Seminars

The DESI DR2 claim of evolving dark energy rests on a frequentist analysis of seven BAO data points in a two-parameter model. I will present a comprehensive Bayesian reanalysis using the unimpeded nested sampling database — 248 dataset & model combinations spanning Planck, DESI, DES, Pantheon, ACT, SPT and SH0ES — showing that the evidence for dynamical dark energy is weaker than advertised and largely driven by inter-dataset tension rather than genuine evolution. Moving beyond w0wa, I will show flexible dark energy reconstructions using transdimensional flexknot models in JAX on GPU, where the data themselves select the model complexity, and argue that a supernova magnitude offset provides a better fit than exotic dark energy. More broadly, I will make the case that GPU-accelerated classical statistical methods — nested sampling, HMC, Laplace approximation — are competitive with neural network approaches across astronomy, from gravitational waves to 21-cm cosmology, and that large language models are transforming how we build and verify these analyses without replacing the science itself.

This talk presents recent developments toward incorporating gravitational wave standard sirens into the portfolio of observable probes used for precision measurements by major experiments in the cosmology community. In particular, we present recent results led by our group using data from the LIGO-Virgo-KAGRA (LVK) and the Dark Energy Survey (DES) collaborations followed by a discussion of the potential of this type of multi-messenger analysis to inform the ongoing Hubble tension debate in the near future.

After more than a quarter of a century as the standard model of Cosmology, the Λ Cold Dark Matter (ΛCDM) paradigm is increasingly challenged by combinations of observations from galaxy clustering, weak lensing, Type Ia supernovae, and the cosmic microwave background. This talk will critically review recent results from DESI++ and DES++, and contrast the evidence supporting ΛCDM with emerging indications for an evolving dark energy component.

The Hubble tension is one of the central topics of cosmology, possibly pointing to new physics beyond LambdaCDM. Furthermore, recent baryon acoustic oscillation (BAO), combined with type Ia supernova, observation unveils a new “tension” indicating possible gaps in our understanding of the recent expansion history of our Universe. One way of explaining the Hubble tension is to introduce new physics in the early Universe during recombination while the BAO “tension” might be explained by evolving dark energy in the late Universe. I will present recent works that study the origin of the cosmological tensions by assuming either early- or late-Universe modification. Based on such analysis, I will discuss how a non-minimally coupled scalar field theory, dubbed thawing gravity, can unify the early- and late-Universe modifications and explain the cosmological tensions.

The most precise inference of the Hubble constant uses a three-rung distance ladder (geometry to Cepheids to supernovae), where the supernovae are needed to probe the Hubble flow where peculiar velocities are negligible. However, recent advances in the Bayesian Origin Reconstruction from Galaxies (BORG) algorithm have provided highly accurate local peculiar velocity fields with precisely-characterised uncertainties, enabling quality constraints on the Hubble constant without the third rung. I will describe a hierarchical Bayesian forward model to infer H0 from SH0ES Cepheids and geometric anchors alone, marginalising over the Cepheid period-luminosity relation, galaxy distances and various nuisance parameters of the velocity field, including a statistically rigorous accounting for selection effects. For the fiducial selection model the result is H0 = 71.7±1.3 km/s/Mpc, slightly lower than (though consistent with) the SH0ES result and discrepant with the CMB-inferred value at 3.3 sigma. Alternative selection models produce at most a 1-sigma shift. As well as supporting supernovae as accurate contributors to the Hubble tension and highlighting the vital importance of robust peculiar velocity fields, this result demonstrates great promise for future two-rung H0 inferences incorporating more data. I will also stress a few general statistical aspects of distance-ladder modelling, particularly the need for an r^2 prior on distances and a principled selection model — and how badly things can go wrong if inadequate statistics are used.

