• Hello, I'm a theoretical physicist based at the Max-Planck Institute for Physics, in Munich.

    The type of topics I work on are:

    • The Swampland
    • Theoretical Cosmology
    • F-Theory
    • String Model Building
  • String theory allows for many different low-energy effective theories, the so-called Landscape.

    However, the Landscape is surrounded by an even vaster Swampland of effective theories that cannot arise in String Theory.

    The constraints on physics coming from the Swampland may impact our understanding of the nature of Dark Energy , early universe Inflation , and even the mass of the Graviton .

    See my Introductory Lectures on the String Theory Swampland if you are interested in learning more.
  • F-theory is a formulation of String Theory in terms of geometry.

    It allows us to learn about non-perturbative physics in string theory from the mathematics of elliptic fibrations, a typically beautiful string-theoretic synergy of physics and mathematics.

    Remarkably, such non-perturbative regions of string theory appear to be the most promising ways to realise so-called Grand Unified Theories which unify the different forces of the Standard Model.

    See my Review of F-theory for more about how geometry and physics combine to teach us about unified theories of particle physics.

Publications

Here is a link to all my publications: Publications on Inspire

Some recent publications:

Recent Talks

Talks on the Swampland

Theory Colloquium, CERN 2018

Conjectures on the Swampland

Slides

Summer Workshop, Simons Center for Geometry and Physics 2018

Emergence and infinite distance in Calabi-Yau moduli spaces

Video

Vistas over the Swampland, Instituto de Física Teórica, Madrid 2018

The Swampland, duality, and de Sitter entropy

Video

Slides

Colloquium, Max-Planck Institute for Physics 2017

Colloquium, University of Utrecht 2017

Quantum Gravity far from the Planck Scale

Slides

PASCOS, Instituto de Física Teórica, Madrid 2017

The Weak Gravity Conjecture and Scalar Fields

Video

Slides

Talks on F-Theory

String Pheno, Instituto de Física Teórica, Madrid 2015

Discrete Symmetries and Torsion in F-theory

Video

Slides

Physics and Geometry of F-theory Workshop, Max-Planck Institute for Physics 2015

On E8 and Global F-theory GUTs

Slides

Students

Thesis: Testing Swampland Conjectures and the Emergence proposal in a two-parameter Calabi-Yau model

This thesis starts in Chapter 1 with a short review of some of the Swampland conjectures, mainly the Distance conjecture and the Weak gravity conjecture. We then discuss the emergence proposal and its relation to the previous conjectures. In Chapter 2 and 3 we take a detour, introducing the relevant points of string compactifications and D-branes on Calabi-Yau manifolds needed for the following discussion, mainly moduli spaces of Calabi-Yaus, their special geometry, and supersymmetric cycles. After the introduction of the necessary concepts in the first three chapters we turn in Chapter 4 to the main body of this thesis which consists in its majority of original work. We introduce first the two-parameter Calabi-Yau manifold P_4^{11222}[8] and its moduli space which has already been studied in the literature. The original work and new results consist of identifying and studying towers of D-branes becoming light at the large volume point in moduli space and their relation to the Swampland conjectures and Emergence. We find that these towers of D-branes satisfy the Distance conjecture and the Weak gravity conjecture and furthermore are in most cases in agreement with the expectations from the emergence proposal. Most notably, and the main result of this thesis is that the leading behaviour of the prepotential of the N = 2 low-energy effective theory at the large volume point can be thought of as emergent from integrating out these towers of branes.

Thesis: Aspects of the Weak Gravity Conjecture

In this thesis we mainly study two aspects of the weak gravity conjecture (WGC). First, we look at an indirect argument in favor of the WGC based on holographic entropy bounds. We make the existing proposals more precise and relate them to recent work on communication bounds. In the second part, we explain the connection between the WGC and the phenomenon of emergence. Here, we make a precise toy-model calculation and discuss similar observations in String Theory. Most of the relevant formalism and concepts are provided either in the main part of the thesis or in the appendix, hence the > 100 pages.

