The goal of my work was to estimate the Hubble constant $H_0$ through the study of the quadruply lensed, variable QSO SDSSJ1433+6007. I combined multi-filter, archival HST data for lens modelling with a dedicated 3-year long time-delay monitoring campaign using the 2.1 m Fraunhofer telescope at the Wendelstein Observatory.\\
The lens modelling was performed with the public lenstronomy Python package individually for the infrared data, utilizing the higher resolution of the optical data to constrain image positions a priori. This approach revealed two luminous contaminants in one of the near-infrared exposures, which would bias the lensing potentials and cosmological inference if not accounted for. After masking these contaminants, I repeated the modelling and combined the lens posteriors, obtaining a constraint on the Fermat potential with a precision of $\sim 3.3\,\%$.\\
The g'-band Wendelstein light-curve data was reduced and then analyzed using a free-knot spline fitting method implemented in the public Python PyCS3 tools, accounting for microlensing correction. I obtained a precision of $\sim6.6\%$ for the time delays between the QSO images.
By combining the posteriors for the Fermat potential differences and time delays, and assuming a flat $\Lambda$CDM cosmology, I inferred a Hubble constant of $ H_0 = 73.7^{+4.3}_{-3.9}\frac{\text{km}}{\text{s Mpc}}$ achieving $\sim 6\,\%$ uncertainty for this single system.

The temperature and polarization anisotropies of the cosmic microwave background (CMB) have been used to set constraints on decaying dark matter models down to keV masses. In this talk, I will discuss recent work to extend these limits down into the sub-keV mass range. I will show how we used principal component analysis to estimate the lower bound on the decay lifetime for a basis of different dark matter masses and Standard Model final states, and then how we validated our principal component analysis using Markov chain Monte Carlo methods and Planck 2018 data. I will then show a separate analysis for models decaying into photons below the hydrogen ionization threshold, where the redshift dependence of the effect on the CMB is entirely different. This work constitutes a step towards a complete understanding of the CMB as a probe of exotic energy injection.

Long-range attractive interactions between dark matter particles can significantly enhance their annihilation, particularly at low velocities. However, the standard method of computing this "Sommerfeld enhancement" is known to break down at special points in parameter space, where it predicts cross sections exceeding general bounds from unitarity. I will show how this issue can be resolved within the framework of non-relativistic quantum mechanics, in the case where the physics relevant to annihilation corresponds to a parametrically shorter distance scale (higher energy scale) compared with the long-range potential, by using the short-range annihilation to modify the boundary conditions for the long-range scattering problem. I will present the resulting prescription for the corrected Sommerfeld enhancement, valid for all partial waves and for systems where multiple states are coupled by the long-range potential, and briefly discuss the phenomenological implications.

Interacting fluid models are a natural candidate for modified cosmological models with an array of interesting phenomenology. For example, cosmologies with an interacting dark sector have seen particular interest recently in relation to the current observational tensions. However, a Lagrangian description is preferable to phenomenological models for a number of reasons (e.g., for unambiguous cosmological perturbations, or identifying pathologies such as ghosts). In this talk, I will give an overview of the perfect fluid Lagrangian approaches for interacting cosmological models, comparing different formulations in the literature. I will then discuss recent extensions from my own work, which allow for interesting physical processes such as particle creation/annihilation. Lastly, I will mention how even more exotic modifications can be implemented in these Lagrangian formulations.

One of the most outstanding problem in today's understanding of the Universe is the nature of the elusive dark-matter (DM) that dominates the matter content. Rotation curves (RC) have been used to put forward the concept of DM as it was difficult to hide that the RC of massive galaxies remain flat at large galactocentric radii. RC have since revealed a potential problem, for the standard CDM model, on small scales (<1kpc) where DM profiles are too flat (cored) compared to predictions. Alternative DM models have been proposed to produce cores naturally. Hence, rotation curves are dark-matter laboratory given that the shape of RC on intermediate scales (1-5kpc) is a measure of the shape of the inner DM profiles. Measuring the shape of DM profiles in distant marginally resolved galaxies (with z>0) was deemed impossible, but recently, thanks to recent innovations, it has become possible to study the shape of RCs of hundreds of distant galaxies. I will present the most recent innovations, and recent results from our large program using the MUSE 3D spectrograph.

