CosmoVerse@Istanbul 2025

24 Jun 2025, 08:00 → 26 Jun 2025, 21:00 Europe/Istanbul

Istanbul Technical University

Our understanding of the Universe is at a turning point with the predictions of the standard cosmological model and the observations from different surveys are showing tensions in several key areas. The disagreement is expressed in the value of cosmic expansion as well as in the growth of large-scale structure in the Universe. New cosmological surveys, many of which are European, may expose tension in additional areas of the concordance model. The question of cosmological tensions can be confronted in a number of ways. Firstly, survey data needs to be further analyzed for potential systematic uncertainties or biases. Secondly, there have been numerous advances in approaches to data analysis and statistics, some of which provide less dependence on cosmological models to make cosmological parameter estimates. Lastly, there are a plethora of new proposals from fundamental physics which range from novel neutrino physics to dark energy proposals (and others) which may contribute to a solution to the cosmological tensions problem. These represent the three research themes through which cosmological tensions will either be alleviated or resolved.

CosmoVerse@Istanbul 2025 (24 June – 26 June 2025, Management Committee meeting on 27 June) is the third annual conference in a series of conferences that aim to establish a link between the different research areas in cosmology with the main focus on challenges of tensions in cosmological survey data.

This conference is organized within the COST Action initiative CA21136 – Addressing observational tensions in cosmology with systematics and fundamental physics (CosmoVerse).

The first conference called CosmoVerse@Lisbon 2023 took place between 30 May and 1st June 2023. All of the recorded talks and presentations are public and available here. The second conference called CosmoVerse@Krakow 2024 took place between 9 and 11 July 2024. All of the recorded talks and presentations are available here.

The conference is organized together with İstanbul Technical University (ITU) in İstanbul.

You may check your visa requirement status. 

Main Topics

● Observational cosmology and Systematics (cross-correlation of data, systematic effects)
● Data analysis (astrostatistics; data science in astronomy; Bayesian analysis; machine learning and artificial intelligence)
● Fundamental Physics (challenge of the cosmological hypothesis, dark energy and modified gravity, neutrino physics, dark energy, and dark matter interaction)

Invited speakers

Angus Wright (German Centre for Cosmological Lensing (GCCL) at the Ruhr Universität Bochum (RUB), Germany)
Asta Heinesen (Niels Bohr Institute, Copenhagen, Denmark)
Brent Tully (University of Hawaii, USA)
Daniel Scolnic (Duke University, Durham, USA)
Eric Linder (Berkeley, USA)
Florian Niedermann (Nordita, Stockholm, Sweden)
Mariam Bouhmadi-López (Basque Foundation for Science, Bilbao, Spain)

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Venue

The conference will take place in İstanbul at the Maslak Ayazaga Campus of the Istanbul Technical University. 

Important Dates

18 November 2024: Registration and abstract submission opens
24 April 2025: Registration and abstract submission deadline
24-26 June 2025: Conference days
24 June 2025: Welcome Drinks/ Boat Trip  
25 June 2025: Poster Session and Conference Dinner
26 June 2025: Public Lecture 
27 June 2025: CosmoVerse COST Action Management Committee Meeting for members

 

CosmoVerse2025 Banner.png

Tuesday 24 June ¶

Registration¶

1 Welcome¶

Speakers: Emre Onur Kahya (Istanbul Technical University), Dr Nihan Katırcı (Doğuş University), Prof. Özgür Akarsu (Istanbul Technical University)

Morning session 1: (Tuesday)¶

Convener: Jackson Levi Said (University of Malta)

2 Cosmic Shear Cosmology with the Completed Kilo-Degree Survey¶

The fifth (and final) data release of the Kilo-Degree Survey (KiDS) provides significantly more than just an increase in survey area. Instead, additional observations couple with improvements in data quality, data reduction, and value-added data-products, to make KiDS-DR5 the most robust and reliable release produced by KiDS to date. We leverage the lensing sample from this state-of-the-art dataset (called KiDS-Legacy) to perform the most robust and precise analysis of cosmic shear to date, finding KiDS-Legacy to be wholly consistent with the results from the Planck Cosmic Microwave Background (CMB) analyses. This talk will provide an introduction KiDS-DR5, discuss how this dataset differs with respect to previous KiDS releases, and focus in particular on how these differences have influenced the analysis of cosmic shear with KiDS-Legacy. I will discuss the causes of systematic variation in the parameter constraints that we observe between KiDS data releases, and discuss the implications that these systematic effects have for future analyses of cosmic shear.

Speaker: Angus Wright (Ruhr University Bochum)

3 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 is available as part of the package unimpeded (https://github.com/handley-lab/unimpeded) and serve as an analogous grid to the Planck Legacy Archive (PLA), but machine learning enhanced and expanded to enable not only parameter estimation (currently available with the MCMC chains on PLA), but also allowing cosmological model comparison and tension quantification, as well as including pairwise comparisons between modern cosmological datasets (CMB, BAO, Supernovae, Weak Lensing). Currently, a systematic coverage of 10 cosmological models and 60 datasets (to be extended) are easily accessible via the unimpeded package using a few lines of code. In addition, we provide machine learning emulators [2205.12841] of these marginalised likelihoods for use in future analyses as a fast and flexible alternative to MCMC chains and nested samples for re-use in future analyses.

