CosmoFONDUE - Cosmological Fundamental Observables and Novel Discoveries in Universe Evolution

10 Jun 2025, 11:00 → 13 Jun 2025, 20:30 Europe/Zurich

Main auditorium (École de physique, Quai Ernest-Ansermet 24, 1205 Genève, Suisse)

Francesco Sorrenti (University of Geneva), Maria Berti (University of Geneva), Nastassia Grimm (University of Geneva), Simona Procacci (University of Geneva), Sveva Castello (University of Geneva)

While new discoveries in cosmology have revolutionized our understanding of the Universe over the past decades, many unresolved mysteries remain, such as the nature of dark energy or the physical processes occurring in the earliest stages of history. To resolve these questions in light of new observations, joint efforts across the whole community are needed.
This conference aims at offering an occasion for scientists across all fields of cosmology to interact in an engaging and inclusive atmosphere, and to reflect on promising new research directions. A special focus is put on fostering exchange among junior and senior researchers. 

Confirmed speakers:

• Luca Amendola
  
• Camille Bonvin
• Chiara Caprini
• Anthony Challinor
• Eleonora di Valentino
• Daniel G. Figueroa
  
• Ruth Durrer
• Julien Lesgourgues
• Michele Maggiore
• Arttu Rajantie
• Fabian Schmidt
• Matteo Viel

 

The event is planned to be in-person, with the number of participants limited. The registration and the call for abstracts are open until April 30, 2025.

To cover the expenses of coffee and lunch breaks, we ask for a contribution of CHF 150 per person.

We received information of several phishing attempts claiming to offer hotel bookings. CosmoFONDUE does NOT have an official hotel booking agent, please do not share personal or payment details with unknown senders. All communications on our part will be made through institutional Unige email accounts.

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CosmoFONDUE is founded by the Société Académique de Genève, the European Research Council, and the Swiss National Science Foundation. The CosmoFONDUE Organizing Committee acknowledges the support of Camille Bonvin and Martin Kunz. 

 

 

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Tuesday 10 June

11:00 Registration and welcome breakfast

Introduction

1 Welcome and introduction

CosmoFONDUE - intro.pdf

2 Questions in Modern Cosmology

Introduction to Modern Cosmology

Speaker: Ruth Durrer (DPT Université de Genève)

Durrer.pdf

3 Discussion session

13:00 Lunch break

CMB

4 Introduction to the Cosmic Microwave Background

Speaker: Anthony Challinor

Challinor_Geneva2025.pdf

5 Cosmology in latent space: a data-driven parametrization of the CMB temperature power spectrum

Finding the best parametrization for cosmological models in the absence of first-principle theories is an open question. We propose a data-driven parametrization of cosmological models given by the disentangled 'latent' representation of a variational autoencoder (VAE) trained to compress cosmic microwave background (CMB) temperature power spectra. We consider a broad range of ΛCDM and beyond-ΛCDM cosmologies with an additional early dark energy (EDE) component. We show that these spectra can be compressed into 5 (ΛCDM) or 8 (EDE) independent latent parameters, as expected when using temperature power spectra alone, and which reconstruct spectra at an accuracy well within the Planck errors. These latent parameters have a physical interpretation in terms of well-known features of the CMB temperature spectrum: these include the position, height and even-odd modulation of the acoustic peaks, as well as the gravitational lensing effect. The VAE also discovers one latent parameter which entirely isolates the EDE effects from those related to ΛCDM parameters, thus revealing a previously unknown degree of freedom in the CMB temperature power spectrum. We further showcase how to place constraints on the latent parameters using Planck data as typically done for cosmological parameters, obtaining latent values consistent with previous ΛCDM and EDE cosmological constraints. Our work demonstrates the potential of a data-driven reformulation of current beyond-ΛCDM phenomenological models into the independent degrees of freedom to which the data observables are sensitive.

