CosmoVerseWorkshop@Naples (21 May – 23 May 2025)
This was the first CosmoVerse workshop and aimed to bring together observational cosmologists working in probing systematics and in connecting survey physics with the exploration of fundamental physics. The workshop provided a forum for discussions on different aspects of observational cosmology and how they may connect with the open question of cosmic tensions. One of the core goals of the CosmoVerse Network is to connect observational cosmologists with data analysts and researchers developing new fundamental physics in order to understand how they may nuance the question of cosmic tensions. This workshop brought those communities together. In order to encourage collaboration and the exchange of concrete ideas, the workshop has been limited to 40 participants.
This workshop was organized within the COST Action initiative CA21136 – Addressing observational tensions in cosmology with systematics and fundamental physics (CosmoVerse). Learn more about CosmoVerse action at https://cosmoversetensions.eu/
The CosmoVerse Network has held two conferences CosmoVerse@Lisbon and CosmoVerse@Krakow which were held in 2023 and 2024 respectively, and there was also a School held in 2024 called CosmoVerseSchool@Corfu.
This workshop aimed to establish a link between the different research areas in observational cosmology with a main focus on possible systematics in recent surveys, and how they connect to the challenges of tensions in cosmological survey data. It had a format with a limited number of talks, and a strong focus on discussions, as well as time for collaborative work.
The workshop was organized together with Scuola Superiore Meridionale in Naples.
- Large Scale Structure
- Distance Ladder
- Distance Ladder Free H0
- Measurements
- Galaxy Clustering
- Cosmic Microwave Background
- High Redshift
Venue
Scuola Superiore Meridionale (Via Mezzocannone 4 – google maps link)
Two public talks were held on Thursday 22 May at 18:30 by Dr Matilde Signorini and Prof. Vincenzo Salzano. For more information, visit this page.
Talks
Large Scale Structure
Dani Leonard
Mitigating astrophysical systematic effects for LSST weak lensing and galaxy clustering
Combined measurements of weak gravitational lensing and galaxy clustering are a powerful cosmological probe. The Rubin Observatory’s LSST will offer incredible statistical power in these observables, and thus the opportunity to shed light on the cosmic tensions which afflict our standard cosmological model. However, to take full advantage of LSST’s incredible data volume, we need to carefully mitigate modelling uncertainties arising from astrophysical processes. In this talk, I will discuss recent work towards ensuring we can adequately mitigate such systematic uncertainties for LSST weak lensing and galaxy clustering, with a focus on galaxy intrinsic alignment, baryonic physics, photometric redshift uncertainties, and galaxy bias.
Jenny Sorce
CLONES: Digital twins of the local Universe for bias-free inference
Clustering
Samuel Brieden
Correcting Systematic Uncertainties in Spectroscopic Large-Scale Structure Surveys
Spectroscopic large-scale structure (LSS) surveys like BOSS, eBOSS, and DESI have significantly advanced our understanding of the universe’s expansion and growth history using the Baryon Acoustic Oscillation (BAO) and full-shape (FS) techniques. The DESI DR1 BAO+FS and in particular the recent DESI DR2 BAO analyses suggest a compelling preference for an evolving dark energy equation of state. The accuracy of these BAO and FS measurements hinges on effectively addressing systematic effects encountered throughout the analysis, including imaging, spectroscopy, incompleteness, and modeling. This talk reviews the systematic corrections applied—from targeting and measuring spectral to LSS catalog creation and blinding. I will highlight the methodologies employed in recent DESI analyses and advancements compared to previous surveys.
Dominique Eckert
Cosmology with galaxy cluster counts: a critical review
The abundance and mass distribution of galaxy clusters is a sensitive cosmological probe through the dependence of the halo mass function on cosmological parameters, most importantly Omegam and sigma8. Recent galaxy cluster surveys yielded highly competitive results on these parameters and provide a stringent test of the LCDM model. However, as for all probes the systematic uncertainties need to be kept under control to avoid spurious conclusions. In this talk, I will provide a critical review of galaxy cluster cosmology. I will describe the method as well as the main recent surveys and modeling techniques. I will then describe the main bottlenecks and systematic uncertainties and provide possible avenues to address the remaining issues. Finally, I will discuss the latest results with this technique in the context of the current cosmological tensions.
Geraint Lewis
Confronting the Cosmological Principle: Surveys, Statistics & Systematics
The Cosmological Principle — that the Universe is homogeneous and isotropic on large scales — is a cornerstone of modern cosmology, but growing survey data are beginning to test its limits. However, subtle features in source catalogues, shaped by selection effects and systematics, are demanding simulation-based inference to capture uncertainties and avoid bias. I will discuss how these methods allow us to move beyond simplistic approximations and confront the true statistical structure of the data. I will also highlight the role of Bayesian tension metrics in assessing consistency across surveys, helping distinguish between new physics, overlooked systematics, and statistical fluctuations. Together, these tools offer a sharper lens through which to evaluate the Cosmological Principle.