Speaker: Dily Duan Yi Ong (University of Cambridge)

presentation_cosmoverse2025.key

presentation_cosmoverse2025.pdf

4 Outer regions of galaxy clusters as a new probe to test modifications to gravity¶

We perform a stacking analysis of galaxy cluster velocity phase space using the caustic technique. By stacking 128 clusters, we create four robust stacked clusters with excellent agreement between caustic masses and binned medians. We model the gravitational potential using the NFW profile, validating the $\mathrm{\Lambda }$CDM mass-concentration relation. Implementing the Chameleon screening model, we find constraints on modified gravity parameters consistent with $\mathrm{\Lambda }$CDM, yielding stringent upper limits of $|f_{\mathrm{R}0}|\lesssim 4\times {10}^{-6}$ at 95% C.L. for $f(\mathcal{R})$ gravity. We also test the caustic technique's effectiveness in constraining MOND in individual clusters.

Speaker: Minahil Adil Butt (SISSA)

CosmoverseITU2025_Presentation.pptx

Coffee Break¶

Morning session 2: (Tuesday)¶

Convener: Eric Linder (UC Berkeley)

5 The TRGB-SBF Project: a Pop II path to H0¶

The path from Cepheid variables to type Ia supernovae gives a value of the Hubble constant which significantly disagrees with the value determined from observations of conditions in the early universe and a cosmological model. A totally independent measurement of H0 from observed redshifts and distances is needed to evaluate the possibilities of systematic errors. A path is being explored that should be as accurate or better than the Cepheid-SNIa way, involving only observations of old evolved stars. Gaia parallaxes ground the absolute values of RR Lyrae stars that establish the absolute magnitudes of stars at the tip of the red giant branch that sets the scale of the power spectrum of surface brightness fluctuations in E/S0 galaxies that are observed with negligible confusion from peculiar velocities. Observations with JWST are fundamental for the success of this program.

Speaker: Brent Tully (University of Hawaii)

6 Hubble tension or distance ladder crisis?¶

I present an up-to-date compilation of published Hubble constant ($H_0$) measurements that are independent of the CMB sound horizon scale. This compilation is split in two distinct groups: A. Distance Ladder Measurements sample comprising of 20 recent measurements, primarily from the past four years, utilizing various rung 2 calibrators and rung 3 cosmic distance indicators. B.One-Step Measurements sample including 33 measurements of $H_0$ that are independent of both the CMB sound horizon scale and the distance ladder approach. These 33 measurements are derived from diverse probes such as Cosmic Chronometers, gamma-ray attenuation, strong lensing, megamasers etc. Statistical analysis reveals a significant distinction between the two samples. The distance ladder-based sample yields a best fit $H_0=72.8\pm 0.5$ km s${}^{-1}$ Mpc${}^{-1}$ with ${\chi }^2/dof=0.51$ indicating some correlations. The one-step measurements result in $H_0=69.0\pm 0.48$ km s${}^{-1}$ Mpc${}^{-1}$ with ${\chi }^2/dof=1.37$ indicating some internal tension. Possible local physics approaches that may explain this finding will be discussed.

Speaker: Prof. Leandros Perivolaropoulos (University of Ioannina)

leandros-istanbul25a.pptx

7 Cosmology with GWxHI Cross-Correlation with Future Observatories¶

We explore the potential of cross-correlation between gravitational waves (GWs) and 21 cm intensity mapping (IM) from neutral hydrogen emission (HI), focusing on its role in view of future high-precision observatories, specifically Einstein Telescope (ET) and the Square Kilometer Array Observatory (SKAO). We model the large scale structure evolution by making use of 3D lightcones of the dark matter density distribution up to redshift $z\sim 3$ from both linear perturbation theory and N-body simulations. Assuming that GW and HI are different tracers of the same underlying density field, we infer cosmological parameters from both the angular power spectrum (auto-correlation) and the angular cross-power spectrum (cross-correlation) of these probes. We show that synergies between future GW and HI experiments will significantly improve standard sirens measurements, thanks to increased sensitivity and high-redshift coverage, allowing to constrain cosmological parameters to a level comparable to that of well-established alternative probes, such as Type Ia Supernovae.

Speaker: Matteo Schulz (Gran Sasso Science Institute)

Schulz_CosmoVerse_presentation.pdf

Lunch¶

Afternoon session 1: (Tuesday)¶

Convener: Tekin Dereli

8 The Hubble Tension and Evolving Dark Energy Measurements: State of the Evidence¶

In this talk, I will review the latest measurements of the Hubble Constant and of dark energy from a variety of techniques. I will discuss differences in sensitivity, particularly with Type Ia supernovae, to different redshift ranges. I will also differentiate what can be producing the Hubble Tension signal versus the evolving dark energy, and offer guidance on various theoretical directions. Finally, I will forecast constraints from the next generation of telescopes like LSST and Roman on answering these problems.

Speaker: Daniel Scolnic

Istanbul.pdf

9 On the Cosmological Time Dilation of High Redshift Quasars¶

One of the fundamental predictions of modern cosmology is that we should see the distant universe run slowly, with time dilated by the expansion of space. Numerous searches for this cosmic time dilation in the light curves of distant quasars, some of the most luminous objects in the universe, have failed to find the expected signal, with claims that this challenges the standard cosmological model. I present a new analysis of a sample of quasars monitored over two decades, testing competing hypotheses within a Bayesian framework to search for the influence of cosmology. The key result is that modern cosmology is safe, quasars are truly luminous objects at immense distances, and I will explore how available data and chosen approaches limited previous studies.

Speaker: Geraint Lewis (Sydney Institute or Astronomy, The University of Sydney)

TimeDilation-Istanbul-Geraint_Lewis.pptx

10 Modern Statistical Methods for Cosmological Inference: Samplers, Likelihoods, and Beyond¶

We present a comprehensive statistical framework for cosmological parameter estimation, focusing on Baryon Acoustic Oscillation (BAO) analyses in the light of the DESI DR2. We focus on two complementary directions: the performance evaluation of various sampling algorithms and the investigation of likelihood formulations. We will discuss the role of the choice of a sampler in both its performance as measured by runtime efficiency and parameter accuracy across different dimensionalities and probability landscapes. Then we will examine statistical nuances in BAO analysis, comparing different likelihood formulations and their effect on the parameter constraints and their higher-order distribution properties.

