Theory group
 Dmitri SEMIKOZ (responsible)
 Eric HUGUET (deputy responsible)
 List of all members of the Theory group
The theorists at APC have a very broad range of interests. Their research is both closely linked with observations and focused on fundamental theories. A summary of the main research interest is given below.
For more information, see the lab's activity report (20172021)
Main scientific interests
String theory and holography
String theory is a theoretical framework that seeks to unify all known physical forces, including gravity, at the quantum level. One of its key insights is the Gauge/Gravity duality, also known as the Holographic correspondence. This conjecture states that a quantum field theory in lower dimensions is equivalent to a gravitational theory in higher dimensions. This has important implications for understanding the relationship between quantum field theory, string theory, and general relativity. Additionally, it can be used as a tool to study strongly coupled field theories, such as QCD, and has been applied to other areas such as hightemperature superconductors and strongly correlated quantum systems. Holographic correspondence has also led to new connections between fluid dynamics and gravity, as well as information theory and entanglement. It has also played a significant role in developing theories of gravity beyond Einstein's General Relativity.

Holographic description of QCD and strongly coupled phases of matter
The gauge/gravity duality, also known as holographic correspondence, is a conjecture in string theory that equates a quantum field theory with a gravitational theory in higher dimensions. It has been used to study strongly coupled field theories such as QCD, and has potential applications in condensedmatter systems and neutron stars. It has also been used to engineer theories that match the properties of QCD, like confinement, mass gap, and running of the coupling constant, allowing the understanding of their phase diagrams at finite temperature and baryonnumber density. This approach may have interesting implications for the study of neutron stars and gravitational waves.

Emergent gravity and the Selftuning of the cosmological constant
The gauge/gravity correspondence suggests that gravity is an effective lowenergy theory of a highenergy ordinary QFT, with the graviton being a composite of highenergy degrees of freedom. This connection allows researchers to attack the cosmological constant problem, in which the observed small value of the cosmological constant today is in contrast with the large value expected from effective field theory. Researchers have proposed a framework, based on holography, which suggests that our observed 4D universe is a defect embedded in a 5D curved spacetime, with the fields of the Standard Model confined to the defect and gravity able to propagate in the bulk. This framework leads to a stabilization of the defect, resulting in a flat spacetime geometry, regardless of the localized vacuum energy.

Holographic Renormalization Group of QFTs on curved manifolds
In gauge/gravity duality, the QFT renormalization group is given a geometric representation as evolution along a radial dimension of the higherdimensional dual spacetime. We have been investigating the nature of these holographic RG flows, and have developed a general framework for analyzing holographic field theories on curved spacetimes.
Quantum Field Theory

Quantum Field Theory in curved spacetime
The study of quantum effects in strong gravitational backgrounds is a subject of topical interest in cosmology and astrophysics. Paradigm examples are the gravitational amplification of primordial density perturbations in inflationary cosmology or the UnruhHawking radiation from black holes which are cornerstones of modern cosmology and of the fundamental understanding of black hole physics. More generally, understanding situations where both quantum and gravitational effects come into play sheds a new light onto the laws of physics at work and may give some insight concerning more fundamental laws.

Conformal methods for fields in curved geometries
The group studies quantization and quantum field theory on manifolds, focusing on conformal scalar and Maxwell fields in de Sitter and Robertson Walker spaces. The goal is to obtain exact and explicit expressions for objects such as twopoint functions, allowing for practical calculations and interpretations in curved backgrounds. A formalism based on differential geometry has been developed, which generalizes the Dirac's sixcone formalism and converts Maxwell equations in a conformal gauge to a set of conformal scalar equations. The group is currently working on applying this formalism and generalizing the use of conformal transformations to non conformallyinvariant equations.

Interacting fields in de Sitter space
The group studies interacting scalar fields in de Sitter space, focusing on light fields in units of the inverse curvature, which are of interest for inflation. Perturbative techniques often result in divergent contributions, which must be resummed to obtain reliable answers. The group develops and applies resummation methods, such as the prepresentation of correlators, largeN techniques, 2PI techniques and renormalization group techniques, to questions of physical interest in the context of inflationary physics. Recent original results include the observation that quantum contributions to nonGaussian correlators can contribute the same as treelevel contributions, and the fact that amplified quantum fluctuations can lead to the restoration of symmetries in O(N) theories in any spacetime dimension.

