String Theory Models Research Articles

Statistical Predictions in String Theory and Deep Generative Models

AbstractGenerative models in deep learning allow for sampling probability distributions that approximate data distributions. We propose using generative models for making approximate statistical predictions in the string theory landscape. For vacua admitting a Lagrangian description this can be thought of as learning random tensor approximations of couplings. As a concrete proof‐of‐principle, we demonstrate in a large ensemble of Calabi‐Yau manifolds that Kähler metrics evaluated at points in Kähler moduli space are well‐approximated by ensembles of matrices produced by a deep convolutional Wasserstein GAN. Accurate approximations of the Kähler metric eigenspectra are achieved with far fewer than h11 Gaussian draws. Accurate extrapolation to values of h11 outside the training set are achieved via a conditional GAN. Together, these results implicitly suggest the existence of strong correlations in the data, as might be expected if Reid's fantasy is correct.

The multi-feature universe: Large parameter space cosmology and the swampland

Under the belief that the universe should be multi-feature and informative, we employ a model-by-model comparison method to explore the possibly largest upper bound on the swampland constant c. Considering the interacting quintessence dark energy as the comparison model, we constrain the large parameter space interacting dark energy model, a 12-parameter extension to the ΛCDM cosmology, in light of current observations. We obtain the largest 2σ (3σ) bound so far, c≲1.62 (1.94), which would allow the existences of a number of string theory models of dark energy such as 11-dimensional supergravity with double-exponential potential, O(16)×O(16) heterotic string and some Type II string compactifications. For inflationary models with concave potential, we find the 2σ (3σ) bound c≲0.13 (0.14), which is still in strong tension with the string-based expectation c∼O(1). However, combining Planck primordial non-Gaussianity with inflation constraints, it is interesting that the Dirac–Born–Infeld inflation with concave potential gives the 2σ (3σ) bound c≲0.53 (0.58), which is now in a modest tension with the swampland conjecture.

Probing a cosmic axion-like particle background within the jets of active galactic nuclei

Axions or more generally axion-like particles (ALPs) are pseudo-scalar particles predicted by many extensions of the Standard Model of particle physics (SM) and considered as viable candidates for dark matter (DM) in the universe. If they really exist in nature, they are expected to couple with photons in the presence of an external electromagnetic field through a form of the Primakoff effect. In addition, many string theory models of the early universe motivate the existence of a homogeneous Cosmic ALP Background (CAB) with 0.1 – 1 keV energies analogous to the Cosmic Microwave Background (CMB), arising via the decay of string theory moduli in the very early universe. The coupling between the CAB ALPs traveling in cosmic magnetic fields and photons allows ALPs to oscillate into photons and vice versa. In this work, we test the CAB model that is put forward to explain the soft X-ray excess in the Coma cluster due to CAB ALPs conversion into photons using the M87 jet environment. Then we demonstrate the potential of the active galactic nuclei (AGNs) jet environment to probe low-mass ALP models, and to potentially constrain the model proposed to better explain the Coma cluster soft X-ray excess. We find that the overall X-ray emission for the M87 AGN requires an ALP-photon coupling gaγ in the range of ∼ 7.50 × 10−15–6.56 × 10−14 GeV−1 for ALP masses ma ≲ 10−13 eV as long as the M87 jet is misaligned by less than about 20 degrees from the line of sight. These values are up to an order of magnitude smaller than the current best fit value on gaγ ∼ 2 × 10−13 GeV−1 obtained in soft X-ray excess CAB model for the Coma cluster. Our results cast doubt on the current limits of the largest allowed value of gaγ and suggest a new constraint that gaγ ≲ 6.56 × 10−14 GeV−1 when a CAB is assumed.

Background field method for nonlinear sigma models in nonrelativistic string theory

We continue the study of nonrelativistic string theory in background fields. Nonrelativistic string theory is described by a nonlinear sigma model that maps a relativistic worldsheet to a non-Lorentzian and non-Riemannian target space geometry, which is known to be string Newton-Cartan geometry. We develop the covariant background field method in this non-Riemannian geometry. We apply this background field method to compute the beta-functions of the nonlinear sigma model that describes nonrelativistic string theory on a string Newton-Cartan geometry background, in presence of a Kalb-Ramond two-form and dilaton field.