Speaker: Denitsa Staicova (INRNE, Bulgarian Academy of Sciences)

BAO_presentation_Istanbul.pdf

Coffee Break¶

Afternoon session 2: (Tuesday)¶

Convener: Mariam Bouhmadi-López (Ikerbasque and University of the Basque Country)

11 The present and the future of quasars in cosmology¶

I will review what the prospects of quasars in the context of observational cosmology are, and I will present recent measurements of the expansion rate of the Universe based on a Hubble diagram of quasars detected up to the highest redshift ever observed (z~7.5).
The derived distances are in agreement with the standard flat ΛCDM model up to a redshift of ~1.5, but they show significant deviations at higher redshifts. Yet, several authors suggested that this discrepancy is due to inconsistencies between the low- and high-redshift sources within the parent sample, or to a redshift evolution of the relation. I will discuss these issues through a quantitative comparison with supernova-derived distances in the common redshift range, complemented by simulations showing that all the claimed inconsistencies would naturally arise from any limitation of the cosmological model adopted for the data analysis, that is, from our ignorance of the true cosmology.
I will finally show that the synergy amongst multi-wavelength facilities (current and future) will provide the needed sample statistics to obtain constraints on the observed deviations from the standard cosmological model which will rival and complement those available from the other cosmological probes.

Speaker: Elisabeta Lusso (University of Firenze - Physics & Astronomy Department)

cosmoverse_Lusso_online.pdf

12 GRB Redshift Estimation using Machine Learning and the Associated Web-App¶

Context. Gamma-ray bursts (GRBs), observed at redshifts as high as 9.4, could serve as valuable probes for investigating the distant Universe. However, this necessitates an increase in the number of GRBs with determined redshifts, as currently, only 12% of GRBs have known redshifts due to observational biases. Aims. We aim to address the shortage of GRBs with measured redshifts, enabling us to fully realize their potential as valuable cosmological probes Methods. Following Dainotti et al. (2024c), we have taken a second step to overcome this issue by adding 30 more GRBs to our ensemble supervised machine learning training sample, an increase of 20%, which will help us obtain better redshift estimates. In addition, we have built a freely accessible and user-friendly web app that infers the redshift of long GRBs (LGRBs) with plateau emission using our machine learning model. The web app is the first of its kind for such a study and will allow the community to obtain redshift estimates by entering the GRB parameters in the app. Results. Through our machine learning model, we have successfully estimated redshifts for 276 LGRBs using X-ray afterglow parameters detected by the Neil Gehrels Swift Observatory and increased the sample of LGRBs with known redshifts by 110%. We also perform Monte Carlo simulations to demonstrate the future applicability of this research. Conclusions. The results presented in this research will enable the community to increase the sample of GRBs with known redshift estimates. This can help address many outstanding issues, such as GRB formation rate, luminosity function, and the true nature of low-luminosity GRBs, and enable the application of GRBs as standard candles

Speaker: Maria Giovanna Dainotti (NAOJ)

Redshift-ML-Istanbul-2025-Dainotti.pptx

13 Future measurements of S8 with tomographic cross-correlations between LSST and CMB experiments¶

The well established ΛCDM model of cosmology suffers from several challenges and disparities like the nature of dark matter and dark energy, accelerated expansion of the Universe and tensions in cosmological parameters derived from different observations. Tomographic cross-correlation measurements from the next-generation CMB experiments and galaxy surveys will allow us to robustly quantify deviations from the ΛCDM model. However, with increasing quantity and quality of observational data, it is crucial to mitigate the systematic errors that may bias these cross-correlation measurements. In this talk, we will present forecasts on the S8 parameter from tomographic cross-correlations between the Vera C. Rubin LSST photometric galaxy survey and CMB experiments (Simons Observatory and Planck). We will discuss the importance and mitigation strategies for redshift bin mismatch of galaxies due to photometric redshift uncertainties. Although this talk will be based on LSST simulations, the results and discussions should be applicable to photometric galaxy surveys and their cross-correlations measurements in general.

Speaker: Chandra Shekhar Saraf (Korea Astronomy and Space Science Institute)

CosmoVerse_24062025.pdf

Boat trip and welcome drinks¶

Wednesday 25 June ¶

Morning session 1: (Wednesday)¶

Convener: Emmanuel Saridakis (National Observatory of Athens)

14 The Hubble Tension as the Signature of a New Phase Transition¶

In this talk, I will argue that the Hubble tension points to new physics in the early universe, between big bang nucleosynthesis and recombination. Specifically, I will propose that it might be the signature of a new phase transition in the dark sector. The latent heat released during the phase transition provides a sizeable energy injection into the cosmic fluid, shortening the sound horizon and raising the early-universe inferred value of the Hubble parameter. I will present two concrete microphysical realizations of this idea: Cold New Early Dark Energy is a triggered vacuum phase transition that occurs shortly before matter-radiation equality and shares phenomenological features with other early dark energy models. Hot New Early Dark Energy, by contrast, is a supercooled phase transition that occurs at higher redshift and heats the dark sector, providing a concrete realization of a (stepped) dark radiation model.