Integral quantization of geometries
Integral quantization is a method of giving a classical object a quantum version by quantizing various geometries like symplectic manifolds. It is based on operatorvalued measures and has probabilistic aspects at each stage of the procedure. Various types of integral quantization exist, like Berezin or Klauder quantization, and coherent state quantization. It is well established that the WeylHeisenberg group underlies the canonical commutation rule. The approach also includes less familiar quantization based on the affine group of the real line, which is useful for dealing with gravitational singularities on a quantum level in quantum cosmology. The main issue of this approach is the appearance of a quantum centrifugal potential that allows for regularization of the singularity, selfadjointness of the Hamiltonian, and unambiguous quantum dynamics.

Nonabelian gauge theories and confinement
The understanding of the long distance aspects of the theory of strong interactions is a major open question in highenergy physics, particularly the quantitative description of confinement in nonabelian gauge theories. Numerical calculations on the lattice are currently the main tool to address this regime, but are limited in their application. The Theory group studies various aspects of YangMills theories and QCD related to the infrared regime, confinement, and the quarkgluon plasma. They also propose a new approach to gaugefixing in YangMills theories, which takes into account Gribov copies, and an alternative approach using holographic Gauge/Gravity duality.
Cosmology

Earlyuniverse cosmology and primordial black holes
Primordial Black Holes (PBHs) are attracting increasing attention due to recent observations of gravitational waves from blackhole mergers and unresolved questions in Cosmology. PBHs could form from large quantum fluctuations in the early universe, offering an opportunity to study the physics during cosmic inflation. This theory group made a number of contribution on this important question. The "stochasticδN formalism" was developed to understand quantum backreaction in shaping the early universe and shows that PBHs could have a heavy exponential tail, deviating from Gaussian statistics. The theory also shows that "metric preheating" could produce ultralight PBHs, which could have dominated the universe before evaporating. The standard cosmological paradigm raises fundamental issues and new approaches are being proposed to better understand the quantum origin of cosmological structures. Extending tools developed in quantum information theory to the realm of quantum cosmological fields, new approaches have thus been proposed to better describe (and hopefully reveal) genuine quantum properties of the primordial fluids.

Dark energy and modified gravity
Dark energy models are often based on scalartensor theories, and the most general framework for these theories is known as Degenerate HigherOrder ScalarTensor (DHOST) theories. However, a constraint on the speed of gravitational waves from the binary neutron star merger GW170817 places severe restrictions on DHOST theories. Some researchers argue that this constraint does not necessarily apply to dark energy models, as it corresponds to a scale much smaller than cosmological scales. Relaxing this constraint allows for a much richer phenomenology in dark energy models. An important aspect of these models is their linear stability, and researchers have used the Effective Theory of Dark Energy to examine this aspect within DHOST theories, providing a powerful tool for studying cosmological perturbations in a wide range of dark energy models.

Inflation and cosmological perturbations
Inflation is a proposed phase of accelerated expansion in the early universe that explains the origin of primordial fluctuations observed in the CMB. The nature of the inflaton field driving inflation is still unknown. The group has studied various models of inflation, focusing on those involving multiple scalar fields and their potential signatures in present or future data, including nonGaussianities and isocurvature perturbations. The group also examines the link between inflation and approximate conformal invariance and scaling in Quantum Field Theory through the gauge/gravity duality. This approach offers a new perspective on the standard problems of inflation.
Gravity and cosmology

Gravitational waves
In 2016, the LIGO interferometers directly detected a gravitational wave signal for the first time. The LISA Pathfinder satellite also demonstrated the technology needed to detect gravitational waves from space. The detection of gravitational waves opens a new window on the universe and allows us to detect objects that are invisible through electromagnetic radiation. The gravitational wave detection also provides us with a unique opportunity to test the universe through a new messenger and the theory group is exploring this opportunity in particular concerning the potential of the LISA mission to probe cosmology.

General relativity and modified gravity theories
The study of possible deformations of general relativity is of major theoretical and phenomenological importance. The group has worked on several theories of modified gravity, including f(R) and chameleon theories, with application to the structures of stars and spherical collapse, as well as Galileon models. Group members worked on the ghostfree formulation of massive gravity, its cosmological solutions, and the Vainshtein mechanism. The dynamics and problems of massive gravity are also investigated by using the holographic link to Quantum Field Theory. The dynamics and problems of massive gravity are also investigated by using the holographic link to Quantum Field Theory. This same link makes massive gravity a model for studying momentum dissipation in finite density strongly coupled systems with potential applications to condensed matter physics.