Speaker: Davide Piras (University of Geneva)

Piras_Geneva_presentation.pptx

6 Generalized neutrino isocurvature

Searches for neutrino isocurvature usually constrain a specific linear combination of isocurvature perturbations. In this talk, realistic cosmological scenarios giving rise to neutrino isocurvature are discussed. We show that in general both, neutrino and matter isocurvature perturbations are generated, whose ratio we parameterise by a newly introduced mixing angle. We obtain the first limits on this new mixing angle from PLANCK data. Since the amount of matter isocurvature can be related to DM production and/or baryogenesis, novel insights into the early universe could be provided by future measurements in this way.

Speaker: Wolfram Ratzinger (Weizmann Institute)

NeutrinoIsocurvatureCosmoFondue.pdf

7 De-lensing the CMB

Gravitational lensing of the Cosmic Microwave Background is a very valuable cosmological signal, detected at very high significance by several experiments, and its SNR is expected to increase by a factor of 10 or so in the next decade.
At the same time, it has also become an hindrance to some important science goals of CMB experiments, most notably for best constraints on a primordial background of gravitational waves, which should also see very significant improvements in the coming years.
After reviewing current lensing estimation techniques and recent results, I’ll discuss how removal of the lensing signal (`delensing') typically helps measuring small parameters that affects the CMB polarization.
Very sensitive experiments must rely on novel more powerful algorithms to optimally extract or remove the lensing signal, and I’ll discuss recent developements.

Speaker: Julien Carron (Université de Genève)

CosmoFondue.pdf

16:00 Coffee break

GWs emitted after the CMB

8 Cosmology and fundamental physics with GWs

Introduction to gravitational waves emitted after the CMB

Speaker: Michele Maggiore (University of Geneva)

cosmofondue.pdf

9 Chirality in Cosmology: testing large-scale mirror symmetry with LIGO-Virgo black hole mergers.

Precessing black-hole mergers can produce an imbalance between right- and left-handed circularly polarized gravitational waves. According to the Cosmological Principle, such chiral emission should average out to zero across all binary mergers in our Universe in order to preserve mirror-reflection symmetry at very large scales. In this talk, I will show how gravitational-wave astronomy enables a unique, observer-independent test of this hypothesis. Specifically, I will introduce a new geometric observable within General Relativity, which is chiral and can be used to quantify the average net circular polarization emitted by an ensemble of binary black hole mergers detected by LIGO-Virgo. I will present current results and limitations, and discuss future prospects with upcoming detections and technical advancements. Notably, we find that this novel measure of circular polarization and the helicity of the remnant black hole are linearly correlated, drawing a conceptual parallel between this cosmological test and Wu’s experiment in particle physics.

Based on:

[1] J. Calderon-Bustillo, A. del Rio, N. Sanchis-Gual, K. Chandra, S. H. W. Leong, Phys. Rev. Lett. 134, 031402 (2025) [arXiv:2402.09861].

[2] S.H.W. Leong, A. Florido-Tomé, J. Calderon-Bustillo, A. del Rio, N. Sanchis-Gual, To appear in Phys. Rev. D (2025) [arXiv:2501.11663].

Speaker: Adrian del Rio (Charles III University of Madrid)

Talk.pdf

10 Gravitational lensing of gravitational waves : new insights from the wave optics regime

Gravitational lensing, i.e. the deflection of propagating signals by spacetime curvature is one key prediction of General Relativity. Besides this intrinsic interest as a fundamental prediction, this effect has become one crucial probe used by the cosmology community to identify and map dark matter.
Relying on the idea that the propagation of both electromagnetic and gravitational waves (GW) signals is analogous, it is tempting to think that these two types of signals obey the same type of gravitational lensing. However, I will argue why this idea is insufficient when wave-optics effects are taken into account, e.g. diffraction. I will illustrate how GW lensing differs from the usual lensing formalism and discuss its relevance, challenges and opportunities for cosmology with GW.