In theories with compact dimensions, Bubbles of Nothing (BoN) are vacuum decays in which the compactification manifold C shrinks to a point, with the instability expanding at the speed of light and leaving nothing (not even spacetime) behind. The Swampland Cobordism Conjecture predicts spacetime-ending configurations (known as bordisms to nothing) such as these being commonplace in a consistent theory of Quantum Gravity. Most constructions of BoN or more general bordisms to nothing found in the literature feature simple instances of C or singular bordisms, which cannot be approached from the effective field theory. Assuming the solution mediating such decay to nothing is homeomorphic to a smooth description, one can go a step further, and obtain topological properties from those of C. Through the use of Morse-Bott theory, these can be translated into information on the number and types of topology changes the compact manifold experiences as we move towards the tip of the bordism, as well as the location of possible cobordism defects. I will illustrate these results with different examples coming from String Theory and, time permitting, study more complicated arrangements such as BoN collisions. The talk is based on the paper https://arxiv.org/abs/24LF.2CR72.

The minimal two-triplet extension of the Standard Model demanding custodial symmetry gives rise to a new model, the extended Georgi-Machacek (eGM) model, not the well-known Georgi-Machacek (GM) model. Via computing one-loop corrections to all 2 to 2 scattering amplitudes in the eGM model, we place NLO unitarity bounds on the quartic couplings. On top of that, we derive stringent conditions on the quartic couplings ensuring there exists no field direction that leads to an unbounded potential. Finally, we perform a global fit for the eGM model using HEPfit to these theoretical bounds together with the latest Run II LHC data on Higgs signal strengths. We delineate the allowed ranges for the heavy Higgs boson masses and their mass differences. We further re-analyse the parameter space of the well known GM model by including NLO unitarity bounds for the first time. The global fit for GM model, with these improved theoretical and latest experimental constraints significantly refines the allowed ranges of the quartic couplings and heavy Higgs masses present in the literature.

In this talk, we will explore entangled relativity, a general theory of relativity that offers a more parsimonious approach compared to general relativity when integrated within a quantum field theory framework. Requiring only two universal dimensionful parameters instead of three, entangled relativity retains the key components of general relativity, including a four-dimensional spacetime manifold, a metric tensor representing the mechanical properties of space, inertia, and gravitation, and matter fields generating curvature. The term « entangled » does not directly refer to quantum entanglement but rather emphasizes that matter fields and gravity are intrinsically intertwined and cannot be treated separately at the level of the formulation of the theory. Entangled relativity converges toward, or reduces to, general relativity and standard quantum field theory in most cases but predicts a variable quantum of action (ħ) in response to gravitational phenomena, particularly in dense environments like neutron stars or the early universe. After presenting and explaining the theory, I will present a few predictions that may become observable in the foreseeable future, and which do not depend on any theoretical parameter.

Berglund Hubsch are Calabi-Yau of chain and loop types. We investigate the case when they are defined in the same weighted projective space. We show that they are deformation of each other. We also show that their mirrors are the same manifold for the 79 “Good” cases but not for the 77 “Bad” cases.

Relations between quantum field theories and vertex operator algebras (VOAs) have proven ubiquitous. I will discuss one such instance, where VOAs arise on the boundary of topologically twisted 3d supersymmetric quantum field theories. These VOAs are defined from twisted non-Abelian quiver gauge theories by restricting to the boundary sector and performing a BRST reduction. The quiver description plays a key role, with parallels between the geometry of the associated quiver variety and structures of the corresponding VOA. There are two interconnected perspectives here: i) physically, BRST closed operators allow to construct an explicit homomorphism from affine W-algebras into the H-twist VOAs of particular quiver gauge theories, while ii) mathematically, the VOAs are defined as a chiralization of an extended quiver variety. The latter point of view is particularly powerful as it allows to implement a reduction procedure for the quiver diagrams, which translates to free-field realisations when lifted to the VOAs.