Thesis: The Axion-Instanton Weak Gravity Conjecture and Scalar Fields

We study the Weak Gravity Conjecture in the presence of scalar fields. The Weak Gravity Conjecture is a consistency condition for a theory of quantum gravity asserting that for a U(1) gauge field, there is a particle charged under this field whose mass is bounded by its charge. It was extended to a statement about any canonical pair of (p-1)-dimensional object and p-form coupling to it, in particular to axion-instanton pairs. The gauge-scalar Weak Gravity Conjecture is a modification of this bound that includes scalar interactions. We propose a similar extension to cases where scalar fields are present for the axion-instanton Weak Gravity Conjecture and provide evidence from Type IIA supergravity.

Thesis: Constraints on Four Dimensional Effective Field Theories From String and F-Theory

This thesis is a study of string theory compactifications to four dimensions and the constraints the Effective Field Theories must exhibit, exploring both the closed and open sectors. In the former case, we focus on axion monodromy scenarios and the impact the backreaction of the energy density induced by the vev of an axion has on its field excursions. For all the cases studied, we find that the backreaction is small up to a critical value, and the proper field distance is flux independent and at most logarithmic in the axion vev. We then move to the open sector, where we use the framework of F-theory. We first explore the relation between the spectra arising from F-theory GUTs and those coming from a decomposition of the adjoint of E8 to $SU(5)\times U(1)^n$. We find that extending the latter spectrum with new SU(5)-singlet fields, and classifying all possible ways of breaking the Abelian factors, all the spectra coming from smooth elliptic fibration constructed in the literature fit in our classification. We then explore generic properties of the spectra arising when breaking SU(5) to the Standard Model gauge group while retaining some anomaly properties. We finish by a study of F-theory compactifications on a singular elliptic fibration via Matrix Factorisation, and find the charged spectrum of two non-Abelian examples.

Thesis: SUPER-PLANCKIAN FIELD DISPLACEMENTS AND CONSISTENT QUANTUM GRAVITY

We study the consistency of super-Planckian scalar field displacements in quantum gravity. The first part of this thesis discusses the cosmology of the early universe and in particular large field inflation, which explains the homogeneous initial conditions. Several models of natural inflation are presented. It is argued that the embedding of these models into a quantum gravity framework can be problematic because of the required super-Planckian scalar field displacements. Concrete obstructions to an implementation in string theory are presented. The second part is devoted to the study of various conjectures on the moduli space of quantum gravity theories which constrain large field displacements. The Weak Gravity Conjecture constrains mass-to-coupling ratios in gauge theories. In string theory these are functions of scalar moduli. We propose a connection between a variant of the Weak Gravity Conjecture and a Swampland Conjecture, which states that as a scalar field displacement asymptotes to infinity an infinite tower of states appears, exponentially light in the displacement. We show that the Weak Gravity Conjecture leads to evidence for this and that the exponential behaviour sets in quickly after the field variation passes the Planck scale. These conjectures can be used to constrain large field inflation models.

Thesis: MODEL BUILDING IN F-THEORY USING HYPERCHARGE FLUXES

One of the challenges in string phenomenology is to find compactifications of superstring theory such that the resulting low-energy effective theory resembles the Minimal Supersymmetric Standard Model of particle physics with its SU(3) × SU(2) × U(1) gauge group, three generations of fermions and one Higgs doublet. An established method that achieves the correct gauge group is to model an SU(5) GUT theory using type IIB superstring theory with D7-branes and then to break the GUT group by including a hypercharge flux. The correct non-perturbative description of type IIB superstring theory with D3- and D7-branes is F-theory. In F-theory, the geometry of the branes and of the compactification space is captured by a four-dimensional elliptically fibered Calabi-Yau. In [1], the authors give a basic example of a realistic model in this context, including however nine generations of fermions and five Higgs doublets. The aim of this thesis is to review the necessary prerequisites in differential geometry, to explain in detail how such a model in F-theory can be built, and finally to improve the model given in [1]. By including a so-called G4-flux, we were able to reduce the number of generations in the model to the desired value of three.

