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Dieter Lust

Publications and source records attributed to Dieter Lust.

At least 37 records · Page 2Linked to original sources
arXiv2024

String dualities and modular symmetries in supergravity: a review

We review the remarkable interplay between modular symmetries and supergravity, which has led to major advances in both physics and mathematics in recent decades. Our focus will be on four-dimensional models with $\mathcal{N}=1$ and $\mathcal{N}=2$ local supersymmetry. We will look at the early articles on the topic, but also touch on recent developments. These results testify to how supergravity, when supplemented with the appropriate assumptions, can be turned into a unique window into quantum gravity.

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arXiv2024

Through the Looking Glass into the Dark Dimension: Searching for Bulk Black Hole Dark Matter with Microlensing of X-ray Pulsars

Primordial black holes (PBHs) hidden in the incredible bulk of the dark dimension could escape constraints from non-observation of their Hawking radiation. Since these five-dimensional (5D) PBHs are bigger, colder, and longer-lived than usual 4D PBHs of the same mass $M$, they could make all cosmological dark matter if $10^{11} \lesssim M/{\rm g} \lesssim 10^{21}$, i.e., extending the 4D allowed region far down the asteroid-mass window. We show that these evasive PBHs could be search for by measuring their $X$-ray microlensing events from faraway pulsars. We also show that future $X$-ray microlensing experiments will be able to probe the interesting range ($10^{16.5} \lesssim M/{\rm g} \lesssim 10^{17.5}~{\rm g}$) where an all dark matter interpretation in terms of 4D Schwarzschild PBHs is excluded by the non-observation of their Hawking radiation.

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arXiv2024

Bulk Black Hole Dark Matter

The dark dimension provides a mechanism to resolve the cosmological hierarchy problem and assembles a colosseum for dark matter contenders. In a series of recent publications we investigated whether primordial black holes (PBHs) perceiving the dark dimension could constitute all of the dark matter in the universe. A key assumption of these investigations is that PBHs remain confined to the brane during the entire evaporation process. As a consequence, the abundance of PBHs evaporating at the present epoch is severely constrained by observations of both the extragalactic and Galactic $γ$-ray backgrounds. As a natural outgrowth of these investigations, herein we relax the assumption of brane localized PBHs and reexamine the evaporation process of PBHs which are allowed to escape into the dark dimension. We show that the escape of PBHs from the brane is almost instantaneous. Armed with this pivotal finding we reassess the allowed mass range of PBHs to assemble all cosmological dark matter, which is estimated to be $10^{11} \lesssim M_{\rm BH}/{\rm g} \lesssim 10^{21}$.

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arXiv2024

From infinite to infinitesimal: Using the Universe as a dataset to probe Casimir corrections to the vacuum energy from fields inhabiting the dark dimension

Promptly after high-resolution experiments harbinger the field of precision cosmology low- and high-redshift observations abruptly gave rise to a tension in the measurement of the present-day expansion rate of the Universe ($H_0$) and the clustering of matter ($S_8$). The statistically significant discrepancies between the locally measured values of $H_0$ and $S_8$ and the ones inferred from observations of the cosmic microwave background assuming the canonical $Λ$ cold dark matter (CDM) cosmological model have become a new cornerstone of theoretical physics. $Λ_s$CDM is one of the many beyond Standard Model setups that have been proposed to simultaneously resolve the cosmological tensions. This setup relies on an empirical conjecture, which postulates that $Λ$ switched sign (from negative to positive) at a critical redshift $z_c \sim 2$. We reexamine a stringy model that can describe the transition in the vacuum energy hypothesized in $Λ_s$CDM. The model makes use of the Casimir forces driven by fields inhabiting the incredible bulk of the dark dimension scenario. Unlike the $Λ_s$CDM setup the model deviates from $Λ$CDM in the early universe due to the existence of relativistic neutrino-like species. Using the Boltzmann solver CLASS in combination with MontePython we confront predictions of the stringy model to experimental data (from the Planck mission, Pantheon+ supernova type Ia, BAO, and KiDS-1000). We show that the string-inspired model provides a satisfactory fit to the data and can resolve the cosmological tensions.

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arXiv2024

Dark dimension with (little) strings attached

We motivate a relation between dark energy and the scale of new physics in weakly coupled string theory. This mixing between infrared and ultraviolet physics leads to a unique corner for real-world phenomenology: barring fine-tunings, we are naturally led to the ``dark dimension'' scenario, a single mesoscopic extra dimension of micron size with the standard model localized on D-branes. Our explicit top-down worldsheet derivation establishes it on a more solid grounding. Allowing some fine-tuning, such that the vacuum energy only arise at higher orders in string perturbation theory, the ``little string theory'' scenario with a very weakly coupled string is an alternative possibility. In this case, the string scale lies at the edge of detectability of particle accelerators.