The nature of the dark matter in the universe remains one of the most important outstanding questions in modern physics. My central research goal is to address this question, which I pursue with a wide range of techniques and data from particle physics, astrophysics, and cosmology. In this talk, I’ll present two lines of work that each offer insight on the particle properties of dark matter. First, I’ll discuss how dark matter may have affected the first stars to form in the universe, known as Dark Stars. These objects, if confirmed, would serve as unambiguous evidence of the particle nature of dark matter. I’ll discuss several detection strategies for Dark Stars, and what we can learn from them. Second, I’ll turn to recent work on a key observational strategy for constraining (or confirming) the properties of particle dark matter: observing gamma-rays produced by its annihilation or decay in nearby dwarf spheroidal galaxies. The Model-Agnostic Dark Halo Analysis Tool (MADHAT) is a publicly-available computational tool that uses data from the Fermi-LAT to constrain gamma ray emission from dwarf satellite galaxies and dwarf galaxy candidates due to dark matter annihilation, dark matter decay, or other nonstandard or unknown astrophysics. I’ll discuss the capabilities of MADHAT, and the constraints it can provide for any model of dark matter particle physics or astrophysics. I’ll also discuss a new Tool for Determining background Emission Empirically (TweedleDEE), which facilitates purely data-driven searches for anomalous localized sources of gamma-ray emission, including new physics, as well as some possible future directions.

Linear-perturbation theory has proven to be an extremely powerful tool to compare inflationary models with observational data. Recently, the newcoming high-precision observations call for predictions beyond linear perturbations. Such effects are known to be relevant for example in the production of primordial black holes or scalar-induced gravitational waves.

The separate-universe approach proposes to capture some of these non-linearities. It describes the universe as a set of causally disconnected homogeneous and isotropic patches (FLRW). In this talk, I will show that by allowing the patches to exhibit constant curvature, the separate-universe approach can be extended to non-slow-roll models. I will discuss the case of ultra-slow roll where this new approach allows to correctly predict the power spectrum of scalar perturbations together with non-Gaussianities.

We study the geometry of alpha' corrections to D-brane superpotentials using open-closed string duality. More specifically, we use F-Theory to study the contribution of D-branes in type IIB string theory wrapped on holomorphic cycles of a Calabi-Yau manifold to the superpotential, including the leading order alpha' corrections. We argue that mirror symmetry of the F-theoretic Calabi-Yau fourfold suggests that on the mirror dual type IIA side, some alpha' corrections of D-branes wrapped on special Lagrangian 3-cycles are topological and given by gravitational Chern-Simons invariants.

Nearly twenty years ago in an experiment at Brookhaven National Laboratory, physicists measured the muon's anomalous magnetic moment, a_mu, with a remarkable precision of 0.54 parts per million. Since that time, the reference Standard Model prediction for a_mu has exhibited a discrepancy with experiment of over 3 standard deviations, raising the tantalizing possibility of elementary particles or fundamental forces as yet undiscovered. On April 7, 2021 the physicists of an ongoing experiment at Fermilab presented first results of a new measurement of a_mu, brilliantly confirming Brookhaven's measurement and bringing the discrepancy with the reference prediction to a near discovery level of 4.2 sigma. This discrepancy was further enhanced to 5.1 sigma this past with the publication of Fermilab’s new result that reduces the measurement uncertainty by a factor of 2. According to usual particle physics standards, such a discrepancy would mean that new fundamental physics has been uncovered. In the meantime a very large-scale supercomputer calculation of the contribution that most limits the precision of the Standard Model prediction was performed by the Budapest-Marseille-Wuppertal collaboration. The results of this lattice quantum chromodynamics (QCD) calculation paint a very different picture, in particular reducing the difference between theory and experiment and suggesting that new physics may not be needed to explain the current, experimental, world-average value of a_mu. However, it does so at the expense of an untenable discrepancy with the data-driven determination of this most uncertain contribution. After an introduction and a discussion of the current experimental and theoretical status of a_mu, I will present this precise lattice QCD calculation and partial confirmations by other teams. I will also present a framework that enables a comparison of the primary ingredients that are used in the lattice QCD and data-driven approaches and discuss the steps that are required to make a Standard Model prediction that will allow determine whether the final results of the Fermilab experiment, expected in 2025, indicate the presence of new fundamental physics.