Distance Ladder
Abby Lee
The J-region Asymptotic Giant Branch (JAGB) method: An Independent Measurement of the Hubble Constant using Carbon Stars as Standard Candles
One of the most critical unresolved issues in cosmology today is the question of whether new physics is needed beyond our current standard model, Lambda Cold Dark Matter (LCDM). In the past several years, a distinct 5-sigma tension has arisen between measurements of the Hubble constant (Ho) determined via the local Type Ia supernova distance ladder calibrated by Cepheids and from values inferred from the cosmic microwave background fluctuations. This discrepancy points to either yet-to-be discovered systematic errors in the local distance scale or a necessary revision of the LCDM model, which would indicate new physics of the Universe. In this talk, I will present an independent route to measuring Ho based on a promising new standard candle that leverages the mean NIR luminosities of carbon-rich asymptotic giant branch (AGB) stars, called the J-region asymptotic Giant Branch (JAGB) method. The JAGB method can measure distances completely independent of Cepheids and the tip of the red giant branch (TRGB). Therefore, it can provide valuable cross-checks on their distances to shed light on potential currently hidden systematic errors in the local distance scale. Furthermore, the JAGB method is less susceptible to extinction by dust and to crowding/blending effects than Cepheid variable stars. In this talk, I will discuss the advantages and current systematic uncertainties of this new standard candle. Finally, I will present a measurement of the Hubble constant derived from the JAGB method using new JWST data and discuss the implications of these results for the Hubble tension.
Siyang Li
Expanding the TRGB & JAGB SNe Ia Calibration Samples to Investigate Distance Ladder Systematics
Distance ladders which calibrate the luminosity of Type Ia supernovae (SNe Ia) currently provide the strongest constraints on the local value of H0. Recent studies from the Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) show good consistency between measurements of SNe Ia host distances. These are calibrated to NGC 4258 using different primary distance indicators (Cepheids, Tip of the Red Giant Branch (TRGB), J-region Asymptotic Giant Branch (JAGB), and Miras). However, some sub-samples of calibrated SNe Ia employed to measure H0 yield noteworthy differences due to small sample statistics but also due to differences in sample selection. This issue is particularly important for TRGB-derived calibrations owing to the smaller volume they reach compared to Cepheids, reducing sample size and enhancing the size of statistical fluctuations. To mitigate this issue, we compile the largest and complete (as currently available) samples of TRGB and JAGB in the hosts of normal SNe Ia. The full TRGB sample together with the Pantheon+ SN catalog gives H0=72.1-73.3 +/- 1.8 km/s/Mpc (depending on methodology), while the JAGB sample yields H0 = 73.3 ± 1.4 (stat) ± 2.0 (sys) km/s/Mpc, both in good agreement with the value of 72.5 +/- 1.5 km/s/Mpc from HST Cepheids in hosts of 42 SNe Ia calibrated by the same anchor, NGC 4258. We also extend the JAGB distance scale to ~40 Mpc using NGC 5468 and characterize variations in the J-region luminosity function.
Lluís Galbany
The Hubble constant from near-infrared observations of type Ia supernovae
I’ll present a measurement of the Hubble constant (H0) using type Ia supernova (SNe Ia) in the near-infrared (NIR) from the recently updated sample of SNe Ia in nearby galaxies with distances measured via Cepheid period-luminosity relations by the SHOES project (astro-ph:2209.02546). We collected public near-infrared photometry of 19 calibrator SNe Ia and further 57 SNe Ia in the Hubble flow (z>0.01), and directly measure their peak magnitudes in the J and H band by Gaussian processes and spline interpolation. Calibrator peak magnitudes together with Cepheid-based distances are used to estimate the average absolute magnitude in each band, while Hubble-flow SNe are used to constrain the zero-point intercept of the magnitude-redshift relation. Our baseline result of H0 is 72.3±1.4 (stat) ±1.4 (syst) km s−1 Mpc−1 in the J band and 72.3±1.3 (stat) ±1.4 (syst) km s−1 Mpc−1 in the H band, where the systematic uncertainties include the standard deviation of up to 21 variations of the analysis, the 0.7\% distance scale systematic from SHOES Cepheid anchors, a photometric zeropoint systematic, and a cosmic variance systematic. Our final measurement represents a measurement with a precision of 2.8\% in both bands. Importantly, we demonstrate stretch and reddening corrections are still useful in the NIR to standardize SN Ia NIR peak magnitudes. I’ll also present further work on systematics not included in the published paper done on the host galaxy environments, including correlations with peak magnitudes and characterization of the global and local environments.