Speaker: Florian Niedermann

Niedermann_Istanbul.pdf

15 Strong progenitor age bias in supernova cosmology and alignment with DESI BAO¶

Supernova (SN) cosmology is based on the key assumption that the luminosity standardization process of type Ia SNe remains invariant with progenitor age. However, our comprehensive age measurements of 360 SN host galaxies reveal a significant (5.5 sigma) correlation between standardized SN luminosity and progenitor age, which is expected to introduce a serious systematic bias with redshift in SN cosmology. After correcting for this age bias with redshift, the SN dataset aligns more closely with the recent DESI BAO result, bringing the revised 'standard candle' (SNe) into concordance with the 'standard ruler' (BAO). When the three cosmological probes (SNe, BAO, CMB) are combined, we find a strong (> 7 sigma) tension with the LCDM model, suggesting a time-varying dark energy equation of state in a currently non-accelerating universe.

Speaker: Prof. Young-Wook Lee (Yonsei University)

Lee_20250625_CosmoVerse.pdf

16 Testing DDE and Constraining high-\texorpdfstring{z}{z} reionization with Cosmological Data¶

I will discuss two research projects about testing DDE and Constraining high-\texorpdfstring{$z$}{z} reionization with cosmological data. First, we explore an extension of the $\mathrm{\Lambda }$CDM model that introduces a time-dependent evolution of the pressure parameter $p$ in the dark energy fluid, where the corresponding energy density $\rho$ is derived by a continuity equation and the equation of state $w\equiv p/\rho$ evolves during the late-time expansion of the Universe. We model the pressure parameter using a Taylor expansion within the dynamical dark energy (DDE) framework, initially truncating at first order with a single additional parameter allowing deviation from the cosmological constant value, then extending to second order to demonstrate that observational data can constrain both parameters. Since no significant deviation from $\mathrm{\Lambda }$CDM was detected for the relevant cosmological parameters, we did not include higher-order terms. We modified the publicly available CAMB code and performed Markov Chain Monte Carlo analysis, extending the $\mathrm{\Lambda }$CDM parameter space to incorporate the pressure parameterizations. Second, we use Gaussian processes to give a model-independent reconstruction of the reionization history of the Universe constrained by the \textit{Planck} CMB measurements, providing consistent reconstruction of standard reionization at $z\lesssim 10$ and strongly constraining any additional high-$z$ reionization. We construct a new derived parameter expressing the high-$z$ contribution to the CMB optical depth, ${\tau }_{\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{z}}$, whose posterior distribution can be used to set accurate constraints on models for exotic energy injection, which we demonstrate for the case of decaying dark matter with particle mass in the range $\mathcal{O}(1\text{MeV})$. Our results can be recast to different models using the provided data and code available at: https://github.com/Cheng-Hanyu/CLASS_reio_gpr.

Speaker: Hanyu Cheng (Tsung Dao Lee institute & SJTU&University of Sheffield)

Istanbul_CosmoVerse.pdf

Istanbul_CosmoVerse.pdf

Coffee Break¶

Morning session 2: (Wednesday)¶

Convener: Prof. Özgür Akarsu (Istanbul Technical University)

17 Dark Energy Dynamics and Data¶

Dark energy away from a cosmological constant Λ – like early universe inflation that ends – can be understood in terms of well defined physical behaviors. These guide dark energy into thawing or freezing classes, with w0–wa arising as a physical calibration of the phase space. Other regions of phase space – zones of avoidance – require violation of some basic principle. We explore these cases, drawing a direct analogy with how non-Gaussianity in inflation can add physics beyond standard dynamics. We examine the physics implications if the best fit of current data is taken to be truth.

Speaker: Eric Linder (UC Berkeley)

LinderCosmoVerse25a.pdf

18 Implications for cosmological expansion models from the final Dark Energy Survey BAO and SN data¶

The Dark Energy Survey (DES) recently released the final results of its two main probes of the expansion history: Type Ia Supernovae (SN) and Baryonic Acoustic Oscillations (BAO). In this work, we explore the cosmological implications of these data in combination with external Cosmic Microwave Background (CMB), Big Bang Nucleosynthesis (BBN), and age-of-the-Universe information. The BAO measurement, which is $\sim 2\sigma$ away from {\tt Planck-2018}'s $\mathrm{\Lambda }$CDM predictions, pushes for low values of ${\mathrm{\Omega }}_m$ compared to {\tt Planck-2018}, in contrast to SN which prefers a higher value than {\tt Planck-2018}. We identify several tensions among datasets in the $\mathrm{\Lambda }$CDM model and try to solve them by including curvature or evolving dark energy. For DES BAO and SN in combination with {\tt Planck-2018}, we find a $3.2\sigma$ deviation from $\mathrm{\Lambda }$CDM when assuming $w_0{w}_a$CDM. For the combination of all the background cosmological probes considered (including ${\theta }_{\star }$ from {\tt Planck-2018}), we still find a deviation of $2.8\sigma$ in the $w_0-w_a$ plane. This work provides further evidence for non-$\mathrm{\Lambda }$CDM physics or systematics, which is consistent with recent claims in support of evolving dark energy.

Speaker: Juan Mena-Fernández (CNRS - LPSC)

CosmoVerse_Istanbul_JuanMena.pdf

19 Model-independent constraints in the context of multi-messenger cosmology¶

Gravitational wave (GW) and multi-messenger (MM) astronomy provides new ways to gain insights into the Dark Energy (DE) phenomenology, through the distance-redshift relation, as well as into potential deviations from General Relativity (GR), via the distance duality relation. Both analyses involve the same astrophysical observables, making MM astronomy a powerful tool to explore simultaneously the nature of DE and gravity.
To this end, provided our current ability to localize $\gamma$-ray bursts (GRBs), we employ a catalog of 38 of these events supposedly originated from binary neutron star (BNS) mergers detected by the Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory. We generate a mock dataset of MM events by means of a prior-informed Fisher matrix approach and forecast the sensitivity of the Einstein Telescope (ET) to constrain cosmological and modified gravity (MG) parameters.
We compare the performance of standard DE and MG parametrizations with a more flexible, model-independent method based on Gaussian Processes (GP). We examine synergies among next-generation GW interferometers and cosmological probes such as Cosmic Microwave Background (CMB), Type IA Supernovae (SnIA) and Baryon Acoustic Oscillation (BAO) data. We capitalize on their complementarity to break parameter degeneracies, showing that fewer than 40 GRB-GW events will provide unprecedented accuracy in constraining the DE phenomenology as well as potential departures from GR.