I will discuss cosmic strings associated with axions arising from extra-dimensional gauge fields, with a focus on their impact on axion dark matter. Unlike more common 4D setups, the string cores are fundamentally singular, demanding a UV-complete description to make sense of their properties and cosmology. We take it to be the string theory axiverse, for which we show that the presence of cosmic strings can lead to overproduction of axion dark matter regardless of the axion mass. We then explore how the multiple axions of the axiverse alleviate that conclusion.

Elaborating on the recent developments concerning Feynman integrals' calculus and Scattering Amplitudes' evaluation, I introduce the Intersection Theory for twisted de Rham co-homology, and discuss how Intersection Numbers rule the vector-space structure of special functions, such as Euler-Mellin integrals, appearing in Mathematics and Physics. Applications in Electromagnetism, Quantum Mechanics, QFT and Cosmology point to the crucial impact of Intersection Theory on the theory of Fundamental Interactions.

Dark sirens are a powerful way to infer cosmological and astrophysical parameters from the combination of gravitational wave sirens and galaxy catalogues. Importantly, the method relies on the completeness of the galaxy catalogues being well modelled. A magnitude-limited catalogue will always be incomplete to some extent, requiring a completion scheme to avoid biasing the parameter inference. Standard methods include homogeneous and multiplicative completion, which have the advantage of simplicity but underestimate or overestimate the amplitude of structure at low completeness, respectively. In this talk, I will introduce a new method to complete galaxy catalogues which uses clustering information to incorporate knowledge of the large scale structure into the dark sirens method. If the structure of the true number of galaxies is sufficiently well preserved in the catalogue, our estimator can perform drastically better than both homogeneous and multiplicative completion. We lay the foundations for a maximally informative dark sirens analysis.

Cosmological surveys are probing extremely large volumes at ever increasing precision. The dramatic improvement in statistical errors allows us to detect even very subtle effects in the large-scale distribution of matter, and to thoroughly test our theory of gravity on cosmological distance scales. I will present a numerical framework for cosmological N-body simulations that offers a consistent treatment of the dynamics of spacetime as described by the full set of Einstein's equations. Such a framework can be used to compute higher-order correlations in a relativistic setting, or to predict the gravitational dynamics of exotic models that do not offer a simple Newtonian description.

It is now established that about 95% of the energy density of the Universe is made up of Dark Matter (DM) and Dark Energy (DE). Given our limited understanding of their physical nature, an intriguing possibility is to test interactions involving the dark components of the model. In this talk, I will focus on two different categories of interactions: scatter-like interactions between DM and neutrinos, and interacting DE. I will show that both offer interesting perspectives from a theoretical and observational standpoint. I will argue that small-scale Cosmic Microwave Background (CMB) observations can open novel observational windows to accurately test interactions, possibly revealing unique signatures that would be challenging to detect on larger angular scales. Interestingly, several independent CMB observations seem to hint at a mild preference for an interacting dark sector that might help with cosmological tensions.

In this talk I will summarize how duality symmetries of supergravity models can potentially lead to global symmetries. These are associated to deformation classes of spaces with non-trivial duality bundle and classified by so-called bordism groups. Since global symmetries are not allowed for consistent theories of quantum gravity, there need to exist certain objects that break these global symmetries. We will discuss S-duality for type IIB supergravity as well as U-duality for maximal 8d supergravities and explore the necessary symmetry-breaking objects in the various setups.

I will present updated constraints on 'light' Dark Matter (DM) particles with masses between 1 MeV and 5 GeV. In this range, we can expect DM-produced e+/e- pairs to up-scatter low-energy ambient photons in the Milky Way via the inverse-Compton process, and produce a flux of X-rays that can be probed by a range of space observatories. Using diffuse X-ray data from XMM-Newton, INTEGRAL, NuSTAR and Suzaku, we set the strongest constraints to date on annihilating and decaying DM with masses between 150 MeV and 5 GeV.

