Seminars

We seem to be faced with an impossible task: to determine the precise microphysical model of dark energy. I will argue that, nevertheless, it may be possible to determine certain features of the nature of dark energy. In particular, and in light of recent cosmological data, I will show that it is unlikely that the dark energy is a simple, minimally coupled, “thawing”, scalar field. This means that there is scant evidence for, for example, rolling scalar fields like axions or other such simple models. A careful analysis seems to indicate a very strong preference (on any measure) for a non-minimally couple scalar field which brings with it a host of undesirable properties: time varying Newton’s constant and fifth forces. We will show that there are ways of getting around these problems, but we are left with the unwelcome requirement of further new physics on non-cosmological scales. I will conclude by surveying the narrow range of options that are available to explain current cosmological data.

The Hubble Tension is often quoted at >5 sigma significance, suggesting definitive evidence for new physics. In this talk, however, I present evidence that suggests uncertainties have been underestimated in the Cepheid-supernova (SN) analysis that strongly influences the Hubble Tension landscape. I then discuss a recent James Webb Space Telescope (JWST) program which undertook a partially blinded, multi-method check on the Hubble Telescope (HST) distance measurements underlying previous distance ladder H0 estimates. The new JWST results yielded a lower H0 (70.4 ± 1.9 km/s/Mpc) than the latest SH0ES estimate (73.17 ± 0.86 km/s/Mpc), despite sharing the same geometric zero point calibration and SNe. Resolving this inconsistency in local distance ladder measurements is paramount before a realistic assessment of the Hubble Tension can be made.
To that end, I introduce a new, blinded analysis of the SNe used to determine H0 within the Union3+UNITY SN cosmology framework. Preliminary findings have revealed discrepancies with (and within) Pantheon+, including a systematic offset in their host masses that both biases their H0 and suppresses their evidence for evolving dark energy. We also see significant disagreements over the colors of SNe when estimated from identical data. Once unblinded, we will report separate H0 values based on either the SH0ES-Cepheid or CCHP-TRGB distances.

In the first part of my talk, I will review the phenomenology required to alleviate the Hubble tension through late-time new physics, focusing on the so-called angular (2D) and anisotropic (3D) BAO data, which themselves are in mutual tension. I will then present a recent model, the wXCDM, which combines quintessence with an exotic component known as “phantom matter.” This component satisfies the strong energy condition but exhibits negative energy density and positive pressure. The wXCDM model outperforms its competitors and resolves the H₀ tension when tested against a comprehensive dataset that includes 2D BAO. However, when angular BAO is replaced by 3D BAO — as expected — the model can no longer yield high values of H₀. Nevertheless, it still produces low chi-squared values, comparable to those found with the CPL parametrization. Finally, I will describe the Weighted Function Regression method, which enables a Bayesian and model-agnostic reconstruction of the effective dark energy properties and the late-time cosmic expansion history. I will also assess the impact of supernova data on quantifying the statistical evidence for dynamical dark energy in the late universe.

Time-delay cosmography with lensed quasars is a one-step method for estimating the Hubble constant in the local Universe independently of the cosmic distance ladder. It does not require any intermediate calibration and relies on measuring the time delays between multiple images of strongly lensed quasars, which are inversely proportional to the Hubble constant.

 In this talk, we present cosmological constraints from eight strongly lensed quasars (hereafter, the TDCOSMO-2025 sample), based on a new blind analysis by the TDCOSMO collaboration designed to prevent experimenter bias. Building on previous work, we have improved our modeling of line-of-sight effects, the surface brightness profiles of lens galaxies, and stellar orbital anisotropy, and we have corrected for projection effects in the lens dynamics. Our uncertainties are maximally conservative, accounting for the mass-sheet degeneracy in the deflectors’ mass density profiles, constrained by new measurements of stellar velocity dispersions from spectra obtained with the James Webb Space Telescope (JWST), the Keck Telescopes, and the Very Large Telescope (VLT), and using improved methods.

 Our primary result, H_0 = 72.1+4.0−3.7 km/s/Mpc, is derived from the TDCOSMO-2025 sample combined with Ω_m constraints from the Pantheon+ Type Ia supernova (SN) dataset. We also present measurements of the Hubble constant combining TDCOSMO-2025 with external datasets from the Sloan Lens ACS (SLACS) and Strong Lenses in the Legacy Survey (SL2S) lens sample, further improving the precision.

The Hubble constant measurement is robust against the addition of external lens samples, the choice of different cosmological models beyond the ΛCDM model, and the use of the Ω_m prior from other datasets, such as the DESI DR2 BAO or the DES Year-5 SN sample.

Spectroscopic galaxy surveys are among the most powerful tools in modern cosmology, allowing us to map the large-scale structure of the Universe and constrain its fundamental parameters. In this talk, I will introduce the principles behind galaxy redshift surveys, focusing on the design and goals of the Dark Energy Spectroscopic Instrument (DESI). I will highlight key cosmological observables, including Baryon Acoustic Oscillations (BAO), and explain how DESI uses them to test models of dark energy and cosmic acceleration. I will then present recent results from DESI’s Year 1 and Year 3 analyses. Finally, I will discuss the emerging evidence for possible evolution in the dark energy equation of state and its implications for the standard cosmological model.