Thermal dark energy

We present a novel source of dark energy, which is motivated by the prevalence of hidden sectors in string theory models and is consistent with all of the proposed swampland conjectures. Thermal effects hold a light hidden sector scalar at a point in field space that is not a minimum of its zero temperature potential. This leads to an effective "cosmological constant", with an equation of state $w=-1$, despite the scalar's zero temperature potential having only a 4D Minkowski or AdS vacuum. For scalar masses $\lesssim \mu$eV, which could be technically natural via sequestering, there are large regions of phenomenologically viable parameter space such that the induced vacuum energy matches the measured dark energy density. Additionally, in many models a standard cosmological history automatically leads to the scalar having the required initial conditions. We study the possible observational signals of such a model, including at fifth force experiments and through $\Delta N_{\rm eff}$ measurements. Similar dynamics that are active at earlier times could resolve the tension between different measurements of $H_0$ and can lead to a detectable stochastic gravitational wave background.

Supersymmetric anti-D3-brane action in the Kachru-Kallosh-Linde-Trivedi setup

An anti-D3-brane plays a crucial role in the construction of semi-realistic cosmological models in string theory. Part of its action provides an uplift term that has been used to lift AdS solutions to phenomenologically viable dS vacua in the KKLT and LVS setups. In the last few years it has been shown that this uplift breaks supersymmetry spontaneously and can be described in the 4d $\mathcal{N}=1$ supergravity language by using constrained supermultiplets. Here we derive the complete 4d $\mathcal{N}=1$ supergravity action for an anti-D3-brane coupled to all closed string background fields. In particular we include the vector field, the scalar fields and all fermions that live on the anti-D3-brane.

Generalized uncertainty principle and corpuscular gravity

We show that the implications of the generalized uncertainty principle (GUP) in the black hole physics are consistent with the predictions of the corpuscular theory of gravity, in which a black hole is conceived as a Bose–Einstein condensate of weakly interacting gravitons stuck at the critical point of a quantum phase transition. In particular, we compute such characteristic thermodynamic quantities as the temperature and the evaporation rate of a black hole. By comparing the results obtained in the two scenarios, we are able to estimate the GUP deformation parameter beta , which turns out to be of order unity, in agreement with the expectations of some models of string theory. We also comment on the sign of beta , exploring the possibility of having a negative deformation parameter when a corpuscular quantum description of the gravitational interaction is assumed to be valid.

BPS Spectra, Barcodes and Walls

BPS spectra give important insights into the non-perturbative regimes of supersymmetric theories. Often from the study of BPS states one can infer properties of the geometrical or algebraic structures underlying such theories. In this paper we approach this problem from the perspective of persistent homology. Persistent homology is at the base of topological data analysis, which aims at extracting topological features out of a set of points. We use these techniques to investigate the topological properties which characterize the spectra of several supersymmetric models in field and string theory. We discuss how such features change upon crossing walls of marginal stability in a few examples. Then we look at the topological properties of the distributions of BPS invariants in string compactifications on compact threefolds, used to engineer black hole microstates. Finally we discuss the interplay between persistent homology and modularity by considering certain number theoretical functions used to count dyons in string compactifications and by studying equivariant elliptic genera in the context of the Mathieu moonshine.

Axion from Quivers in Type II Superstrings

AbstractWe investigate a string‐inspired axion extension of the standard model obtained from Type II superstrings using quiver method. In the first part, we discuss intersecting Type IIA D6‐branes wrapping non trivial 3‐cycles in the presence of the Peccei‐Quinn symmetry U(1). Concretely, a complex scalar field , where σ is a stringy axion generates a general fermion Yukawa coupling weighted by a flavor‐dependent power taking specific values. Using string theory and standard model data with adequate scales, we show that the corresponding axion window could lie in the allowed range matching with the recent cosmological results.

Inflation as an information bottleneck: a strategy for identifying universality classes and making robust predictions

In this work we propose a statistical approach to handling sources of theoretical uncertainty in string theory models of inflation. By viewing a model of inflation as a probabilistic graph, we show that there is an inevitable information bottleneck between the ultraviolet input of the theory and observables, as a simple consequence of the data processing theorem. This information bottleneck can result in strong hierarchies in the sensitivity of observables to the parameters of the underlying model and hence universal predictions with respect to at least some microphysical considerations. We also find other intriguing behaviour, such as sharp transitions in the predictions when certain hyperparameters cross a critical value. We develop a robust numerical approach to studying these behaviours by adapting methods often seen in the context of machine learning. We first test our approach by applying it to well known examples of universality, sharp transitions, and concentration phenomena in random matrix theory. We then apply the method to inflation with axion monodromy. We find universality with respect to a number of model parameters and that consistency with observational constraints implies that with very high probability certain perturbative corrections are non-negligible.