The Drell-Yan processes pp->ll and pp->lnu at high transverse momentum can provide important probes of semileptonic transitions that are complementary to low-energy flavor physics observables. We derive constraints on the New Physics contributions to these processes by recasting the latest ATLAS and CMS searches for mono- and di-lepton resonances. We then study the interplay between low-energy observables and collider constraints, using the "HighPT" package. Moreover, we investigate the limits of validity of the Standard Model Effective Field Theory (SMEFT) in these regimes by comparing the constraints obtained for some explicit tree-level mediators and their EFT equivalent.

In this talk I show many aspects of gauged L_{\mu-\tau} symmetry. The symmetry gauges {the muon number – the tau number} in the standard model so as to be anomaly free. First I will show how it is interesting for phenomenology. It includes IceCube Gap, Hubble tension etc. Then I show our phenomenological model for lepton mass and mixing with the symmetry. In this model the symmetry appears as the family symmetry. Finally I will
present my trial to unify the symmetry with the Standard gauge group in terms of coset space family unification.

The existence of nonbaryonic dark matter (DM) in the Universe is compelling, as suggested by astrophysical and cosmological observations. The most commonly assumed production mechanism for DM in the early universe corresponds to the weakly interacting massive particle (WIMP) paradigm, in which DM has mass and couplings at the electroweak scale. However, the current null experimental results and severe constraints on the natural parameter space are forcing us to search beyond the standard WIMP paradigm. In this talk, I will review alternative DM production mechanisms in the early universe, both thermal and non-thermal, like the FIMP and the SIMP paradigms. The possible impact of alternative non-standard cosmological scenarios will also be analyzed. Finally, experimental avenues for DM detection are discussed.

The observed clustering of quasars on large angular scales is imprinted with information of our motion with respect to the cosmological rest-frame. Using the Catwise catalogue, based upon data from the Wide-field Infrared Survey Explorer (WISE), we infer posteriors for the kinematic dipole and the intrinsic quasar clustering dipole. We present a methodology for simulating dipole measurements including survey systematics such as; the survey mask, variable photometric errors, biased dust extinction modelling and contamination along the ecliptic plane. We then use Simulation-based Inference to infer our posteriors. We examine a range of different prior hypotheses, dipole estimators and magnitude cuts . When compared to measurements inferred from the Cosmic Microwave Background dipole and with a $\Lambda$CDM prior on the expected intrinsic quasar clustering dipole in agreement with past measurements we get a greater than 6 $\sigma$ tension with the CMB measured kinematic dipole when estimating the dipole for the entire sample and a greater than 9 $\sigma$ tension with a spectrally binned dipole estimator.

Scale separation is the important phenomenological property for a given vacuum to have an internal compact space much "smaller" than the extended spacetime, so that a lower-dimensional effective description indeed makes sense. At a practical level, this is measured by the decoupling or not of the magnitude of the cosmological constant from the Kaluza-Klein scale. The status of scale separation in AdS vacua obtained from string theory is under debate. On the one hand bottom-up constructions in type IIA string theory seem to achieve this hierarchy of scales parametrically. However on the other hand some Swampland arguments as well as holographic considerations cast a pessimistic shadow on scale separation and about its presence in the Landscape. After an overview of the evolution of scale separation in string theory over the last twenty years, I will present new families of scale separated vacua obtained via a 4d EFT analysis in massless type IIA flux compactifications on elliptic fibrations with metric fluxes. Parametric scale separation is achieved by an asymmetric flux rescaling which, however, in general is not a simple symmetry of the 4d equations of motion. At this level of approximation the vacua are stable but, unlike in the Calabi-Yau case, they display a non-universal mass spectrum of light fields.

Symmetries are a powerful tool in the physicist’s toolkit, and when it comes to symmetric functions, rarely can one find functions as symmetric as automorphic forms. What makes certain automorphic forms powerful is their connection to arithmetic, via the theory of L functions. They are core to some of the most challenging and ambitious open problems in modern mathematics, and played a central role for example in the proof of Fermat’s last theorem. These arithmetic structures are valuable to physicists too. The “how” will be addressed in this talk. I will go over what automorphic forms are, how they give rise to arithmetic structures in string theory and quantum field theory, and some applications. Due to the technical nature of this subject, most of it will be an overview with a few new insights and WIP towards the end.