Speaker: Martin Pijnenburg

Talk_GW_wave_optics_COSMOFondue.pdf

11 Deviations from CDM in SMBH, bounds from JWST and predictions for lISA

In the talk, I will introduce a new semianalytical model (SAM) for supermassive black hole (SMBH) growth that departs from traditional EPS-based merger tree methods by directly tracking differential SMBH growth via mergers. I will show that this model reveals a clear preference for heavy SMBH seeds across diverse datasets—including recent JWST observations—except in cases of extremely efficient merging, where a light-seed scenario uniquely fits the JWST data. I will also discuss the interplay between galaxy–SMBH co-evolution and dark matter (DM) models. I will show that, by incorporating the effects of warm and fuzzy DM on large-scale structure formation and halo evolution, the latest JWST measurements of the UV luminosity function and SMBHs can be used to place novel constraints on DM properties. Notably, this uncovers a previously unexplored link between the initial SMBH seed mass and DM characteristics, establishing SMBH evolution as a remarkably sensitive tracer of DM. Finally, I will also comment on what are the prospects of future GW observatories, namely LISA and the atom interferometers, to complement current observations and severely constrain or discover deviations from cold DM.

Speaker: Juan Urrutia (KBFI, Tallin)

COSMOgeneva-SMBH-as-DM-TRACERS.pdf

Apèro and Poster session

12 4MOST Cosmology Redshift Survey: BG and LRG Target Selection

The 4-metre Multi-Object Spectroscopic Telescope is currently being installed on the VISTA Telescope (Paranal) and will start observations of multiple science cases at the end of this year. One of the 18 experiments is the Cosmology Redshift Survey (CRS) which is composed of 4 samples: Bright Galaxies (BG, 0.15 < z < 0.4), Luminous Red Galaxies (LRG, 0.4 < z < 1.0), Quasars (QSO, 0.9 < z < 2.2) and Lyman-forest Quasars (QSO-Lyalpha, 2.2 < z < 3.5).
These samples will be used to perform spectroscopic clustering measurements but also provide spectroscopic redshifts for other photometric surveys as the LSST, or future southern radio telescopes as the Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) or the Square Kilometre Array (SKA).
In this talk, I will present an overview of 4MOST as well as a description of the BG and LRG samples

Speaker: Aurélien Verdier (EPFL)

13 A new self-consistent 2nd order alternative gravity theory

I present a new gravity theory based on the Schouten and Codazzi tensors. Field equations are second order and generalize Einstein's equations with a well defined correspondence limit. Exact solutions with static spherical symmetry and with the Robertson-Walker metric contain extra terms with the potential to account for dark energy effects. The theory is still under development but it is logically self-consistent. Its predictions should be further examined and tested.

Speaker: Roberto Sussman (Institute of Nuclear Sciences, National Autonomous University of Mexico)

CosmoFONDUE.pdf

14 Bayesian Optimisation for cosmological model selection

Cosmological model selection, in the framework of Bayesian inference requires the calculation of the Bayesian evidence. This can often be quite challenging, especially if the underlying likelihood function is expensive to evaluate. I will show how a technique called Bayesian Optimisation, based on Gaussian Process regression, can be used to calculate this evidence in far fewer likelihood evaluations, offering a much more efficient approach compared to traditional methods. I will discuss applications to CMB, GW and 21cm cosmology.