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arXiv2024

Matching the Vilkovisky-DeWitt Effective Action of Quantum Gravity to String Theory

In this work, we discuss the matching of the Vilkovisky-DeWitt effective action of quantum gravity to an example of an ultra-violet complete theory of quantum gravity. We show how this matching enables a calculation of the local Wilson coefficients of the effective action. We provide several examples in string theory. Moreover, we discuss the matching conditions within the context of the swampland program.

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arXiv2024

Shedding black hole light on the emergent string conjecture

Asymptotically massless towers of species are ubiquitous in the string landscape when infinite-distance limits are approached. Due to the remarkable properties of string dualities, they always comprise Kaluza-Klein states or higher-spin excitations of weakly coupled, asymptotically tensionless critical strings. The connection between towers of light species and small black holes warrants seeking a bottom-up rationale for this dichotomoy, dubbed emergent string conjecture. In this paper we move a first step in this direction, exploring bottom-up constraints on towers of light species motivated purely from the consistency of the corresponding thermodynamic picture for small black holes. These constraints shed light on the allowed towers in quantum gravity, and, upon combining them with unitarity and causality constraints from perturbative graviton scattering, they provide evidence for the emergent string scenario with no reference to a specific ultraviolet completion.

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arXiv2024

Minimal Black Holes and Species Thermodynamics

The species scale provides a lower bound on the shortest possible length that can be probed in gravitational effective theories. It may be defined by the size of the minimal black hole in the theory and, as such, it has recently been given an interpretation along the lines of the celebrated black hole thermodynamics. In this work, we extend this interpretation to the case of charged species. We provide working definitions of minimal black holes for the case of uncharged and charged species constituents. Then, examining the modifications in the thermodynamic properties of near-extremal charged species compared to the uncharged case, we uncover interesting implications for the cosmology of an expanding universe, particularly within the context of the Dark Dimensions Scenario. Finally, we explore possible microscopic constructions in non-supersymmetric string theories in which towers of charged near-extremal species may arise.

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arXiv2024

Anti-de Sitter $\to$ de Sitter transition driven by Casimir forces and mitigating tensions in cosmological parameters

Over the last few years, low- and high-redshift observations set off tensions in the measurement of the present-day expansion rate $H_0$ and in the determination of the amplitude of the matter clustering in the late Universe (parameterized by $S_8$). It was recently noted that both these tensions can be resolved if the cosmological constant parametrizing the dark energy content switches its sign at a critical redshift $z_c \sim 2$. However, the anti-de Sitter (AdS) swampland conjecture suggests that the postulated switch in sign of the cosmological constant at zero temperature seems unlikely because the AdS vacua are an infinite distance appart from de Sitter (dS) vacua in moduli space. We provide an explanation for the required AdS $\to$ dS crossover transition in the vacuum energy using the Casimir forces of fields inhabiting the bulk. We then use entropy arguments to claim that any AdS $\to$ dS transition between metastable vacua must be accompanied by a reduction of the species scale where gravity becomes strong. We provide a few examples supporting this AdS $\to$ dS uplift conjecture.

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arXiv2024

More on Black Holes Perceiving the Dark Dimension

In the last two years the dark dimension scenario has emerged as focal point of many research interests. In particular, it functions as a stepping stone to address the cosmological hierarchy problem and provides a colosseum for dark matter contenders. We reexamine the possibility that primordial black holes (PBHs) perceiving the dark dimension could constitute all of the dark matter in the universe. We re-assess limits on the abundance of PBHs as dark matter candidates from $γ$-ray emission resulting from Hawking evaporation. We re-evaluate constraints from the diffuse $γ$-ray emission in the direction of the Galactic center which offer the best and most solid upper limits on the dark matter fraction composed of PBHs. The revised mass range which allows PBHs to assemble all cosmological dark matter is estimated to be $10^{15} \alt M_{\rm BH}/{\rm g} \alt 10^{21}$. We demonstrate that due to the constraints from $γ$-ray emission, quantum corrections due to the speculative memory burden effect do not modify this mass range. We also investigate the main characteristics of PBHs which are localized in the bulk. We show that PBHs localized in the bulk can make all cosmological dark matter if $10^{11} \alt M_{\rm BH}/{\rm g} \alt 10^{21}$. Finally, we comment on the black holes that could be produced if one advocates a space with two boundaries for the dark dimension.

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arXiv2024

Landscape, Swampland, and Extra Dimensions

By combining swampland conjectures with observational data, it was recently suggested that the cosmological hierarchy problem (i.e. the smallness of the dark energy in Planck units) could be understood as an asymptotic limit in field space, corresponding to a decompactification of one extra (dark) dimension of a size in the micron range. In these Proceedings we examine the fundamental setting of this framework and discuss general aspects of the effective low energy theory inherited from properties of the overarching string theory. We then explore some novel phenomenology encompassing the dark dimension by looking at potential dark matter candidates, decoding neutrino masses, and digging into new cosmological phenomena.