Gauge gravity
In Teleparallel Equivalent to General Relativity (TEGR) theory, gravity is encoded in torsion rather than curvature, through the Weitzenböck connection. The theory leads to the same predictions as General Relativity. With collaborators, we proposed a formulation of TEGR using a Cartan connection, showing how the usual gauge theory formalism must be modified to interpret it as a gauge theory for the translation group. This allows for a coherent mathematical framework for describing TEGR, including matter coupling.
Astroparticles

Neutrino physics
Neutrinos tell us stories from far away in space and time, and have key unknown properties to reveal. They include their absolute mass and mass ordering, leptonic CP violation, the neutrino Majorana or Dirac nature and the existence of a fourth sterile neutrino. These measurements will bring fundamental bricks for physics beyond the Standard Model of particles and interactions. The theory group works at the forefront of this research, investigating fundamental aspects and conceiving new avenues in close connection with experiments.

UHE neutrinos
UltraHigh Energy (UHE) cosmic rays produce secondary charge pions on background fields in the sources or in intergalactic space. Neutrinos produced during propagation are called "cosmogenic neutrinos". Flux of those neutrinos depends on the unknown distribution of sources and on the initial proton spectrum produced at those sources. Experiments like ANITA, AUGER and Icecube will be able to study this flux. For direct neutrino flux from the sources even backgrounds are unknown or at least model dependent. This make predictions of neutrino flux from sources even more difficult. We are developing theoretical models of the UHE neutrino sources.

Cosmological neutrinos at the epoch of bigbang nucleosynthesis
Big Bang Nucleosynthesis (BBN) is one of key stones of cosmology. When the Universe cools down to subMeV temperatures, the plasma is not hot enough anymore to destroy light nuclei, produced from protons and remaining neutrons. The abundances of light elements is then governed by the neutrontoproton ratio, which in turn depends on reactions with electron neutrinos and antineutrinos as well as neutron decay, and on the total energy density of Universe at that time. The observed abundances of light elements strongly restrict any new physics connected with MeV scales. This offers a poweful tool to constrain the parameter spaces of exotic models or novel particles such as sterile neutrinos.

Low energy weak interaction and neutrino physics
Low energy weak interaction and neutrino physics has brought milestone in the build up of the Standard Model. Nowadays it is a powerful tool to search for new physics beyond it. The group has been strongly contributing to this domain by predicting neutrinonucleus cross sections crucial for the interpretation of oscillation experiments, exploring the connection with the leptonflavor violating neutrinoless doblebeta decay, proposing experiments nearby existing facilities such as spallation source ones (ESS), investigating other low energy weak processes. The group is also renown for the proposal of a novel neutrino facility in the 100 MeV energy range based on a new concept : the low energy betabeam. This facility is of great interest for nuclear physics, neutrino and supernova physics.

Cosmic Rays
Despite cosmic rays was discovered more than 100 years ago, their origin is still unknown. We suggested that local supernovae of 2 Myr can explain the number of anomalies in cosmic ray data. The same supernovae can be a reason for the evolution of the climate and life diversity on Earth. The group developed a theory of galactic and extragalactic cosmic ray propagation. We discovered new types of cosmic ray diffusion around their sources. We developed a theory of anisotropic cosmic ray diffusion in galaxy.

Intergalactic magnetic fields
The signal excess recently discovered by several Pulsar Timing Arrays experiments can be explained in terms of a stochastic gravitationalwave background due to a primordial magnetic field at the QCD scale. The Intergalactic Magnetic Field (IMF) in the voids of large scale structure is dominated by the contribution of primordial magnetic fields, possibly created during inflation or at phase transitions in the Early Universe. Primordial magnetic fields can be probed via measurements of secondary gammaray emission from gammaray interactions with extragalactic background light. Lower bounds on the magnetic field in the voids were derived from the nondetection of this emission.

Multimessenger astrophysics
Astrophysical neutrinos have been discovered about 10 years ago, but the nature of their source is still unknown. We worked on multimessenger astrophysics which neutrinos and gammarays at energies above TeV. In particular we constructed models of galactic and extralactic neutrino sources and compared their predictions with gammaray and neutrino observations.