Journal Club

We explore the notion that cosmological models that modify the late-time expansion history cannot simultaneously fit the SH0ES collaboration’s measurements of the Hubble constant, DESI baryon acoustic oscillations data, and Type Ia supernova distances. Adopting a few simple phenomenological models, we quantitatively demonstrate that a satisfactory fit with a model with late-time expansion history can only be achieved if one of the following is true: 1) there is a sharp step in the absolute magnitude of Type Ia supernovae at very low redshift, $z\sim 0.01$, or 2)  the distance duality relation, $d_L(z)=(1+z)^2d_A(z)$, is broken. Both solutions are trivial in that they effectively decouple the calibrated SNIa measurements from other data, and this qualitatively agrees with previous work built on studying specific dark-energy models. We also identify a less effective class of late-time solutions with a transition at $z\simeq 0.15$ that lead to a more modest improvement in fit to the data than models with a very low-z transition. Our conclusions are largely unchanged when we include surface brightness fluctuation distance measurements, with their current systematic uncertainties, to our analysis. We finally illustrate our findings by studying a  physical model which, when equipped with the ability to smoothly change the absolute magnitude of Type Ia supernovae, partially resolves the Hubble tension.

Ongoing tensions in cosmology have motivated the study of extensions to Lambda-CDM. A promising route is to consider interactions in the dark sector, with a transfer of energy-momentum between dark matter and dark energy. In this talk, I will discuss the cosmological effects of new classes of interacting dark sector models, which couple the intrinsic entropy of dark matter fluids with scalar field dark energy. I show that these models give rise to a pure-momentum transfer, modifying only the Euler equation up to linear order, while leaving the background expansion history unchanged. The effects of these interactions can be seen in the suppression of the matter power spectrum and structure growth at late times. Meanwhile, key features of the primary CMB anisotropies are unchanged, with only small ISW effects occurring on large scales. I conclude by outlining ongoing work on potential observational signatures associated with intrinsic entropy perturbations.

The recent observational evidence of deviations from the Λ-Cold Dark Matter (ΛCDM) model points towards the presence of evolving dark energy, which is most naturally modeled as a cosmological scalar field driving the accelerated expansion. Despite this, I argue that the simplest scalar field proposals, quintessence, are not a good description of the cosmological data and that, if dark energy is driven by a scalar field, the cosmologically data strongly prefer more exotic scalar field proposals involving non-minimal couplings or non-canonical kinetic terms. However, such proposals invariably imply significant ancillary gravitational consequences which are disfavored by other (non-cosmological) data. In light of these results, I offer an assessment of the present and future of scalar field dark energy.

Cosmological parameters represent fundamental quantities that offer insights into the structure, composition, and dynamics of the universe. Despite the remarkable progress of modern cosmology, significant challenges persist, with cosmological tensions emerging as a focal point. In this talk, I will discuss the results obtained using geometric probes from DESI and galaxy–CMB lensing cross-correlations to test extensions of the standard cosmological model. I will focus on scenarios with either dynamical or interacting dark energy, showing that current observations reveal signatures pointing toward possible new physics in the dark sector beyond the standard cosmological model, while also potentially helping to address existing cosmological tensions.

In order to derive model-independent observational bounds on dark energy/modified gravity theories, a typical approach is to constrain parametrised models intended to capture the space of dark energy theories. Here we investigate in detail the effect that the nature of these parametrisations can have, finding significant effects on the resulting cosmological dark energy constraints. In order to observationally distinguish well-motivated and physical parametrisations from unphysical ones, it is crucial to understand the theoretical priors that physical parametrisations place on the phenomenology of dark energy. To this end we discuss a range of theoretical priors that can be imposed on general dark energy parametrisations, and their effect on the constraints on the phenomenology of dynamical dark energy. More specifically, we investigate both the phenomenological mu-Sigma parametrisation as well as effective field theory (EFT) inspired approaches to model dark energy interactions. We compare the constraints obtained in both approaches for different phenomenological and theory-informed time-dependencies for the underlying functional degrees of freedom, discuss the effects of priors derived from gravitational wave physics, and investigate the interplay between constraints on parameters constraining only the background evolution vs. parameters controlling linear perturbations.

