Journal Club

The late-time linear Integrated Sachs-Wolfe (ISW) effect directly probes the dynamics of cosmic acceleration and the nature of dark energy. Detecting these weak, secondary temperature anisotropy signals of the Cosmic Microwave Background requires accurate theoretical predictions of their amplitude across cosmological models. By extending the pyGenISW package, previously limited to ΛCDM, we aim to generate full-sky ISW maps for a suite of 791 wCDM cosmologies using the Gower Street N-body simulations, thereby enabling ISW analyses across a broader dark-energy parameter space. We make our code and ISW data publicly available. We compute the ISW signals by tracing the time evolution of the gravitational potential across large-volume simulations that span dark energy equation of state parameters from phantom to quintessence, −1.79 < w < −0.34. These data are projected onto the sphere using HEALPix to obtain full-sky temperature maps. We validate our pipeline by comparing the measured ISW angular power spectra and ISW-density cross-correlations against linear theory expectations (2 ≤ ℓ ≤ 200) computed with benchmarks from the pyCCL library. The agreement is excellent across the multipole range where the ISW contribution is expected to dominate, confirming the reliability of our modelling of gravitational potential evolution. With additional tests of the ISW signal’s strength in density extrema, as well as comparing all models to a reference ΛCDM cosmology using power ratios, we found that quintessence-like models (w > −1) show higher ISW amplitudes than phantom models (w < −1), consistent with enhanced late-time decay of gravitational potentials. The consistency of our wCDM ISW maps and their agreement with theory predictions confirm the robustness of our methodology, establishing it as a reliable tool for theoretical and observational ISW-LSS analyses. This includes applications to next-generation surveys in the context of covariance calculations and various map-based statistics.

The Hubble tension and the unknown origin of dark energy motivate the exploration of alternative mechanisms for late-time cosmic acceleration. We investigate gravitationally induced particle creation (PC) as a non-equilibrium process that can effectively mimic dynamical dark energy. Within the thermodynamic framework of open systems, we adopt an agnostic approach to the extra created component, leaving its equation-of-state parameter $w_E$ free. We consider four phenomenological parametrisations of the PC rate, allowing deviations from the standard cosmological model (ΛCDM) only at late times (0<z<3). The PC models are constrained using a joint analysis of cosmic chronometers, Type Ia supernovae, local $H_0$ measurements, baryon acoustic oscillations, and cosmic microwave background data. The constraints on $w_E$ are consistent with dark energy, while particle creation of pressureless matter is disfavoured. All PC scenarios provide fits comparable to ΛCDM, with one showing an effective dynamical dark-energy behaviour. When early- and late-time datasets are analysed separately, the PC models reduce the Hubble tension to ≃ 2.4σ-3σ, compared to 4.3σ in ΛCDM. Gravitationally induced dark energy thus offers a consistent late-time extension of ΛCDM and a viable theoretical framework for dynamical dark energy.

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.

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.

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.

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.