Speaker: Ameek Malhotra (Swansea University)

Poster_BayOp_Malhotra.pdf

15 Consistency of Strange Quark Matter within f(R,Lm) Gravity

In this study, we explored the FLRW model within f(R,Lm) gravity, focusing on Strange Quark Matter and its role in cosmic evolution. Using 57 observational data points, we derived the best-fit H(z) curve with an impressive R^2 value of 0.9527, showing strong alignment with the ΛCDM model. The deceleration parameter q(z) highlights a smooth transition from deceleration to acceleration, consistent with dark energy-driven expansion. Additionally, our analysis of energy density ρsq(z), pressure psq(z), and the equation of state ωsq(z) reveals Strange Quark Matter evolving from dark energy-like behavior to a matter-dominated phase. The statefinder diagnostic showcases the model’s capability to capture cosmic evolution, transitioning from a high-density Chaplygin Gas state to the ΛCDM point (r,s) = (1,0) and aligning with the de Sitter phase (q,r) = (−1,1). Furthermore, the Ω(z) diagnostic rein forces the model’s strong alignment with ΛCDM. Energy conditions, including NEC, WEC, and DEC, are satisfied, while the SEC transitions to consistency with cosmic acceleration.

Speaker: PRAMOD GAIKWAD (School of Mathematical Sciences swami Ramanand Teerth Marathwada University Nanded India)

16 Cubic parametrization of the deceleration parameter within \( f(T) \) gravity

In this study, we used the ( f(T) ) gravity framework with the energy-momentum tensor for a perfect fluid to derive key cosmological parameters, including the Hubble parameter ( H ), deceleration parameter ( q ) and Statefinder diagnostics. Model parameters were optimized using an ( R^2 ) test, resulting in ( \beta = 1.312^{+0.013}{-0.014} ), ( \xi = 1.273^{+0.0065}{-0.0071} ), and ( H_0 = 72.60^{+0.50}_{-0.49} ), with an ( R^2 ) of 0.9527. Our model aligns closely with the (\Lambda)CDM model and shows good performance based on AIC and BIC criteria. Analyzing the ( q(z) ) curve revealed the transition from deceleration to acceleration in the universe’s expansion. Additionally, we examined pressure, energy density, and equation of state parameter for two models, ( f(T) = \lambda T ) and ( f(T) = T + \beta T^2 ), both aligning well with observational data. The ( r )-( s ) and ( r )-( q ) diagnostics further confirm our model's consistency with (\Lambda)CDM, making it a strong alternative for explaining cosmic expansion. The evolution of (\Omega(z)) shows strong consistency with the (\Lambda)CDM model, with the Om parameter approaching 0.3 at lower redshifts and parameter uncertainties highlighting the model's reliability.

Speaker: Nitesh Ghungarwar (Swami Ramanand Teerth Marathwada University, Vishnupuri, Nanded, India)

17 Enhancing Cold Dark Matter Simulations with Physics-Informed Neural Networks

Dark Matter (DM) is a cornerstone of the standard cosmological model, yet its fundamental nature remains elusive. Accurate numerical simulations are essential to test competing DM models against observational data. In this work, we propose a novel approach to DM simulations by replacing traditional N-body methods with Physics-Informed Kolmogorov-Arnold Networks (PIKANs). Specifically, we apply PIKANs to solve the one-dimensional cosmological Vlasov-Poisson equation for Cold Dark Matter. Our model well captures core collapse, the emergence of singularities at shell-crossing times, and the slope of the density profile. We further show that our method achieves lower residuals compared to N-body simulations. These results demonstrate the potential of physics-informed neural networks as a precise tool for DM simulations. This approach is particularly relevant in the context of upcoming Epoch of Reionization experiments, such as with the Square Kilometre Array, which will require high-resolution simulations to distinguish between competing dark matter scenarios.

Speaker: Nicolas Cerardi (EPFL, Swizerland)

18 Evolution of gauge-invariant scalar perturbations from inflation to reheating

We assume that at a late stage of inflation, a scalar inflaton field, a thermal plasma, and a spacetime metric coexisted and interacted with each other. We expand them to the linear order around a homogeneous background and combine the perturbations into a set of gauge invariant variables. For the latter we derive evolution equations in the framework of smooth reheating. Having resolved some numerical challenges, we provide solutions for a set of benchmarks, from inflation all the way until radiation domination. In particular, our solution exhibits some key features of the inflationary paradigm, for example: gauge invariant 'curvature perturbations' obtain the same constant value when they are outside of the Hubble horizon and experience acoustic oscillations upon re-entry. The talk is based on 2407.17074.