CMB
Frode K. Hansen
A new >5sigma tension: Why are the CMB fluctuations much colder than expected around galactic dark matter halos?
In a recent series of papers, we have shown that CMB photons passing through galactic dark matter halos in the nearby universe (z<0.02) appear significantly colder than expected. The effect appears similar to the kinetic SZ-effect and the Rees-Sciama effect in that the cooled CMB photons maintain their blackbody spectrum, but the observed cooling is an order of magnitude larger than expected from these well known effects. I will speculate about possible mechanism that could give rise to the observed cooling and discuss possible implications: Is dark matter involved? What about dynamical dark energy? And what happens at larger redshift? How are the cosmological parameters affected?
Will Handley
Scanning for cosmological tensions across a DiRAC-enabled grid of models, datasets and samplers
Recent cosmological surveys have revealed persistent discrepancies within the context of the concordance model regarding the values of the H0 [1907.10625], σ8 [1610.04606] and ΩK [1908.09139, 1911.02087] when estimated with different datasets. Determining the level of disagreement between multidimensional fits is called “tension quantification” [1902.04029]. We approach this problem by producing a re-usable library of MCMC chains, Nested sampling runs, and machine learning emulators across a grid of cosmological models through detecting cosmological tensions between datasets from the DiRAC allocations DP192 & DP264. This library will be released at this conference as part of the package unimpeded (https://github.com/handley-la
Authors: Dily Ong & Will Handley
Oliver Fabio Piatella
Varying equation of state and matter
Dust is pressureless matter, a key ingredient of the cosmological recipe, effectively describing, at present, about one third of the whole content of the universe. On the other hand, dust at early times, before recombination, is substantially different from dust at late times, when structures are formed and the universe becomes highly inhomogeneous at small scales. We show that such a difference induces, via a backreaction mechanism, a small change in the equation of state of dust, which effectively gains a small positive pressure. We show that such modification can ease the tensions in determining the Hubble constant and the clustering parameter S8 without the need to introduce additional degrees of freedom.
Distance Ladder Free H0 Measurements
Frédéric Courbin
Time delay cosmography with strongly lensed quasars: where do we stand ?
Time delays in lensed quasars offer a single-step measurement of H0 from a geometrical method completely independent of any other cosmological probe in the late Universe. However, the method is not free of systematics and most of the effort to turn the ~50 time delays available into H0 now depends on our ability to minimize and/or mitigate the systematics due to the mass models of lens galaxies. This talk will remind the time delay methodology and point to the different avenues to achieve the goal of H0 to 1% percent, given the systematics.
Geza Csörnyei
The ladder-free path to H0: an overview of existing techniques and the perspective from a Type II supernova angle
The main question shaping today’s cosmology is the Hubble tension, the apparent discrepancy between the measured near- and far-Universe Hubble constants, H0, potentially suggesting physics beyond Lambda-CDM. Most local measurements to date use the concept of the cosmic distance ladder, the empirical path to H0. However, with the prevalence of the Hubble tension, there has been a growing interest in techniques that allow for an H0 estimation in a single step without the need for empirical calibrations, which provide an independent glimpse of the tension.
To date, three methods allow measuring H0 free of the distance ladder: masers, strong lensing of either quasars or supernovae, and the radiative transfer modeling of Type II supernovae. In this presentation, I will focus on Type II supernovae and discuss the improvements made over the last decade that have eliminated the issues associated with distance estimations based on these transients, establishing them as high-precision probes for cosmology. Beyond the description of the method, I will showcase its application on a dedicated dataset obtained through an ESO Large Programme, which enables an H0 estimation on the percent level, demonstrating how SNe II can be used to add new, valuable puzzle pieces to the Hubble tension picture.
Teresa Sicignano
Type II and Anomalous Cepheids: An Alternative Route to the Hubble Constant
Classical Cepheids (CCs) are the young, massive, pulsating stars that have been established as the primary calibrators of the distance ladder for measuring the Hubble constant in the local Universe. The Cepheid variables class also includes Type II Cepheids (T2Cs) and Anomalous Cepheids (ACs), which have been less studied. These less massive and more metal-poor stars are relatively frequent in dwarf galaxies as well as in the Milky Way. Moreover, every class of Cepheid obeys a different Period Luminosity (PL) and Period Wesenheit (PW) relation, which are particularly tight in near-infrared bands. In the era of the `Hubble tension’, AC and T2C variables together with the RR Lyrae stars and the tip of the red giant branch can potentially provide non-CC-dependent calibration of the Cosmic Distance Ladder. In this contribution, we present new accurate multi-wavelength PL and PW relations for T2Cs subclasses (BLHer, WVir, pWVir and RVTau) and both for fundamental and first overtone ACs in the Magellanic Clouds (MCs). We adopted literature optical photometry from the OGLE survey and the Gaia mission, while for the near-infrared we took advantage of data collected in the context of the VISTA near-infrared YJKs survey of the Magellanic Clouds System (VMC, PI M.-R.L. Cioni). We analysed YJKs time-series for a sample of 200 ACs and more than 300 T2Cs in the MCs. The derived PL/PW relations were adopted to estimate the distances of several stellar systems in the Local Group. The comparison between distances based on ACs and T2Cs PL/PW relations and the ones based on other classical pulsators, such as CCs and RR Lyrae stars, shows generally good agreement within the errors. This strengthens the case for the Hubble tension, independently from the details of the Cosmic Distance Ladder.