Speaker: Andrea Cozzumbo (Gran Sasso Science Institute)

CosmoVerse_Cozzumbo.pdf

20 Conference Photo¶

Lunch¶

Afternoon session 1: (Wednesday)¶

Convener: Eleonora Di Valentino (University of Sheffield)

21 Modified gravity & friends vs current CMB data: departures from the standard model, their link with non-zero curvature and massive neutrinos.¶

I will present a series of works looking for evidence of the effects induced by seemingly different extensions to the standard cosmological model in the cosmic microwave background (CMB) primary and secondary anisotropies: modified gravity, massive neutrinos and non-vanishing spatial curvature. By looking at their correlations and the differences in the impact they have on cosmological observables, such as the CMB lensing potential spectrum, we assess whether such additions, according to current CMB data, can be told apart.

Speaker: Enrico Specogna (University of Sheffield)

presentation_istanbul.pdf

22 BAO miscalibration cannot rescue late-time solutions to the Hubble tension¶

Baryon Acoustic Oscillations (BAO), together with Type Ia Supernovae (SNeIa), increasingly disfavor late-time modifications as viable resolutions to the Hubble tension. A key argument against post-recombination solutions—the so-called “no-go theorem”—relies critically on BAO data: since BAO constrain angular scales proportional to rdH0​, a higher H0​ implies a smaller rd, in tension with the value inferred from ΛCDM fits to Planck or BBN+BAO. However, the BAO analysis pipeline involves several steps that assume a fiducial cosmology, raising concerns about potential model dependence. In this proof-of-principle work, we explore the impact of a possible miscalibration (whose origin we remain agnostic about) in the BAO scale that effectively shifts the inferred BAO positions closer to the observer, leading to larger values of rdH0. We find that, even if such a modification were present, “popular” late-time modifications would still fall short of resolving the Hubble tension, to no small extent because of tight constraints on the shape of the expansion history arising from high-redshift uncalibrated SNeIa.

Speaker: Davide Pedrotti (TIFPA-INFN, Università di Trento)

Davide_Istanbul_2025.pdf

23 Dark Energy Crosses the Line: Quantifying and Testing the Evidence for Phantom Crossing¶

Combinations of the most recent CMB, BAO, and SNeIa datasets, when analyzed using the CPL parametrization, $w(a)=w_0+(1-a)w_a$, exclude $\mathrm{\Lambda }$CDM at $\gtrsim 3\sigma$ in favor of a dark energy equation of state (EoS) parameter that crosses the phantom divide. We confirm this behavior and show that it persists when DESI BAO data are replaced by SH0ES $H_0$ measurements, despite the known tension between these probes in the presence of CMB data. In both cases, the constraints favor a transition from an early-time phantom-like phase to a late-time quintessence-like phase, with the crossing occurring at different redshifts depending on the dataset combination. The probability that a phantom divide line (PDL) crossing does not occur within the expansion history is excluded at significance levels ranging between $3.1\sigma$--$5.2\sigma$. To investigate whether the apparent PDL crossing is a genuine feature preferred by the data or an artifact of the linear form of the CPL parametrization, we isolate the PDL crossing feature by introducing two modified versions of CPL that explicitly forbid it: CPL${}_{>a_{\mathrm{c}}}$ and CPL${}_{

Speaker: Emre Ozulker (University of Sheffield)

EmreOzulker_CosmoVerse_Istanbul.pdf

Coffee Break¶

Afternoon session 2: (Wednesday)¶

Convener: Agnieszka Pollo (Jagiellonian University)

24 Cosmic Dipole Tensions: Confronting Planck, NVSS, RACS and CatWISE¶

The cosmic dipole measured in surveys of cosmologically distant sources is generally in disagreement with the kinematic expectation of the Cosmic Microwave Background (CMB). This discrepancy represents severe tension with the Cosmological Principle and challenges the standard model of cosmology. We present a Bayesian analysis that quantifies the tension between datasets used to measure the cosmic dipole. Here, we examine active galaxies in the NRAO VLA Sky Survey (NVSS), the Rapid ASKAP Continuum Survey (RACS) and the Wide-field Infrared Survey Explorer catalogue (CatWISE), and jointly analyse them with the Planck LFI observations of the CMB. Whilst we find that the Planck–quasar-dipole tension is strong, we conclude that deeper radio observations are required to disentangle the Planck–radio-dipole tension, which may be detectable at high significance in the near-future.