Observations of type Ia supernovae (SN Ia) play a starring role in two cosmological surprises: the accelerated expansion of the Universe driven by dark energy and the discrepancy between the measured and inferred Hubble constant from the late and early Universe. I will describe the contemporary use of SN Ia to measure cosmological distances, with an emphasis on the limiting factors in their application. With ongoing and upcoming surveys, we are passing a threshold beyond which systematic uncertainties limit the cosmological utility of SN Ia. However, as the number of SN Ia we can study grows, and we broaden the way we study them, we are also gaining new insights that we can apply to the problem. I will describe recent advances in our understanding of the progenitors and explosions of SN Ia and their correlations with their host-galaxy and larger-scale environments that are pointing the way make better use of SN Ia samples in measuring the Hubble constant and the properties of dark energy.

Scalar fields are ubiquitous in string theory compactifications, arising from geometric moduli and as descendants of higher-dimensional form fields. In cosmology, they provide well-motivated frameworks for dynamical dark energy and interacting dark sectors, allowing for time-varying equations of state and an effective phantom behaviour, features that have recently attracted attention in connection with late-time deviations from ΛCDM.

At the same time, theoretical consistency places important conditions on the scalar potentials, couplings, and field-space geometry that can be realised in such models. Swampland considerations, fine-tuning issues, and stability criteria play a central role in evaluating their viability and offer a useful complement to purely phenomenological approaches.

In this talk I will present recent progress on single- and multi-scalar dark-energy models, including coupled dark sectors, and discuss their cosmological implications within a theoretically consistent framework. I will highlight how theoretical limitations may shape the space of viable models and where new dynamics may remain possible. I will also comment on how reconstruction approaches, including modern data-driven techniques, may help connect theoretical constraints to emerging observational trends.

I explore how cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) measurements constrain cosmological models. The CMB angular scale provides robust constraints on the ratio of sound horizon to angular diameter distance, limiting possible deviations from the standard ΛCDM model. The null energy condition applied to a separate dark energy component imposes strict inequalities on BAO observables relative to ΛCDM predictions, restricting the freedom to fit new data within standard cosmological frameworks.  I’ll discuss what this means for latest BAO results and other possible interpretations.

The last few years have seen a substantial increase in the number and quality of distance indicators that can be used in the nearby Universe.  Combined, these indicators strengthen the determination of the local Hubble constant; but the combination must properly account for their interdependence. Enter the Distance Network: a new framework to incorporate all relevant distance indicators in a robust, statistically rigorous formalism, with full error propagation and statistical tests to identify possible outliers, developed during a workshop at the International Space Science Institute (Bern) with the participation of many experts in all such methods.

The Distance Network yields an improved determination of the Hubble constant, with a baseline value of 73.50+/-0.81 km/s/Mpc, over 7 sigma from recent Lambda-CDM based estimates.  We test many variants that include, exclude, or modify various methods; all yield values between 72.5 and 74.0 km/s/Mpc.  We will share both the code and the data for the Distance Network, enabling the inclusion of future measurements in this framework.

The cosmic microwave background (CMB) anisotropies remain the cleanest, most powerful probe of fundamental physics in the cosmos.  Measurements of the small-scale CMB temperature and polarization fields have recently undergone transformative improvements with Data Release 6 (DR6) of the Atacama Cosmology Telescope (ACT) and will soon improve further with the Simons Observatory, which will open new windows into physics beyond the standard models (BSM) of particle physics and cosmology.  I will first discuss our recent cosmological parameter constraints from the ACT DR6 CMB power spectra, with a particular emphasis on constraining BSM physics operating just prior to recombination, including new relativistic particles and new pseudo-scalar fields.  I will then turn to novel searches for BSM physics in CMB secondary anisotropies, as could be imprinted by the screening of CMB photons by massive dark photons (DPs) or axion-like particles.  I will show the first results of searches for these signals in CMB data, enabled by our state-of-the-art needlet internal linear combination code, yielding leading bounds on kinetically mixed DPs and axion-photon couplings covering two decades in DP or axion particle mass.  I will conclude with a look ahead to the prospects for BSM physics from the Simons Observatory.