High Redshift
Matilde Signorini
Are quasars reliable standard candles? Study of possible systematics
The non-linear relation between the X-ray and UV luminosities in quasars has been proposed as a way to derive distances and therefore test cosmological models up to redshift ∼4. Previous studies demonstrated that the slope of this relation does not evolve with redshift, making it a promising tool for distance measurements. When applying this method to construct a Hubble diagram, we find that quasar-derived distances are consistent with the predictions of a flat ΛCDM model up to z∼1.5, but show significant deviations at higher redshifts, from which a new tension with the ΛCDM model has been claimed. However, this discrepancy has also been interpreted in alternative ways: some suggest that it arises due to inconsistencies in the parent sample of quasars between low and high redshift, or due to an unaccounted evolution of the X-ray/UV relation. I will present a quantitative assessment of these claims by comparing quasar-based distances with supernova data in their common redshift range and by testing the impact of different cosmological assumptions through simulations. I will show that apparent inconsistencies naturally emerge if the underlying cosmological model used for the analysis is incomplete, meaning that there is no current evidence of true systematics in quasars. I will also discuss how a cosmology-free approach in the study of systematics is fundamental for the validation not only of quasars, but of any possible standard(izable) candle. Finally, I will also discuss how we need to achieve a better understanding of the physical process behind the observed relation and/or an independent observational proof of the tension, such as future supernova measurements at z∼2 or higher, in order to fully understand the Hubble diagram at high redshift.
Maria Giovanna Dainotti
The Hubble constant tension from SNe Ia to Gamma-Ray Bursts: what is the best bet?
Modern cosmological research still thoroughly debates the discrepancy between local probes and the Cosmic Microwave Background observations in the Hubble constant (H0H0) measurements, ranging from 4σ to 6σ. In the current study we examine this tension using the Supernovae Ia (SNe Ia) data from the Pantheon, PantheonPlus, Joint Lightcurve Analysis (JLA), and Dark Energy Survey (DES) catalogs together with their combination called Master Sample containing 3789 SNe Ia, and dividing all of them into redshift-ordered bins. We find a decreasing trend as a function of (1+z)^alpha similarly to previous studies. In this analysis, we introduce the likelihood different from the Gaussians. We tackle the Hubble constant tension, by adding other probes, such as Baryon Acoustic Oscillations, Gamma-Ray Bursts and Quasars. We discuss the problem of making GRBs standard candles and how theoretical models can help in this regard. We introduce other calibrators as cosmic chronometers and using Gaussian Processes and Artificial Neural Network to calibrate GRBs.
We show the analysis of GRBs and Planck with simulated data giving an estimate of how many GRBs are needed to achieve a similar precision on Omega_M of the SNe Ia and what are the constraints we can achieve on the wCDM models.
Ana Luisa González Morán
Unveiling the Cosmic Expansion from z ≈ 0 to z ≈ 13 with HII galaxies
An independent determination of cosmological parameters is important to ascertain the possible need for new physics beyond the standard cosmological model and to have better control over systematic uncertainties, given the tensions in current cosmological parameter measurements. My contribution focuses on providing observations of HII galaxies using a single distance estimator over a vast redshift range (0 < z < 13). HII galaxies are young outbreaks of star formation with a spectrum that shows intense emission lines. Thanks to them and the use of the largest optical-infrared space and ground telescopes, we can observe the first formed galaxies in our Universe that play a fundamental role in the formation of massive galaxies as well as measure with high precision cosmological parameters. I will give an overview of these first discoveries and the cosmological results found. Also, I will highlight a new program aimed at finding and studying the physical properties of new HII galaxies with the JWST.
Local organizing committee (LOC)
Francesco Bajardi (Scuola Superiore Meridionale, IT)
Micol Benetti (Scuola Superiore Meridionale, IT)
Emmanuele Battista (Istituto Nazionale di Fisica Nucleare, IT)
Scientific organizing committee (SOC)
Jackson Levi Said (University of Malta, MT)
Eleonora Di Valentino (Sheffield University, UK)
Marika Asgari (Newcastle University, UK)
Richard I. Anderson (EPFL, Lausanne, CH)
Ariel Goobar (Stockholm University, SE)