Speaker: Mali Land-Strykowski (The University of Sydney)

mali_land_strykowski_cosmoverse_2025.pdf

25 CMB Lensing Imprints of Cosmic Voids: A test of ΛCDM¶

Cosmic voids, the largest observable structures in the Universe and characterized by a scarcity of galaxies, impact the Cosmic Microwave Background (CMB) through gravitational lensing, leaving a negative imprint on the CMB convergence (κ) map. This imprint offers insights into matter distribution within voids and the growth of cosmic structures. Furthermore, voids produce secondary CMB anisotropies via the Integrated Sachs–Wolfe (ISW) effect, which manifest as colder regions in the CMB and serve as probes of the Universe’s accelerated expansion. Motivated by debates on the ISW effect’s strength from voids and its potential link to the CMB Cold Spot, we explore the CMB lensing signatures of voids to test the ΛCDM model using N-body simulations. For this reason, we cross-correlated Planck CMB κ maps with voids from DES Y3, covering approximately 4 200 square degrees, employing both the 2D void-finder and the new Voxel 3D void-finder. Applying optimal matched filtering based on MICE N-body simulations, we measured the lensing amplitude Aκ = 1.03 ± 0.22 (4.6σ) for Voxel and Aκ = 1.02 ± 0.17 (5.9σ) for 2D voids, aligning with ΛCDM predictions. Inverting the void-finding process to identify superclusters yielded Aκ = 0.87 ± 0.15 (5.9σ). Our analysis indicates good agreement with the consensus ΛCDM model, with no deficit or excess signals identified. We also show that Planck noise is the primary limitation, suggesting that future data from the Atacama Cosmology Telescope (ACT), Simons Observatory (SO), South Pole Telescope (SPT), and CMB-S4 will enhance sensitivity and enable more precise measurements. Upcoming observational programs such as the Roman Space Telescope, Euclid, and LSST are expected to provide data sets that will allow the statistics of cosmic voids to be exploited as a stringent probe of the ΛCDM and to deliver significantly tighter constraints on cosmological parameters.

Speaker: Umut Emek Demirbozan (IFAE- Universitat Autonoma de Barcelona)

Cosmoverse Talk Umut.pdf

26 A low-redshift preference for an interacting dark energy model¶

We explore an interacting dark sector model in trace-free Einstein gravity where dark energy has a constant equation of state, $w=-1$, and the energy-momentum transfer potential is proportional to the cold dark matter density. Compared to the standard $\mathrm{\Lambda }$CDM model, this scenario introduces a single additional dimensionless parameter, $ϵ$, which determines the amplitude of the transfer potential. Using a combination of \textit{Planck} 2018 Cosmic Microwave Background (CMB), DESI 2024 Baryon Acoustic Oscillation (BAO), and Pantheon+ Type Ia supernovae (SNIa) data, we derive stringent constraints on the interaction, finding $ϵ$ to be of the order of $\sim \mathcal{O}({10}^{-4})$. While CMB and SNIa data alone do not favor the presence of such an interaction, the inclusion of DESI data introduces a mild $1\sigma$ preference for an energy-momentum transfer from dark matter to dark energy. This preference is primarily driven by low-redshift DESI BAO measurements, which favor a slightly lower total matter density ${\mathrm{\Omega }}_m$ compared to CMB constraints. Although the interaction remains weak and does not significantly alleviate the $H_0$ and $S_8$ tensions, our results highlight the potential role of dark sector interactions in late-time cosmology.

Speaker: Yuejia Zhai (University of Sheffield)

CosmoVerse-Zhai.pdf

Poster session¶

Conference dinner¶

Thursday 26 June ¶

Morning session 1: (Thursday)¶

Convener: Marika Asgari (Newcastle University)

27 Anisotropic cosmography of the local Universe¶

Cosmography is conventionally used to analyze data in an FLRW metric while being agnostic to the field equations of gravity.

I will talk about how we can go one step further to obtain a cosmography that is free of assumptions about the space-time metric as well.

With this approach, we can in principle obtain model-independent information about the expansion and curvature of our anisotropic cosmic neighbourhood, given sufficiently accurate observations of luminosity distance, angular diameter distance, redshift drift, position drift, etc. Such cosmography methods are complementary to the conventional more model-dependent approaches, and while they require high-precision data they hold a great promise for enabling model-independent constraints in cosmology. I will describe the formalism for doing anisotropic cosmography with luminosity distance, redshift drift, and position drift data, and I will discuss things that are good to be aware of when using cosmography methods in practice.

Speaker: Asta Heinesen

Istanbul.pdf

28 Alleviating H0 and σ8 tensions from topological dark energy¶

We formulate topological holographic dark energy and we present how it can alleviate both $H_0$ and $\sigma 8$ tensions. The modified cosmological model is obtained by the application of the gravity thermodynamics conjecture when the corresponding Wald Gauss Bonnet entropy term is added to the standard Bekenstein Hawking one. Thus, one obtains modified Friedmann equations that depend on the rate of horizon topology change, which is directly linked to the formation and merging rate of black holes by the conservation of the second thermodynamical law. By employing the star formation rate per redshift from observations as a first approximation to the BH formation and merging rate, we obtain a phantom effective dark energy which is known to be a sufficient mechanism that can lead to the $H_0$ tension alleviation. Indeed, we show that for particular regions of the parameter space the scenario at hand does lead to higher $H_0$ values, and we verify this result performing a confrontation with observational data from Supernovae Type Ia (SNIa) and Cosmic Chronometers (CC) measurements. Additionally, analyzing the growth of structures and the evolution of the matter overdensity we show that the redshift behavior of the $f{\sigma }_8$ observable obtains smaller values than the ones arising in $\mathrm{\Lambda }$CDM cosmology, and thus the scenario at hand can alleviate the ${\sigma }_8$ tension too. This is an advantage of Wald-Gauss-Bonnet cosmology, since the simultaneous alleviation of both tensions cannot be easily achieved within other entropic scenarios.

Speaker: Stylianos A. Tsilioukas (University of Thessaly, National Observatory of Athens)

Tsilioukas_Istanbul_Cosmoverse2025_final.pdf

29 Confronting the cosmic dipole tension¶

I discuss our ongoing efforts to measure the cosmic dipole in new galaxy surveys, referencing the systematic effects which can influence its measurement and forecasting the contribution of forthcoming surveys. This is motivated by the nascent cosmic dipole tension: the disagreement between the CMB dipole and counts of extragalactic sources, which has reached a high level of statistical significance ($>5\sigma$). It is key that we understand whether this anomaly genuinely means that the rest frames of the CMB and extragalactic sources do not coincide. If so, it would throw doubt on the implicit assumption of the cosmological principle in the FLRW spacetime metric, uprooting our cosmic paradigm.

