Journal Club

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.

A new unknown extragalactic foreground of the cosmic microwave background (CMB) has recently been detected. This signal shows a systematic decrease in CMB temperatures around nearby large spiral galaxies, pointing to an interaction with CMB photons up to several projected Mpc around these galaxies. We investigate to what extent this foreground may impact the CMB fluctuation map and the cosmological parameters.

Since their use in the discovery of dark energy, Type Ia Supernovae (SNe Ia) have remained one of our most powerful tools to constrain the expansion history of our universe. Modern cosmological measurements using SNe Ia have largely relied on spectroscopy to both confirm the type of SN and obtain reliable redshifts. While this has been feasible for our current datasets (~2k SNe), our spectroscopic resources will be vastly outpaced by the troves of SNe (O(100k)) soon to be discovered by imminent surveys such as the Vera Rubin Observatory Legacy Survey of Space and Time (LSST). Without efforts to address the limitations imposed by requiring spectroscopy, the power of these surveys to constrain dark energy and its time evolution will be drastically diminished. In this talk, I will highlight work using data from the Dark Energy Survey (DES) that shows photometric SNIa cosmology using photometric redshifts rather than spectroscopic redshifts is a promising avenue for next-generation constraints.

Recent data releases from the Atacama Cosmology Telescope (ACT DR6) and the Dark Energy Spectroscopic Instrument (DESI DR2) provide new insights into the viability of Early Dark Energy (EDE) as a resolution to the Hubble tension. In this talk, I will explore the implications of these recent measurements for EDE models. In particular, I will show that while ACT DR6 does not statistically prefer EDE over the standard ΛCDM model, it allows for a significantly larger maximum EDE contribution compared to previous constraints from Planck NPIPE, despite ACT’s improved precision on small angular scales. I will also discuss the role of prior volume effects in Bayesian analyses and highlight the importance of complementing Bayesian inference with frequentist approaches

The σ8 parameter is commonly used to quantify the amplitude of matter fluctuations at linear cosmological scales. However, its intrinsic dependence on h can introduce biases and couples the growth and Hubble tensions in an intricate way, when comparing the predictions of different models and/or datasets. For example, the bias found in models with large values of H0 is more prominent, artificially complicating the search for a model that can resolve the Hubble tension without exacerbating the growth tension. To address these challenges, an alternative parameter has been proposed: σ12. In this scenario, the worsening of the growth tension in different cases is much less pronounced than previously thought or may even be non-existent

Gravitational waves (GWs) from compact binary coalescences (CBCs) offer a novel method to probe cosmic expansion, particularly the Hubble constant H0. A key technique in GW cosmology is the spectral sirens method, which utilizes GW luminosity distance and source-frame mass distribution to infer redshift. With GW detectors, populations of CBCs can be either observed as resolved individual sources or implicitly as a stochastic gravitational-wave background (SGWB) from the unresolved ones. This study explores how both resolved and unresolved CBCs contribute to constraining cosmic expansion within the spectral siren framework. The SGWB provides additional constraints on CBC population properties, potentially enhancing precision in cosmic expansion measurements. Using a five-detector network at O5-designed sensitivity, we find that incorporating the SGWB helps exclude lower values of H0​ and the dark matter energy fraction Ωm. It also helps in refining the redshift distribution of CBCs, improving the determination of a possible CBC peak in redshift. However, while SGWB improves constraints on low values of H0​ and Ωm​, resolved spectral sirens remain the dominant source of precision for H0. We also performed a spectral siren analysis for 59 resolved binary black hole sources detected during the third observing run with an inverse false alarm rate higher than 1 per year jointly with the SGWB. We obtain that with current sensitivities, the cosmological and population results are not impacted by the inclusion of the SGWB.

Axion-dilaton models provide a well-motivated, minimal class of models for which kinetic interactions between multiple scalar fields and their predictions can be explored, particularly in late-time cosmology. I will present the cosmological implications of these interactions when prescribing an axion and a dilaton field to describe dark matter and dark energy, respectively, including the predicted effects on the CMB, late time structure growth, and particle mass evolution.