Since more than 10 years, an unexpected gamma-ray component over astrophysical backgrounds has been detected at GeV energies towards the Galactic center in the data of the Fermi-Large Area Telescope. Initially, this excess was considered to be hinting at GeV thermal relics annihilating in the Galactic dark matter halo. However, recent works have demonstrated that the excess is better explained by a population of millisecond pulsar-like sources in the Galactic bulge. While the excess photon flux is peaked at about few GeV, a significant high energy tail extending up to tens of GeV has been detected. Such high-energy photons are naturally explained by the inverse Compton emission of electrons and positrons emitted by millisecond pulsars.
I will present a novel study of the morphology and photon statistics of the Galactic center excess at energies above 10 GeV. This is based on an innovative method, which combines adaptive template fitting and pixel-count statistics in order to assess the role of sub-threshold point sources while minimizing the mis-modelling of Galactic diffuse emission backgrounds.

Using trajectories in quantum cosmology.
Cosmological perturbation theory aims at producing the primordial spectrum
by setting quantum vacuum initial conditions over a classically evolving background.
Although appearing potentially inconsistent, such an approach can be justified in
canonical quantum gravity for inflationary models by decohence. The situation may
not always be so clear-cut, but defining quantum perturbations in a computational
useful manner seems to require some way of defining a trajectory. Various proposals
are available, and I’ll describe two of them, namely the well-known de Broglie Bohm
eikonals and the coherent state approach with phase space formulation. I’ll introduce
both and discuss some consequences that can be more easily worked out.

In this talk, I will show you my investigation of the effect of significant small-scale primordial non-Gaussianity on structure formation and the galaxy formation process. Generically, I find a distinct and potentially detectable feature in the matter power spectrum around the non-linear scale. The feature might have interesting consequences for the S8 tension. I then show in particular that a positively skewed distribution with fNL of the order of 1000 implies that typical galaxy-sized halos reach half of their present-day mass at an earlier stage and have a quieter merging history at z < 3 than in the Gaussian case. Their environment between 0.5 and 4 viri al radii at z = 0 is less dense than in the Gaussian case and may impact the universality of the NFW profile. This quieter history and less dense environment has potentially interesting consequences in terms of the formation of bulges and bars. In particular, I show that all feedback prescriptions being otherwise identical, simulations with a positively skewed distribution , hence forming galaxies a bit later than in vanilla LCDM, allow to form simulated galaxies with more disky kinematics than in the vanilla case. Therefore, such small-scale primordial non-Gaussianities could potentially help alleviate, simultaneously, tensions in cosmology and galaxy formation. These simulations on small scales will need to be complemented with larger box "zoom-in" simulations with scale-dependent non-Gaussianities, to statistically confirm these trends and explore their observational consequences in further detail.

Over the last two decades, the standard cosmological model "Λ cold dark matter" (ΛCDM) has been firmly established by a variety of observations. Nevertheless, the nature of the dominant components of the Universe – namely dark matter and dark energy – and the process of creation of its initial conditions – namely inflation – are not yet known. Furthermore, as the precision of the data has increased, intriguing anomalies have appeared within the standard cosmological model. These anomalies arise from a discrepancy between the value of some cosmological parameters predicted from their calibration in the distant Universe, for example with the cosmic microwave background (CMB), and the measurement of these parameters from the local Universe. The two most significant tensions in modern cosmology concern the parameter that determines the expansion rate of the Universe, i.e., the Hubble parameter H0, and the parameter that quantifies the amplitude of local matter fluctuations, i.e., the parameter S8. A number of surveys dedicated to measuring large-scale structures (LSS) of the Universe (BOSS and eBOSS in particular, and the forthcoming DESI and EUCLID) can be used to weigh in on these cosmological tensions. The first axis of this thesis is based on a semi-analytical method, known as effective field theory of large-scale structures (EFTofLSS), which provides an accurate description of the galaxy power spectrum, and aims at improving cosmological constraints from LSS surveys, a major challenge in the context of the forthcoming data from DESI and Euclid. In particular, we apply an EFTofLSS analysis to describe the BOSS and eBOSS data, and demonstrate that these surveys can be used to obtain constraints that are competitive with those coming from CMB data. We also establish the self-consistency of this theory within the ΛCDM model, as it appears at first sight that the different EFTofLSS parametrizations proposed in the literature seem to provide different constraints on cosmological parameters. We thus identify these discrepancies as originating from subtleties of the commonly used Bayesian framework. In addition, we develop a systematic method for applying the EFTofLSS to different LSS data and obtain robust results contrasting the Bayesian framework with the frequentist approach, for an in depth cosmological inference. The second aspect of my work aims to establish the possible theoretical implications of cosmological tensions for our understanding of the dark sector of the Universe. In particular, we confront various models that can resolve these tensions with the BOSS and eBOSS data analysed under the EFTofLSS. We conclude that current clustering data are not in tension with ΛCDM, and can be used to set important constraints on model suggested to resolve cosmological tensions.