Deviations from the cosmic distance duality (CDD) relation may result from systematic errors in distance measurements or hint at new physics. Furthermore, it can be linked to the Hubble constant tension as a cosmic calibration tension emerging when luminosity and angular-diameter distances are compared. Based on this, we adopt two treatments to investigate possible departures from the CDD relation: model-dependent and -independent approaches mapping both low and intermediate/high redshift epochs. In the first we consider as background cosmological models first the ΛCDM and then the ω0ω1CDM scenarios to assess how deviations from the CDD relation affect the cosmological parameters while the latter focus on adopting the parameterization of the Hubble rate via Bézier polynomials. We seek possible departures from the relation considering i) a Taylor expansion, ii) a power-law parameterization, iii) a logarithmic correction, iv) a Padé parameterization and v) a second order Chebyshev parameterization. Additionally, when adopting the model-dependent treatment we also assess the statistically favored cosmological model through model selection criteria.

The cosmic dipole measured in surveys of cosmologically distant sources consistently diverges from the expectation set by the Cosmic Microwave Background (CMB), posing a serious challenge to the Cosmological Principle and the standard model of cosmology. These inferences rely on our understanding of the source counts and underlying systematics. For many systematics, it is not generally possible to write down the analytical likelihood. Here, simulation-based inference (SBI) is a powerful tool that enables Bayesian inference when the likelihood is intractable. We present a flexible SBI framework that quantifies the cosmic dipole tension using neural ratio estimation. We show that the recovered tensions between Planck, NVSS, RACS and CatWISE are comparable to those in the literature. These may be extended with any number of systematics, setting the stage for the future. If we are to resolve the anomaly or strengthen the challenge against ΛCDM, modelling and quantifying systematics to rule them out as culprits of tension will be essential as we enter the SKA-LSST era.

Robust Bayesian model comparison and tension quantification are essential for interpreting the wealth of modern cosmological data, yet they remain computationally prohibitive bottlenecks. High-dimensional nested sampling runs often require thousands of CPU hours, limiting the community’s ability to explore model extensions or validate new datasets rapidly. To address this, I present unimpeded [2511.04661], a new public resource designed to democratise access to these expensive calculations. Acting as a “Planck Legacy Archive” for nested sampling, unimpeded provides a massive grid of pre-computed chains covering 8 cosmological models (including $\Lambda$CDM and extensions) across 69 observational datasets. The accompanying Python package transforms what used to be months of supercomputer time into seconds on a laptop, enabling instant access to Bayesian evidences, parameter estimates, and robust tension metrics. I will demonstrate the power of this framework by applying it to the recent controversy surrounding DESI DR2 and evolving dark energy [2511.10631]. While frequentist approximations have suggested a preference for dynamic dark energy (w0waCDM), our direct Bayesian analysis reveals that the combination of DESI BAO and Planck CMB actually favours the simpler LambdaCDM model. Using unimpeded to dissect this result, we show that the apparent preference for evolving dark energy is driven primarily by resolving a statistical tension between DESI and the DES-Y5 supernova catalogue, rather than an intrinsic signal within the BAO data itself, warranting a cautious interpretation of its statistical significance.

Hints of dynamical dark energy (DE) have strengthened under the combination of data from CMB, SNIa and BAO from DESI. This evidence is typically quantified using the well-known CPL parameterization of the DE equation-of-state, w_DE=w0+wa(1-a). However, this truncation may bias our interpretation of the data, potentially leading us to mistake spurious features of the best-fit CPL model for genuine physical properties of DE. We keep more terms in the expansion and apply the Weighted Function Regression (WFR) method to eliminate the subjectivity associated with the choice of truncation order. Using this model-agnostic approach we reconstruct cosmological functions and quantify that evidence for a crossing of the phantom divide is statistically significant, with confidence levels ranging from 96.21% to 99.97%, depending on the SNIa dataset. Finally, I will show that the effective DE fluid from a combination of standard and negative quintessence can reproduce the reconstructed shapes of w_DE(z) and f_DE(z).

Classical Cepheids (CCs) are the standard calibrators of the cosmic distance scale, but the current “Hubble tension” motivates the use of alternative tracers. Anomalous Cepheids (ACs) and Type II Cepheids (T2Cs), being older and less massive, are widespread in the Milky Way and dwarf galaxies and follow tight Period–Luminosity (PL) and Period–Wesenheit (PW) relations, especially in the near-infrared.

We present new multi-wavelength PL and PW relations for T2C subclasses and for both modes of ACs in the Magellanic Clouds, using OGLE, Gaia, and VMC data. Based on over 500 variables, these relations provide distances to Local Group systems consistent with those from CCs and RR Lyrae. Ongoing work extends the analysis across a wider metallicity range with photometry and high-resolution spectroscopy.