Speaker: Alica Rogelj (University of Bern (AEC, ITP))

CosmoFondue Poster (4).pdf

19 Finite-temperature bubble nucleation with shifting scale hierarchies

Scale hierarchies are required to reliably describe the thermodynamics of cosmological first-order phase transitions using perturbation theory. At finite temperature, such a hierarchy is provided naturally. One can then use this hierarchy to construct a three-dimensional effective field theory (EFT) that systematically includes thermal resummations to all orders.

Using this EFT framework, I focus on supercooled phase transitions in models with classical scale symmetry [1]. By computing the bubble nucleation rate and accounting for the presence of varying energy scales, I examine the limitations of derivative expansions in constructing a thermal effective field theory for bubble nucleation. In particular, for gauge-field fluctuations, the derivative expansion breaks down beyond the first two terms due to strong variations in gauge-field masses between the high- and low-temperature phases.

By directly computing these contributions using the fluctuation determinant, I demonstrate how this approach significantly improves nucleation rate calculations compared to leading-order results, providing a more robust framework for predicting gravitational-wave signals from supercooled phase transitions.

[1] M. Kierkla, P. Schicho, B. Swiezewska, T. V. I. Tenkanen, and J. van de Vis, Finite-temperature bubble nucleation with shifting scale hierarchies, (2025), [2503.13597].

Speaker: Philipp Schicho (University of Geneva)

cosmoFondue25_schicho.pdf

20 Gravitational waves from pion dark matter?

QCD-like dark sectors may undergo a first-order chiral phase transition in the early Universe that may lead to production of stochastic gravitational wave background. We consider a class of such dark sectors that feature also dark pions as viable dark matter candidates and study the corresponding strength of the gravitational-wave signal. Importantly, the chiral phase transition is of first order only if the pion mass is much smaller than the critical temperature. One way how to achieve correct dark matter abundance for small pion masses is to consider a secluded dark sector. We show that for such scenario, the gravitational wave signal is, unfortunately, strongly suppressed. Alternative scenarios with possibly stronger gravitational wave signal are also discussed.

Speaker: Helena Kolesova (University of Stavanger)

CosmoFondue_Kolesova.pdf

21 Gravity and the Bag Model

The QCD Bag model is used to show that, on the grounds of some gravitational arguments, a proton seems to behave like a microscopic black hole, with de Sitter spacetime as the inner geometry and a regular Schwarzschild spacetime outside it [1]. The basic idea is to assume that, for masses $m$ smaller than the Planck mass, the Newton constant $G_{N}$ may be given by $G_{s} = \hbar c/m^{2}$, where $m$ is the mass of the physical system under consideration, $s$ subscript means ‘’strong’’, $c$ is the velocity of light and $\hbar$ stays for the Planck constant.
If $m$ represents the Higgs mass $m_{H} \approx 125 GeV/c^{2}$, we get $G_{s} = \hbar c/m_{H}^{2} = 10^{27}$ in CGS units, the same value obtained by Onofrio [2], who considers weak interactions as short distance manifestation of gravity.
One is also shown that, due to the proposed strong gravity in microphysics, the quantum cosmological constant (related to the vacuum energy density) and the classical one (resulting from Cosmology) are directly related.
References:
1.H. Culetu, , Int. J. Theor. Phys. 54, 2855 (2015).
2.R. Onofrio, Mod. Phys. Lett. A 28, 1350022 (2013).