The history of the universe between inflation and the onset of BBN - covering half its lifetime on a log scale - is essentially unconstrained by observations. In this talk, I will discuss how general features of String Theory motivate an unusual cosmology in this period, (co-)dominated by the energy density of scalar fields such as moduli and axions. After reviewing the physics of moduli and axions in realistic string compactifications, I will describe how exotic epochs such as kination and trackers can arise in this setting, and present a complete, alternative history of the early universe based on such considerations. In the second part of the talk, I will discuss some observational consequences, and in particular show how perturbation growth in these stringy cosmologies is substantially enhanced compared to the standard ones. If time permits, I will conclude with some applications to baryogenesis, dark matter and gravitational wave production. Based on 2311.12429, 2401.04064 + work in progress

Finding a theoretical explanation to the Hubble tension is extremely challenging, since any serious solution must also account for all other astrophysical and cosmological tests, along with experimental constraints on physics. The gravitational laws in the dark sector are much less constrained, mainly due to our ignorance of the nature of its physical origin. I will show how a non-minimally coupling of dark matter only, which satisfies all the tests in the solar system by construction, can alleviate the Hubble tension. However, given that the constraints on the expansion rate span a large range of redshifts and thus distances, this model cannot fully resolve the issue, just as the other contenders. Furthermore, one of its main features is that it predicts a lower fraction of cold dark matter today, a conclusion which is challenged by the most recent weak-lensing and supernovae results.

In the fully nonlinear regime, gravitational waves can reveal a drastic new phenomenology absent in the linearized theory. This is best illustrated by investigating exact non-perturbative radiative solutions of General Relativity. When new fields are introduced, such as in modified scalar-tensor theories, the scalar-tensor mixing can drastically modify the phenomenology of gravitational waves. So far, most of the efforts have focused on investigating the effects of these scalar-tensor mixing at the perturbative level.
In this talk, I will present a new exact radiative solution in Horndeski gravity describing the nonlinear gravitational waves emitted by a scalar pulse. I will use this opportunity to describe the non-perturbative methods allowing to analyze the gravitational wave content of a given spacetime geometry (Petrov classification, Penrose limit). Then, I will discuss how this exact new solution can be constructed using the disformal solution-generating map and comment on its new phenomenology. In particular, this analysis will show that in such higher order scalar-tensor theory, shear (tensorial modes) can be generated by a purely (time-dependent) scalar monopole, a feature which only emerges at the fully nonlinear level.
This work aims at providing a tractable toy model to explore the nonlinear radiative regime of modified gravity theories. The talk will be based on the following article: https://arxiv.org/abs/2401.05099

In this talk, I discuss recent progress in constructing explicit examples of de Sitter vacua at leading order in the \alpha' and g_s expansions as envisioned by Kachru, Kallosh, Linde and Trivedi 20 years ago. Initially, I will specify explicit Calabi-Yau orientifolds, and choices of quantised fluxes leading to exponentially small flux superpotential. I derive the four-dimensional effective supergravity theories, incorporating the leading non-perturbative superpotential terms from Euclidean D3-branes, and the leading corrections to the Kähler potential that are inherited from N=2 supersymmetry. Subsequently, I will demonstrate how warped Randall-Sundrum throats and SUSY breaking uplifts from anti-D3 branes can be incorporated in this construction. Lastly, I will briefly comment on the robustness of the construction against genuinely N=1 corrections in the \alpha’ and g_s expansions which are however not fully known.