The Landscape, the Swampland and the Era of Precision Cosmology

AbstractWe review the advanced version of the KKLT construction and pure de Sitter supergravity, involving a nilpotent multiplet, with regard to various conjectures that de Sitter state cannot exist in string theory. We explain why we consider these conjectures problematic and not well motivated, and why the recently proposed alternative string theory models of dark energy, ignoring vacuum stabilization, are ruled out by cosmological observations at least at the 3σ level, i.e. with more than 99.7% confidence.

Quantum No-Scale Regimes and String Moduli

We review that in no-scale models in perturbative string theory, flat, homogeneous and isotropic cosmological evolutions found at the quantum level can enter into “quantum no-scale regimes” (QNSRs). When this is the case, the quantum effective potential is dominated by the classical kinetic energies of the no-scale modulus, dilaton and moduli not involved in the supersymmetry breaking. As a result, the evolutions approach the classical ones, where the no-scale structure is exact. When the one-loop potential is positive, a global attractor mechanism forces the initially expanding solutions to enter the QNSR describing a flat, ever-expanding universe. On the contrary, when the potential can reach negative values, the internal moduli induce in most cases some kind of instability of the growing universe. The latter stops expanding and eventually collapses, unless the initial conditions are tuned in a tiny region of the phase space. Finally, in QNSR, no gauge instability takes place, regardless of the details of the potential.

Nonrelativistic string theory and T-duality

Nonrelativistic string theory in flat spacetime is described by a two-dimensional quantum field theory with a nonrelativistic global symmetry acting on the worldsheet fields. Nonrelativistic string theory is unitary, ultraviolet complete and has a string spectrum and spacetime S-matrix enjoying nonrelativistic symmetry. The worldsheet theory of nonrelativistic string theory is coupled to a curved spacetime background and to a Kalb-Ramond two-form and dilaton field. The appropriate spacetime geometry for nonrelativistic string theory is dubbed string Newton-Cartan geometry, which is distinct from Riemannian geometry. This defines the sigma model of nonrelativistic string theory describing strings propagating and interacting in curved background fields. We also implement T-duality transformations in the path integral of this sigma model and uncover the spacetime interpretation of T-duality. We show that T-duality along the longitudinal direction of the string Newton-Cartan geometry describes relativistic string theory on a Lorentzian geometry with a compact lightlike isometry, which is otherwise only defined by a subtle infinite boost limit. This relation provides a first principles definition of string theory in the discrete light cone quantization (DLCQ) in an arbitrary background, a quantization that appears in nonperturbative approaches to quantum field theory and string/M-theory, such as in Matrix theory. T-duality along a transverse direction of the string Newton-Cartan geometry equates nonrelativistic string theory in two distinct, T-dual backgrounds.

Supersymmetric embedding of antibrane polarization

We study the supersymmetry breaking induced by probe anti-D3-branes at the tip of the Klebanov-Strassler throat geometry. Antibranes inside this geometry polarize and can be described by an NS5-brane wrapping an $S^2$. When the number of antibranes is small compared to the background flux a metastable state exists that breaks supersymmetry. We present a manifestly supersymmetric effective model that realizes the polarized metastable state as a solution, spontaneously breaking the supersymmetry. The supersymmetric model relies crucially on the inclusion of Kaluza-Klein (matrix) degrees of freedom on the $S^2$ and two supersymmetric irrelevant deformations of ${\cal N}=4$ super-Yang-Mills (SYM), describing a large number of supersymmetric D3-branes in the IR. We explicitly identify the massless Goldstino and compute the spectrum of massive fluctuations around the metastable supersymmetry-breaking minimum, finding a Kaluza-Klein tower with masses warped down from the string scale. Below the Kaluza-Klein scale the massive tower can be integrated out and supersymmetry is realized nonlinearly. We comment on the effect of the Kaluza-Klein modes on the effective description of de Sitter vacua in string theory and inflationary model building.