Speaker: Hristu Culetu

22 Induced gravitational waves from second-order scalar perturbations

We investigate the generation of gravitational waves from scalar perturbations at second order in a matter-dominated Friedmann–Robertson–Walker (FRW) universe. Since the representation of gravitational waves in perturbation theory is gauge-dependent, identifying gauge-invariant quantities becomes essential for a physically meaningful interpretation. This leads us to define a new tensor amplitude that is both transverse-traceless and gauge-invariant at second order, and can be interpreted as the physical gravitational wave. Finally, we are attempting to study the geometrical interpretation and hypothesize that it acts as a potential for the B-modes of the Weyl tensor.

Speaker: Stylianos Papadopoulos

Papadopolous-Poster_Cosmo_Fondue.pdf

23 Limits of EFTs at finite temperature for strong phase transitions

Phase transitions are violent and interesting phenomena that could have occurred in the early stages of the universe. Possible perturbative techniques to study these phenomena and predict their gravitational wave background can be used in the presence of a hierarchy of scales, leading to the construction of Effective Field Theories at finite temperature by integrating out the heavier scales. These EFTs are actually reliable when the dynamics are mainly encoded in the most relevant operators. I will then discuss the limits of such EFTs, showing how higher-dimensional operators affect the theoretical prediction of stronger transitions, including those detectable by LISA. These considerations impact the applicability of effective theory techniques, including their use in non-perturbative lattice studies.

Speaker: Fabio Bernardo (Universitè de Genève)

24 Line Intensity Mapping in GLASS

The large-scale structure of the universe provides valuable information on the fundamental
laws governing its evolution. This structure consists of a network of immense galaxy filaments,
separated by vast cosmic voids, forming an intricate pattern known as the cosmic web, which
is shaped by gravity. To study and better understand these phenomena, cosmologists employ
computational simulations utilizing sophisticated numerical codes to reproduce the evolution
of the universe. One such code is GLASS.
GLASS (Generator for Large Scale Structure) is a novel simulation code for galaxy surveys
in cosmology. It iteratively constructs a light cone, incorporating matter, galaxies, and weak
gravitational lensing signals as a sequence of nested shells.
This project focuses on the implementation of Line-Intensity Mapping (LIM) in GLASS,
with particular emphasis on the HI 21 cm emission line. LIM is an emerging field in physical
cosmology that measures the integrated emission from spectral lines in galaxies and the diffuse
intergalactic medium (IGM) to trace the growth and evolution of cosmic structures.
We model the HI 21 cm signal using the mean neutral hydrogen temperature to generate fullsky
maps of brightness temperature fluctuations. From these maps, we compute the angular
power spectrum Cℓ and compare it with theoretical expectations. We then correlate these
spectra with those from galaxy clustering, revealing a strong complementarity between the
distributions of galaxies and neutral hydrogen.
As a next step, we plan to extend the analysis to more realistic observational scenarios by
incorporating real survey data distributions, such as those expected from MeerKLASS.

Speaker: Luca Capaldo (University of Turin)

Poster.pdf

25 Local limit of nonlocal gravity: observational viability

Motivated by analogies with the electrodynamics of media, we propose a non-local extension of Einstein’s theory of gravitation formulated within the framework of teleparallel gravity. Employing a specific localizing kernel, we analyze the local limit of the theory at both the background and linear perturbation levels, and confront it with cosmological observations. We demonstrate that the model is capable of alleviating the Hubble tension.

Speaker: Abdolali Banihashemi (University of Oslo)

26 Modified gravity vs dark sector interactions: settling the dispute through the distortion of time

Combining measurements of the growth rate of cosmic structure with gravitational lensing is considered as the optimal way to test for deviations from General Relativity on cosmological scales. In my poster, I will demonstrate that this standard method suffers from an important limitation, since models of dark matter with additional interactions can lead to exactly the same signatures as modified gravity in these two observables. Luckily, I will show that the coming generation of large-scale structure surveys, like the Square Kilometre Array, will allow us to break this degeneracy through measurements of the distortion of time.

