Talks

We study the weak-gravity regime of higher-order scalar-tensor theories that are degenerate in the unitary gauge. In a certain subset of theories analogous to Lorentz-violating scalar-tensor theories, we show that the Vainshtein mechanism due to nonlinear derivative interactions does not work. For this family of theories we determine all the PPN parameters in terms of the EFT of dark energy parameters and discuss the experimental bounds.

In recent years, it has been discovered that inversion symmetry broken systems can exhibit non-linear Hall effects even under time-reversal symmetric conditions [1]. The underlying quantum objects leading to this phenomena are the moments of the Berry curvature, termed the Berry curvature multipoles. This opens up avenues for exploring fundamental physics and possible applications [2,3]. However, despite such promise, the Berry curvature multipole induced non-linear Hall effect has been experimentally realized only in a handful of materials. It is, therefore, of vital importance to find materials with large and controllable Berry curvature multipoles.

In this talk, I will give examples from our work where such a controllable Berry curvature dipole has been predicted. First, we propose Janus monolayers of transition metal dichalcogenides as a promising materials platform to explore the non-linear Hall effect and Berry curvature dipole physics [4]. Here the topology and the Berry curv...

Fluctuations and disorder effects are substantially enhanced in reduced dimensionalities. While they are mostly considered as the foe for long-range orders, fluctuations and disorders can also stimulate the emergence of novel phases of matter, for example, vestigial orders. Taking 2D magnetism as a platform, existing efforts have been focused on maintaining 2D long-range magnetic orders by suppressing fluctuations, whereas the other side, exploiting fluctuations for realizing new 2D magnetic phases, remains as an uncharted territory. We will use a suite of optical spectroscopy techniques to demonstrate the magnetic phase evolution of NiPS3 as functions of temperature and dimensionality and demonstrate the crossover from the 3D zigzag AFM to the 2D Potts nematic phase.

We propose twisted van der Waals heterostructures as an efficient, reliable and scalable quantum platform that enables the seamless realization and control of a plethora of interacting quantum models in a solid state framework. These new materials hold great promise to realize novel and elusive states of matter in experiment. We survey these systems as platform to study strongly correlated physics and topology that is notoriously difficult to study computationally [1]. Among the features that make these materials a versatile toolbox are (i) tunability of properties via readily accessible external parameters (such as gating, straining, packing and twist angle), (ii) ability to realize and control a large number of fundamental many-body quantum models relevant in the field of condensed matter physics and beyond and (iii) state-of-the-art experimental readouts exist to directly map out their rich phase diagrams in and out of equilibrium. This general framework, besides unraveling new phas...

We present key cosmological findings from the Dark Energy Spectroscopic Instrument (DESI)’s first year baryon acoustic oscillations (BAO) measurements. DESI's BAO provide robust measurements of the transverse comoving distance and Hubble rate across seven redshift bins, spanning a redshift range of 0.1 < z < 4.2. DESI BAO data alone align well with the flat ΛCDM model with Ωm=0.295±0.015. Paired with a baryon density prior from Big Bang Nucleosynthesis and the acoustic angular scale from the cosmic microwave background (CMB) data, we find H0=68.52±0.62 km/s/Mpc. Combined analyses with CMB anisotropies and lensing from Planck and ACT yield Ωm=0.307±0.005 and H0=67.97±0.38 km/s/Mpc. Extending the baseline model with a constant dark energy equation of state parameter, w, results in w=−0.99+0.15−0.13. In a dark energy model with time-varying equation of state parametrized by w0 and wa, combined with various supernovae data, indicate deviations from ΛCDM at significance levels up to 3.9σ. For flat ΛCDM with the sum of neutrino mass free, DESI and CMB establish an upper limit of ∑ mν <0.072 (0.113) at 95% confidence for a ∑mν>0 (0.059) eV prior. We will also show forecasts for Y3 and Y5 results as well as prospects with DESI II.

We present a model that modifies general relativity on cosmological scales, specifically by having a 'glitch' in the gravitational constant between the cosmological (super-horizon) and Newtonian (sub-horizon) regimes. This gives a single-parameter extension to the standard ΛCDM model, which is equivalent to adding a dark energy component, but where the energy density of this component can have either sign. Fitting to data from the Planck satellite, we find that negative contributions are, in fact, preferred. Additionally, we find that roughly one percent weaker superhorizon gravity can somewhat ease the Hubble and clustering tensions in a range of cosmological observations. Therefore, the extra parametric freedom offered by our model deserves further exploration, and we discuss how future observations may elucidate this potential cosmic glitch in gravity, through a four-fold reduction in statistical uncertainties.

Measurements of the large-scale distribution of matter in the Universe are one of our primary tools for testing the predictions of general relativity on cosmological scales. I will describe how we pursue this using data from galaxy imaging surveys, focusing on Dark Energy Survey galaxy clustering and weak lensing analyses as an example. I will highlight results from the DES Year 3 analysis that are relevant for testing gravity, some practical aspects of extending survey analyses beyond ΛCDM, as well as ongoing work to address these challenges to prepare for future surveys.

I will introduce mochi_class, a refined version of the popular Einstein-Boltzmann solver hi_class optimised for calculations within Horndeski's gravity framework. Thanks to (i) a re-parametrisation of Horndeski functions, (ii) a numerically stable quasi-static approximation, and (iii) support for time-dependent inputs, mochi_class enhances hi_class capabilities and nicely complements other public Einstein-Boltzmann solvers. Additionally, I will present a non-parametric approach that, when integrated with Principal Component Analysis, can effectively reconstruct Horndeski functions from well-studied modified gravity models, extending the exploration of scalar-tensor theories beyond conventional parametrisations. I will conclude by highlighting practical applications where mochi_class can prove instrumental in analysing current and forthcoming large-scale structure data.

We present a comprehensive joint analysis of two distinct methodologies for measuring the masses of galaxy clusters: hydrostatic measurements and caustic techniques. We show that by including cluster-specific assumptions obtained from hydrostatic measurements in the caustic method, the potential mass bias between these approaches can be significantly reduced. While this may appear to diminish the caustic method as a technique independent of the dynamical state of a cluster, it provides a means to refine mass constraints and offers an avenue for scrutinizing modifications to gravity. Applying this approach to two well-observed massive galaxy clusters A2029 and A2142, we find no discernible mass bias, affirming the method's validity. We draw a similar conclusion when applying this approach to modified gravity models. Specifically, our implementation allows us to investigate Chameleon and Vainshtein screening mechanisms, enhancing our understanding of these modified gravity scenarios. Furthermore, we explore the prospect of achieving more precise constraints with fewer systematic errors by exclusively employing the caustic method to constrain screening mechanisms on a larger scale, encompassing several hundred stacked galaxy clusters.