Thesis: Aspects of E8 in heterotic F-theory duality

In this thesis we study heterotic duality of the SU(5)xU(1)xU(1) F-theory models introduced in [1] in compactifications to four and six dimensions. We investigate the relationship between GUT singlets in the F-theory geometry and singularities in the heterotic compactification. Moreover, we derive constraints on the models as to render the heterotic geometry smooth. In four dimensions, we find that two of the four models do not admit a smooth heterotic dual. We proceed to consider how these singularities are related to possible embeddings of the F-theory spectrum into a Higgsed E8 and show that for the two models allowing for a smooth heterotic geometry, there are multiple embeddings such that all non-singular states are embeddable whereas the singular ones are not. In six dimensions all four models allow for a smooth heterotic dual, but the geometries are severely restricted by said constraints.

Thesis: Anomaly Constraints on Hypercharge Flux in F-Theory GUTs

The aim of this thesis is to derive the consistent choices for hypercharge flux in the F-Theory GUT models, that were constructed in [1]. Their constraints are imposed by emerging gauge anomalies in four dimensions, so beforehand the chiral anomaly in quantum field theory as well as the the concept of flux is introduced and discussed. Initially, a short review of the representation theory of the standard model and the Georgi-Glashow model of SU(5) grand unification will be given.

Thesis: Symmetries, GUTs and Complete Networks

In this thesis, the Standard Model and the Higgs Mechanism are presented and their connection to group theory and symmetries is explored. In the following chapter, the SU(5) Georgi-Glashow model is examined as a possible Grand Unified Theory (GUT) that further unifies the concepts introduced in the Standard Model. Advancing after this model, there is an ongoing search for other theories going beyond the Standard Model, possibly unifying all of physics in a ”Theory of Everything”. The most promising candidate for this to date is String Theory, and the rest of the thesis deals with structures called Complete Networks, which find some applications in type IIB String Theory.

Thesis: "Running Couplings in the Standard Model and their Implications to possible Physics beyond the Standard Model

This thesis uses and derives the methods for the calculation of the running couplings in the Standard Model of Particle Physics. On the basis of the running couplings for the three gauge couplings and the Higgs quartic coupling, the phenomena of coupling unification and the instability of the electroweak vacuum are discussed in search of potential implications which hint to beyond the Standard Model Physics. For this, the concepts of Supersymmetric Theories and Grand Unification Theories are discussed in comparison to the predictions of the Standard Model. The effect of adding a pair of fermions charged under SU(3) to the Standard Model is calculated to quantify the change of running couplings by postulating additional particles and to relate this to the previous discussion.

Teaching Material

An Introduction to the String Theory Swampland

Lectures for the British Universities Summer School in Theoretical Elementary Particle Physics 2018 (Oxford)

Lecture Notes

String Theory

Lecture course on String Theory given at the University of Heidelberg in 2014, 2015 and 2017

Lecture Notes

Can String Theory Explain Dark Matter?

Lecture given in "A Matter of Flavor": 1st PRISMA Interactive Research Symposium, 2017 (Mainz)

Slides

Curriculum Vitae

2017 Long-term Researcher (5-Year)

Max-Planck Institute for Physics, Munich

2013-2017 Junior Research Group Leader

University of Heidelberg

2009-2013 Postdoctoral Researcher (Marie Curie Fellowship)

Ecole Polytechnique, Paris

2006-2009 STFC Postdoctoral Fellowship

University of Oxford

Education

2003-2006 PhD Theoretical Physics

University of Sussex

1998-2002 Master of Physics

University of Oxford

Contact

palti [at] mppmu.mpg.de

Föhringer Ring 6 D-80805 Munich

+49-89-32354-532