Speaker: Oliver Oayda (The University of Sydney)

slides.pdf

Coffee Break¶

Morning session 2: (Thursday)¶

Convener: Geraint Lewis (Sydney Institute or Astronomy, The University of Sydney)

30 The Dark Side of the Cosmos: Exploring Dark Energy and Modified Gravity¶

We are currently witnessing a remarkable era in the fields of cosmology and gravitation, as an ever-increasing amount of observational data continues to be collected. One of the most perplexing questions confronting cosmologists today concerns the mysterious source responsible for the Universe’s recent accelerated expansion. The most widely accepted explanations for this phenomenon involve either a dark energy component or a modified theory of gravity. In this talk, we present, on the one hand, several dark energy models, ranging from axion-like scalar fields and kinetic gravity braiding models to 3-form fields. On the other hand, we explore modified theories of gravity, in particular those based on $f(Q)$ gravity, as alternative explanations for the Universe’s current accelerated expansion.We also discuss the observational constraints on these models. Furthermore, we examine how they may contribute to a potential resolution of the ongoing $H_0$ and $S_8$ tensions.

Speaker: Mariam Bouhmadi-López (Ikerbasque and University of the Basque Country)

Bouhmadi-Lopez-istanbul_2025.pdf

31 The distance duality relation in light of the cosmic calibration tension¶

The standard Lambda Cold Dark Matter (ΛCDM) cosmological model has proven remarkably successful in describing a broad range of observational data, ranging from the cosmic microwave background (CMB) radiation to the large-scale structure of the Universe. However, recent advances in precision cosmology have revealed persistent statistical discrepancies between independent data sets and observational methods. One prominent example is the "Hubble tension," which refers to the irreconcilable predictions of the present expansion rate of the Universe when inferred from early-Universe measurements (such as the CMB) compared to local observations. Low-redshift observables like Baryon Acoustic Oscillations (BAO) and Type Ia Supernovae (SN1a) are used to build the cosmological distance ladder, which relies on calibrations using either early- or late-Universe data. Therefore, the Hubble tension is also reflected in the incompatibility between these distances and how they are calibrated. However, this comparison assumes that the distance-duality relationship (DDR) holds and can be used to compare measurements of the luminosity and angular diameter distances. In this talk, we will examine the implications of relaxing this assumption to more general relations, its implications to the current cosmic tensions and how it could potentially explain the apparent need for the introduction of new physics to address current cosmic tensions.

Speaker: Elsa Teixeira (Université de Montpellier & CNRS)

Cosmoverse2025_Teixeira_linearized.pdf

32 On the microphysics of the Dark Energy¶

We propose a novel dark energy mechanism rooted in the microscopic structure of spacetime, where quantum-induced topological transitions give rise to an effective cosmological constant. Within the framework of Euclidean Quantum Gravity, gravitational instantons emerging at the spacetime foam level induce changes in topology, activating the Gauss-Bonnet term in four dimensions. This leads to a time-evolving dark energy density proportional to the instanton density. Using quantum field theory techniques, we derive the corresponding modified Friedmann equations and analyze the evolution of the Universe under this framework. The model successfully reproduces the thermal history of the Universe and yields a fit to observational data - including type Ia supernovae, Baryon Acoustic Oscillations, and cosmic chronometers - that is statistically comparable to the standard $\mathrm{\Lambda }$CDM model. Moreover, the scenario shows promise for addressing the $H_0$ and ${\sigma }_8$ tensions. These results position topological dark energy as a compelling alternative to the concordance model.

Speaker: Fotios Anagnostopoulos (University of the Peloponnese)

Microphysics_of_DE-FA.pdf

Lunch¶

Afternoon session 1: (Thursday)¶

Convener: Florian Beutler

33 Augmenting Galaxy Catalogues with Machine Learning¶

Hydrodynamic simulations will be instrumental in attempting to resolve cosmological tensions such as the $S_8$ tension as they can assist with modelling key sources of uncertainty such as galaxy bias. However, for these purposes, the simulations need both large box size and high resolution, which is computationally prohibitive. In this work we instead use the connection between galaxies and their host dark matter halo to populate a dark-matter-only simulation. We use the Extremely Randomized Trees machine learning algorithm with the FLAMINGO suite of simulations - a set of hydrodynamic and counterpart dark-matter-only simulations with box side length ranging from 1-2.8 Gpc and particle mass from ${10}^8{M}_{\odot }$ to ${10}^{10}{M}_{\odot }$. We use a two-tiered model that decides whether or not a halo hosts a galaxy, then models the stellar mass of said galaxy. We find that this method produces reliable results not only for the total stellar mass and correlation functions of galaxies, but also models the distribution of stellar masses within haloes of a give mass with higher accuracy than the standard approaches used to date.