A new avenue was recently developed for analyzing large-scale structure data which does not depend on assumptions about the power spectrum shape, the specific background expansion, or the growth function. In this talk I discuss how this model-independent methodology can be applied to answer three fundamental questions: a) is space curve?; b) is gravity Einsteinian?; c) is the equivalence principle violated?

Cosmology is currently in a crisis known as the Hubble tension, the observation that redshift increases with distance about 10% faster than expected in the ΛCDM standard cosmological paradigm with parameters calibrated to fit the CMB anisotropies. A promising explanation for this is that we live near the centre of a large local underdensity or void. This is suggested by observations of source number counts across the whole electromagnetic spectrum, with near-infrared results implying that the density is about 20% below average out to 300 Mpc across 90% of the sky and most of the galaxy luminosity function (ApJ, 775, 62). Outflows from this KBC void can induce enough extra redshift to plausibly solve the Hubble tension (MNRAS, 499, 2845). I will discuss various tests of this proposal. At low redshift, the bulk flow of galaxies traces the average velocity of matter within a sphere centred on our location. The observed bulk flow curve is in good agreement with the void model predictions (MNRAS, 527, 4388). Looking further out, it is possible to infer the H₀ parameter from data in a narrow redshift range centred on z. Such an empirical H₀(z) curve agrees quite well with expectations in the void model, which predicts a return to the CMB-derived H₀ beyond the void (MNRAS, 536, 3232). This result is related to recently submitted work on baryon acoustic oscillations (BAOs), which show a deviation from ΛCDM expectations (Arxiv:2501.17934). I will explain how the BAO observables would be affected by a local void. I will then present BAO results compiled over the last twenty years. These results fit better if the local void is included, thanks to good agreement with ΛCDM at high redshift but a persistent anomaly at lower redshift.

The cosmic microwave background (CMB) serves as a unique backlight for tracing the growth of cosmic structures. By measuring arc-minute-scale deflections experienced by CMB photons due to gravitational lensing, we can map matter distributions at high redshifts. This lensing signal provides a powerful probe of fundamental physics—such as the sum of neutrino masses—and enables consistency tests of the standard cosmological model by comparing observed and predicted large-scale structure growth.
In this talk, I will review recent and forthcoming advances in this rapidly evolving field, with a particular focus on the DR6 CMB lensing results from the Atacama Cosmology Telescope (ACT). I will also discuss ongoing efforts to produce state-of-the-art lensing maps using the final data release of ACT by combining night-time, daytime, and large-scale CMB observations from Planck. I will then explore the transformative potential of the Simons Observatory and discuss the implications of these lensing measurements in the context of understanding cosmic structure growth and addressing the S₈ tension.

The Dark Energy Survey Supernova sample is the largest and deepest Type Ia Supernova (SN Ia) sample from a single telescope to date. It includes 1600 photometrically identified SNe Ia with high-quality multi-band light curves and spectroscopic redshifts. With a redshift range spanning between 0.1 to 1.2 and a well-defined selection function, this SN sample constitutes an ideal dataset for cosmology. In my talk, I will present the cosmological results from this unique sample and show that DES SNe, combined with publicly available low-z SN samples, provides excellent constraints on the Dark Energy equation of state from SN Ia. I will briefly discuss how the DES-SN compares to previous SN cosmological results and how these measurements, combined with BAO and CMB, seem to provide tantalizing evidences for evolving dark energy. Looking towards the future, I will discuss how future SN surveys like Rubin and Roman will revolutionise SN Ia cosmology and potentially answer some of the most pressing questions of modern cosmology.

The present-day expansion rate of the universe, known as the Hubble Constant, is one of the few directly measurable cosmological parameters. In recent years, the persistent disagreement between the value of the Hubble Constant obtained from direct distance-ladder measurements to nearby galaxies and the value inferred from observations of the Cosmic Microwave Background (CMB), assuming a standard Lambda-CDM cosmological model, has become one of the strongest indications of new physics. While the most precise distance ladder currently uses Cepheids, independent measurements of distances made with other standard candles can serve as a cross-check for systematics in distance measurements, helping to either solidify or resolve the tension. 

One such alternative precision distance indicator and calibrator of Type Ia supernovae (SNe) is Mira variables – highly evolved, asymptotic giant branch stars – which are astrophysically distinct from the more commonly used Cepheids or the Tip of the Red Giant Branch (TRGB). In addition to being highly luminous and ubiquitous, Miras can be detected and characterized using only near-infrared and infrared observations, which is particularly advantageous in the era of the JWST and the Roman. In my talk, I will discuss the current progress in the development of the Mira distance ladder and its Hubble Constant measurements, with a focus on recent HST and JWST observations of Mira variables in M101, the nearest recent host galaxy of a Type Ia supernova.