Seminars

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.

The Kilo Degree Survey (KiDS) has collected all of its images which form the basis of its 5th and final data release (KiDS-Legacy). In this talk I will summarise the resulting cosmic shear analysis from KiDS-Legacy and detail the various systematic and consistency tests that we performed to ensure its robustness. We find that with our new analysis choices and extra data the tension in S8 with respect to CMB data from Planck-Legacy has reduced to less than 1 sigma (assuming a flat-LCDM model), rendering it insignificant. At the end of this talk, I will point to the remaining cosmological analyses that are expected to be released in the near future.

The South Pole Telescope (SPT) and its third-generation camera (SPT-3G) are a highly sensitive facility for observing cosmic microwave background (CMB) temperature and polarization anisotropies at small angular scales. The power spectra of CMB primary anisotropies and lensing obtained from SPT-3G data are providing new critical information on the composition and evolution of the universe, shedding new light on long-lasting cosmological tensions such as the Hubble tension, and discovering potentially new ones like the discrepancy of the matter density constraint from CMB and BAO data. In this talk, I will give an overview of the most recent SPT-3G results obtained from the two-year survey of the SPT Main field (SPT-3G D1). I will then present near-future prospects, showing that an upcoming extended survey with SPT-3G will deliver constraints on key cosmological parameters up to a factor of two more tightly than Planck, which will soon further advance our understanding of the cosmos.

The Hubble tension points to possible gaps in our understanding of physics around the epoch of recombination. A key aspect of the tension is its sensitivity to the sound horizon at decoupling, r_*, whose value depends on the microphysics of recombination. It is therefore highly desirable to obtain empirical constraints on both H_0 and r_* without relying on model-dependent assumptions. I will present recent work that demonstrates how combining baryon acoustic oscillations with CMB lensing enables such sound-horizon-agnostic measurements of the Hubble constant. I will also provide an update on primordial magnetic fields as a potential mechanism for alleviating the Hubble tension.

Cepheid variables remain the most robust and widely used primary distance indicators. In this talk, I will review recent developments in the Cepheid-SNIa distance ladder, focusing on key improvements in geometric distance measurements in anchor galaxies, particularly from eclipsing binaries and Gaia parallaxes, which calibrate the Cepheid Period-Luminosity relation. The current status of the Cepheid metallicity dependence — which has long been debated — will also be presented. I will discuss new results from JWST observations that provide an independent check on HST-based Cepheid distances to a subset of SNIa host galaxies, effectively ruling out crowding as the source of the Hubble tension. Lastly, I will review progress from an ongoing HST program designed to cross-calibrate distances derived from Cepheids, the Tip of the Red Giant Branch (TRGB), and the JAGB method, aiming to better understand and constrain systematics affecting each technique.