Speaker: Elliot Scott (Newcastle University)

Istanbul Presentation New.pptx.pdf

34 Impact of galactic foregrounds on delensing of CMB maps and detection of the primordial gravitational waves¶

In the era of high precision Cosmic Microwave Background (CMB) anisotropy measurements, detection of the primordial graviational waves (PGW) will be one of the main goals of the upcoming CMB experiments. To make it possible precise cleaning of the CMB maps from diffused contamination coming from polarised galactic emission and CMB gravitational lensing effect is needed. We investigate the impact of residual galactic foregrounds in lensing reconstruction and delensing for CMB-S4 like experiment providing a complete pipeline for such experiment. This impact can be also important for the constraint on the ${\sigma }_8$ parameter estimated using the lensing potential. In this work, we pay special attention to forecast constraints for the amplitude of the primordial gravitational waves and their senstivity to different foreground models with varying complexity. As we will show, non-Gaussian galactic foregrounds are seen to have significant impact on the gravitational wave amplitude constraints. Accurate modelling of the foregorunds will be crucial for unbiased estimation of the amplitude.

Speaker: Kishan Deka (National Center for Nuclear Research,Warsaw)

cosmoverse_Kishan_26June.pdf

35 Cosmological analysis from the cross-correlation of the CMB gravitational lensing and galaxy surveys¶

The effect of gravitational lensing of the Cosmic Microwave Background
(CMB) provides a unique opportunity to obtain a picture of the gravitational potential of the large-scale structure of the Universe at very high redshifts. Tomographic cross-correlation of the gravitational potential with other tracers of the large-scale structure at known redshifts allows tracing the evolution of the structure through estimation of the matter fluctuation amplitude ${\sigma }_8$, shedding new light on the observed tensions between different estimates of the amplitude. It can also be used for to remove the lensing effect from CMB anisotropy maps and improve constraints on cosmological parameters, in particular the amplitude of the primordial gravitational waves. However, the analysis of the upcoming data requires a very good understanding of any systematic errors that may bias the cross-correlation measurements. In this talk I will show how much the reconstruction of the CMB lensing potential is improved by the tomographic analysis and how much this will affect the estimation of the ${\sigma }_8$ parameter for the upcoming CMB experiments and galaxy surveys.

Speaker: Dr Pawel Bielewicz (National Centre for Nuclear Research)

bielewicz.pdf

Coffee Break¶

Afternoon session 2: (Thursday)¶

Convener: Dr Nihan Katırcı (Doğuş University)

36 The effective running Hubble parameter in SNe Ia as a marker for the dark energy nature¶

We investigate a theoretical framework for the Universe dynamics, based on the possible creation of dark energy constituents by the time-varying gravitational field of the expanding Universe. The proposed scenario is compared to specific cases of the reduced Chevallier-Polarski-Linder (CPL) parameterization, with the aim of probing the nature of dark energy.
We construct, from our model and the reference models, a theoretical running Hubble constant, i.e. a function of the redshift, which highlights the difference between modified dynamics and the $\mathrm{\Lambda }$CDM-one. Then, by this function, we provide a fit of the corresponding running Hubble constant as it emerges from the binned analysis of SNe Ia data sample, i.e. the Pantheon and the Master samples, that is a collection of SNe Ia from 4 catalogs: Pantheon, Pantheon+, JLA, and DES without duplicated SNe Ia.
The main result of our study is the identification of a strong sensitivity of the running Hubble constant and the demonstration that the binned supernova data indicates a phantom nature of Dark Energy for $0

Speaker: Ms Elisa Fazzari (Sapienza University of Rome)

Fazzari_Istanbul.pdf

37 The Ph-LsCDM Model: Towards a Solution to the H0 tension Problem.¶

In my talk, I will present a model where the Universe is filled with a phantom scalar field with a potential in the form of a hyperbolic tangent, as well as cold dark matter and radiation. This model is free from the shortcomings usually attributed to phantom models and undergoes a rapid Anti-de Sitter to de Sitter transition. At the background level, a detailed analysis of the model is performed. The numerical integration of the equations of motion is performed in such a way that both the CMB and SH0ES data are satisfied. Thus, this model points to a possible solution to the problem of cosmological tensions in theories where the cosmological constant changes its sign.

Speaker: Alexander (Oleksandr) Zhuk (Astronomical Observatory, Odessa I.I. Mechnikov National University, Dvoryanskaya St. 2, Odessa 65082, Ukraine; Center for Advanced Systems Understanding, Untermarkt 20, 02826 G¨orlitz, Germany)

Istanbul CosmoVerse_Zhuk.pdf

38 Primordial Black Holes, Charge, and Dark Matter: Rethinking Evaporation Limits¶

Earlier studies investigating the allowed fraction of dark matter as primordial black holes (PBHs) tend to completely rule out PBHs with masses smaller than \~10⁻¹⁵ solar masses. This is due to the lack of evidence for Hawking radiation coming from the final evaporation stages of such small PBHs. These limits, however, make the key assumption that these PBHs can be modelled as uncharged, non-rotating Schwarzschild black holes. This talk concerns changes to these lower mass bounds when charge is included i.e., by going to Reissner–Nordström black holes as models for PBHs. In particular, the analysis presented here assumes a "dark" U(1) charge (assumed to be present in the early universe), and employs the Hiscock and Weems model for charged black hole evaporation to properly account for the Schwinger effect. By assigning a heavier dark electron mass and/or increasing its charge, this approach not only suppresses Hawking radiation but also mitigates Schwinger pair production, effectively extending the lifespan of PBHs beyond the age of the universe, bypassing the conventional wisdom that such small-mass black holes would have evaporated entirely by now.

Based on https://arxiv.org/abs/2503.20696

Speaker: Jessica Santiago (National Taiwan University)

Jessica_Santiago_slides.pdf

Panel discussion¶

Istanbul Public.pdf

PublicTalk_Akarsu_CosmoverseIstanbul2025.pdf

39 ITU Turkish Musical State Conservatory Closing Ceremony¶

40 ITU Turkish Musical State Conservatory Concert (open to the public)¶

Public talks¶

Istanbul Public.pdf

PublicTalk_Akarsu_CosmoverseIstanbul2025.pdf