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arXiv2024

Topology change and non-geometry at infinite distance

The distance conjecture diagnoses viable low-energy effective realisation of consistent theories of quantum gravity by examining their breakdown at infinite distance in their parameter space. At the same time, infinite distance points in parameter space are naturally intertwined with string dualities. We explore the implications of the distance conjecture when T-duality is applied to curved compact manifolds and in presence of (non-)geometric fluxes. We provide evidence of how divergent potentials signal pathological infinite distance points in the scalar field space where towers of light states cannot be sustained by the curved background. This leads us to suggest an extension to the current statement of the Swampland distance conjecture in curved spaces or in presence of non-trivial fluxes supporting the background.

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arXiv2024

The Dark Dimension, the Swampland, and the Dark Matter Fraction Composed of Primordial Near-Extremal Black Holes

In a recent publication we studied the decay rate of primordial black holes perceiving the dark dimension, an innovative five-dimensional (5D) scenario that has a compact space with characteristic length-scale in the micron range. We demonstrated that the rate of Hawking radiation of 5D black holes slows down compared to 4D black holes of the same mass. Armed with our findings we showed that for a species scale of ${\cal O} (10^{9}~{\rm GeV})$, an all-dark-matter interpretation in terms of primordial black holes should be feasible for black hole masses in the range $10^{14} \lesssim M/{\rm g} \lesssim 10^{21}$. As a natural outgrowth of our recent study, herein we calculate the Hawking evaporation of near-extremal 5D black holes. Using generic entropy arguments we demonstrate that Hawking evaporation of higher-dimensional near-extremal black holes proceeds at a slower rate than the corresponding Schwarzschild black holes of the same mass. Assisted by this result we show that if there were 5D primordial near-extremal black holes in nature, then a PBH all-dark-matter interpretation would be possible in the mass range $10^{7}\sqrtβ \lesssim M/{\rm g} \lesssim 10^{21}$, where $β$ is a parameter that controls the difference between mass and charge of the associated near-extremal black hole.

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arXiv2024

Fuzzy Dark Matter and the Dark Dimension

We propose a new dark matter contender within the context of the so-called ``dark dimension'', an innovative 5-dimensional construct that has a compact space with characteristic length-scale in the micron range. The new dark matter candidate is the radion, a bulk scalar field whose quintessence-like potential drives an inflationary phase described by a 5-dimensional de Sitter (or approximate) solution of Einstein equations. We show that the radion could be ultralight and thereby serve as a fuzzy dark matter candidate. We advocate a simple cosmological production mechanism bringing into play unstable Kaluza-Klein graviton towers which are fueled by the decay of the inflaton.

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arXiv2024

Starobinsky Inflation in the Swampland

We argue that the Starobinsky model of inflation, realised via an $R^2$ term in the Lagrangian, can originate from quantum effects due to a tower of light species. By means of two separate arguments, we show how this implies that the scale of the $R^2$ term must be of order of the species scale $Λ_s$, namely the energy at which gravity becomes strongly coupled. We discuss the implications and challenges of this scenario for inflation, inflationary reheating, and string theory embeddings. In this context, we collect strong evidence to conclude that Starobinsky inflation lies in the Swampland.

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arXiv2024

Searching for neutrino-modulino oscillations at the Forward Physics Facility

We make use of swampland conjectures to explore the phenomenology of neutrino-modulino mixing in regions of the parameter space that are within the sensitivity of experiments at the CERN's Forward Physics Facility (FPF). We adopt the working assumption of Dirac mass terms which couple left- and right-handed neutrinos. We further assume that the 3 right-handed neutrinos are 0-modes of bulk 5-dimensional states in the dark dimension, a novel scenario which has a compact space with characteristic length-scale in the micron range that produces a natural suppression of the 4-dimensional Yukawa couplings, yielding naturally light Dirac neutrinos. We formulate a specific realization of models with high-scale supersymmetry breaking that can host a rather heavy gravitino ($m_{3/2} \sim 250$ TeV) and a modulino with mass scale ($m_4 \sim 50$ eV) within the FPF discovery reach.

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arXiv2023

On the Degrees of Freedom of $R^2$ Gravity in Flat Spacetime

We study the degrees of freedom of $R^2$ gravity in flat spacetime with two approaches. By rewriting the theory a la Stueckelberg, and implementing Lorentz-like gauges to the metric perturbations, we confirm that the pure theory propagates one scalar degree of freedom, while the full theory contains two tensor modes in addition. We then consider the degrees of freedom by directly examining the metric perturbations. We show that the degrees of freedom of the full theory match with those obtained with the manifestly covariant approach. In contrast, we find that the pure $R^2$ gravity has no degrees of freedom. We show that a similar discrepancy between the two approaches appears also in a theory dual to the three-form, and appears due to the Lorentz-like gauges, which lead to the fictitious modes even after the residual gauge redundancy has been taken into account. At first sight, this implies a discontinuity between the full theory and the pure case. By studying the first-order corrections of the full $R^2$ gravity beyond the linear regime, we show that at high-energies, both scalar and tensor degrees of freedom become strongly coupled. This implies that the apparent discontinuity of pure and full $R^2$ gravity is just an artefact of the perturbation theory, and further supports the absence of degrees of freedom in the pure $R^2$ gravity.

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