second, the equations of motion tell us that a timedependent axion field in a background magnetic field sources an effective electric current, but a timedependent axion field in a background electric field has no effect. These properties, which manifestly violate electricmagnetic duality, play a crucial role in experimental searches for axions. Recently, electricmagnetic duality has been used to motivate the possible existence of nonstandard axion couplings, which can both violate the usual quantization rule and exchange the roles of electric and magnetic fields in axion electrodynamics. We show that these nonstandard couplings can be derived from SL(2,ℤ) duality, but that they come at a substantial cost: in nonstandard axion electrodynamics, all electrically charged particles become dyons when the axion traverses its field range, in a dual form of the standard Witten effect monodromy. This implies that there are dyons near the weak scale, leads to a large axion mass induced by Standard Model fermion loops, and dramatically alters Higgs physics. We conclude that nonstandard axion electrodynamics, although interesting to consider in abstract quantum field theory, is not phenomenologically viable.more » « lessFree, publiclyaccessible full text available January 1, 2025

The Sharpened Distance Conjecture and Tower Scalar Weak Gravity Conjecture are closely related but distinct conjectures, neither one implying the other. Motivated by examples, I propose that both are consequences of two new conjectures: 1. The infinite distance geodesics passing through an arbitrary point ϕ in the moduli space populate a dense set of directions in the tangent space at ϕ. 2. Along any infinite distance geodesic, there exists a tower of particles whose scalarchargetomass ratio (–∇log m) projection everywhere along the geodesic is greater than or equal to 1/√(d2). I perform several nontrivial tests of these new conjectures in maximal and halfmaximal supergravity examples. I also use the Tower Scalar Weak Gravity Conjecture to conjecture a sharp bound on exponentially heavy towers that accompany infinite distance limits.more » « lessFree, publiclyaccessible full text available January 1, 2025

The coupling between a pseudoscalar inflaton and a gauge field leads to an amount of additional density perturbations and gravitational waves (GWs) that is strongly sensitive to the inflaton speed. This naturally results in enhanced GWs at (relatively) small scales that exited the horizon well after the CMB ones, and that can be probed by a variety of GW observatories (from pulsar timing arrays, to astrometry, to spaceborne and groundbased interferometers). This production occurs in a regime in which the gauge field significantly backreacts on the inflaton motion. Contrary to earlier assumptions, it was later shown that this regime is characterized by an oscillatory behavior of the inflaton speed, with a period of O ( 5 ) efolds. Bursts of GWs are produced at the maxima of the speed, imprinting nearly periodic bumps in the frequencydependent spectrum of GWs produced during inflation. This can potentially generate correlated peaks appearing in the same or in different GWs experiments. While recent lattice studies show that the inclusion of inflaton gradients can modify significantly the dynamics of this system in the strong backreaction regime, this is not the case for the first oscillation or two of the inflaton speed, so that we expect our results to be robust for modes that were excited during that epoch.more » « lessFree, publiclyaccessible full text available January 1, 2025

We study towers of light particles that appear in infinitedistance limits of moduli spaces of 9dimensional 𝒩=1 string theories, some of which notably feature decompactification limits with running string coupling. The lightest tower in such decompactification limits consists of the nonBPS KaluzaKlein modes of Type I′ string theory, whose masses depend nontrivially on the moduli of the theory. We work out the modulidependence by explicit computation, finding that despite the running decompactification the Distance Conjecture remains satisfied with an exponential decay rate ⍺ ≥ 1/√(d2) in accordance with the sharpened Distance Conjecture. The related sharpened Convex Hull Scalar Weak Gravity Conjecture also passes stringent tests. Our results nontrivially test the Emergent String Conjecture, while highlighting the important subtlety that decompactifcation can lead to a running solution rather than to a higherdimensional vacuum.more » « lessFree, publiclyaccessible full text available December 1, 2024

We derive formulas for the leading mass, entropy, and longrange selfforce corrections to extremal black holes due to higherderivative operators. These formulas hold for black holes with arbitrary couplings to gauge fields and moduli, provided that the leadingorder solutions are static, sphericallysymmetric, extremal, and have nonzero horizon area. To use these formulas, both the leadingorder black hole solution and the higherderivative effective action must be known, but there is no need to solve the derivativecorrected equations of motion. We demonstrate that the mass, entropy and selfforce corrections involve linearlyindependent combinations of the higherderivative couplings at any given point in the moduli space, and comment on their relations to various swampland conjectures.more » « lessFree, publiclyaccessible full text available December 1, 2024

We present a method to construct the extended Kähler cone of any CalabiYau threefold by using GopakumarVafa invariants to identify all geometric phases that are related by flops or Weyl reflections. In this way we obtain the Kähler moduli spaces of all favorable CalabiYau threefold hypersurfaces with h1,1 ≤ 4, including toric and nontoric phases. In this setting we perform an explicit test of the Weak Gravity Conjecture by using the GopakumarVafa invariants to count BPS states. All of our examples satisfy the tower/sublattice WGC, and in fact they even satisfy the stronger lattice WGC.more » « lessFree, publiclyaccessible full text available December 1, 2024

Axion inflation coupled to Abelian gauge fields via a ChernSimonslike term of the formmore » « less
represents an attractive inflationary model with a rich phenomenology, including the production of magnetic fields, black holes, gravitational waves, and the matterantimatter asymmetry. In this work, we focus on a particular regime of axion inflation, the socalled AnberSorbo (AS) solution, in which the energy loss in the gaugefield production provides the dominant source of friction for the inflaton motion. We revisit the AS solution and confirm that it is unstable. Contrary to earlier numerical works that attempted to reach the AS solution starting from a regime of weak backreaction, we perform, for the first time, a numerical evolution starting directly from the regime of strong backreaction. Our analysis strongly suggests that, at least as long as one neglects spatial inhomogeneities in the inflaton field, the AS solution has no basin of attraction, not even a very small one that might have been missed in previous numerical studies. Our analysis employs an arsenal of analytical and numerical techniques, some established and some newly introduced, including (1) linear perturbation theory along the lines of ref. [1], (2) the gradient expansion formalism (GEF) developed in ref. [2], (3) a new linearized version of the GEF, and (4) the standard modebymode approach in momentum space in combination with input from the GEF. All these methods yield consistent results confirming the instability of the AS solution, which renders the dynamics of axion inflation in the strongbackreaction regime even more interesting than previously believed.$$ \phi F\overset{\sim }{F} $$ $\mathrm{\varphi F}\stackrel{~}{F}$