DS Supergravity from 10d

AbstractWe consider flux compactification of type II string theory with local sources on SU(3)‐structure manifolds. By adding pseudo‐calibrated anti‐‐branes wrapped on supersymmetric cycles we generalize all existing models so that the effective , supergravity now includes a nilpotent multiplet. We present a new dictionary between string theory models and Kähler potential K and superpotential W for these dS supergravities with a nilpotent multiplet and non‐linearly realized local supersymmetry. In addition to KKLT and LVS with uplifting ‐branes, we have now new models with uplifting , , , ‐branes. The new uplifting contribution to the supergravity potential due to Volkov‐Akulov supersymmetry is universal. As one application of our general result, we study classical flux compactifications of type IIA supergravity and find that a previously discovered universal tachyon is now absent.

Field redefinitions and K\xe4hler potential in string theory at 1-loop

Field redefinitions at string 1-loop order are often required by supersymmetry, for instance in order to make the Kähler structure of the scalar kinetic terms manifest. We derive the general structure of the field redefinitions and the Kähler potential at string 1-loop order in a certain class of string theory models (4-dimensional toroidal type IIB orientifolds with mathcal{N} = 1 supersymmetry) and for a certain subsector of fields (untwisted Kähler moduli and the 4-dimensional dilaton). To do so we make use of supersymmetry, perturbative axionic shift symmetries and a particular ansatz for the form of the 1-loop corrections to the metric on the moduli space. Our results also show which terms in the low-energy effective action have to be calculated via concrete string amplitudes in order to fix the values of the coefficients (in the field redefinitions and the Kähler potential) that are left undetermined by our general analysis based on (super)symmetry.

Towards the Raychaudhuri equation beyond general relativity

In general relativity, gravity is universally attractive, a feature embodied by the Raychaudhuri equation which requires that the expansion of a congruence of geodesics is always non-increasing, as long as matter obeys the strong or weak energy conditions. This behavior of geodesics is an important ingredient in general proofs of singularity theorems, which show that many spacetimes are singular in the sense of being geodesically incomplete and suggest that general relativity is itself incomplete. It is possible that alternative theories of gravity, which reduce to general relativity in some limit, can resolve these singularities, so it is of interest to consider how the behavior of geodesics is modified in these frameworks. We compute the leading corrections to the Raychaudhuri equation for the expansion due to models in string theory, braneworld gravity, $f(R)$ theories, and loop quantum cosmology, for cosmological and black hole backgrounds, and show that while in most cases geodesic convergence is reinforced, in a few cases terms representing repulsion arise, weakening geodesic convergence and thereby the conclusions of the singularity theorems.

The refined Swampland Distance Conjecture in Calabi-Yau moduli spaces

The Swampland Distance Conjecture claims that effective theories derived from a consistent theory of quantum gravity only have a finite range of validity. This will imply drastic consequences for string theory model building. The refined version of this conjecture says that this range is of the order of the naturally built in scale, namely the Planck scale. It is investigated whether the Refined Swampland Distance Conjecture is consistent with proper field distances arising in the well understood moduli spaces of Calabi-Yau compactification. Investigating in particular the non-geometric phases of Kähler moduli spaces of dimension h11 ∈ {1, 2, 101}, we always find proper field distances that are smaller than the Planck-length.

SO(32) heterotic line bundle models

We search for the three-generation standard-like and/or Pati-Salam models from the SO(32) heterotic string theory on smooth, quotient complete intersection Calabi-Yau threefolds with multiple line bundles, each with structure group U(1). These models are S- and T-dual to intersecting D-brane models in type IIA string theory. We find that the stable line bundles and Wilson lines lead to the standard model gauge group with an extra U(1)B−L via a Pati-Salam-like symmetry and the obtained spectrum consists of three chiral generations of quarks and leptons, and vector-like particles. Green-Schwarz anomalous U(1) symmetries control not only the Yukawa couplings of the quarks and leptons but also the higher-dimensional operators causing the proton decay.

Searching for Axion-Like Particles with X-ray Polarimeters

X-ray telescopes are an exceptional tool for searching for new fundamental physics. In particular, X-ray observations have already placed world-leading bounds on the interaction between photons and axion-like particles (ALPs). ALPs are hypothetical new ultra-light particles motivated by string theory models. They can also act as dark matter and dark energy, and provide a solution to the strong CP problem. In a background magnetic field, ALPs and photons may interconvert. This leads to energy dependent modulations in both the flux and polarisation of the spectra of point sources shining through large magnetic fields. The next generation of polarising X-ray telescopes will offer new detection possibilities for ALPs. Here we present techniques and projected bounds for searching for ALPs with X-ray polarimetry. We demonstrate that upcoming X-ray polarimetry missions have the potential to place world-leading bounds on ALPs.