Speaker: Sveva Castello (University of Geneva)

SvevaCastello_Distortion_of_time_poster.pdf

27 Primordial black holes with logarithmic non-gaussianities and their gravitational wave signals

Primordial black holes (PBHs) are a unique probe of the early Universe and offer a potential link between inflationary dynamics and dark matter. In this talk, I will present our recent work investigating PBH formation in the presence of local non-Gaussianities, exploiting a logarithmic duality relation to study a variety of inflationary mechanisms that locally deviate from slow-roll dynamics. Using numerical relativity simulations, we accurately compute the formation thresholds and resolve the expected PBH abundances, along with their associated Scalar Induced Gravitational Waves (SIGWs). We provide signal predictions for LISA-like experiments and discuss the interpretation of SIGWs in the context of the nanoHz stochastic gravitational wave background recently reported by Pulsar Timing Array observations.

Speaker: Cristian Joana (Institute of Theoretical Physics, Chinese Academy of Science)

Poster_PBHwNGs_CJoana.pdf

28 Relic Graviton Background from Gravitational Cherenkov Radiation

Scalar particles traveling faster than a subluminal gravitational wave generate gravitons via gravitational Cherenkov radiation. We investigated graviton production by the primordial plasma within the framework of modified gravity in the early Universe, generating a relic graviton background. By requiring the relic graviton background to remain consistent with the Big Bang Nucleosynthesis constraint, we derived limits on the gravitational wave speed at early times in certain modified gravity theories.

Speaker: Roxane Theriault (Jagiellonian University)

29 SPINN: Advancing Cosmological Simulations of Fuzzy Dark Matter with Physics Informed Neural Networks

Physics-Informed Neural Networks (PINNs) have emerged as a powerful tool for solving differen-
tial equations by integrating physical laws into the learning process. This work leverages PINNs to
simulate gravitational collapse, a critical phenomenon in astrophysics and cosmology. We introduce
the Schr¨odinger-Poisson informed neural network (SPINN) which solve nonlinear Schr¨odinger-Poisson
(SP) equations to simulate the gravitational collapse of Fuzzy Dark Matter (FDM) in both 1D and
3D settings. Results demonstrate accurate predictions of key metrics such as mass conservation, den-
sity profiles, and structure suppression, validating against known analytical or numerical benchmarks.
This work highlights the potential of PINNs for efficient, possibly scalable modeling of FDM and other
astrophysical systems, overcoming the challenges faced by traditional numerical solvers due to the
non-linearity of the involved equations and the necessity to resolve multi-scale phenomena especially
resolving the fine wave features of FDM on cosmological scales.

Speaker: Ashutosh Kumar Mishra (EPFL)

30 Testing Gravity with cross-correlations of CMB and LSS

The accelerated expansion of the Universe is canonically attributed to the Dark Energy (DE), encapsulated in the Lambda factor in the Einstein field equations of gravity, but its nature is still not understood. While observations supply strong evidence in favor of the standard model of cosmology Lambda-CDM, a plethora of different modified gravity models (MG) can still arise and describe gravity and DE in another way than a Lambda-constant. In our work, we exploit the Effective Field Theory (EFT) framework which allows us to describe gravity and DE in a general way, encompassing single-field models. The strength of this approach is that we can describe not only general features of gravity but also recover model-dependent results through a mapping procedure. Upon this theoretical setting, we combine CMB and galaxy-clustering observables to discriminate between Lambda-CDM and MG/DE models. One of our main aims is to specifically assess the constraining power of cross-correlations between different probes from wide galaxy surveys, like Euclid, and the high sensitivity maps of the microwave sky delivered by Planck.

Speaker: Guglielmo Frittoli (Università Roma Tor Vergata)

31 Tests of gravity: a few phenomenological and theory-specific study cases.

In this work we reflect on the departure from the standard growth of structures induced by two seemingly different, yet comparable extensions to the standard cosmological model: modified gravity and massive neutrinos. Can current Cosmic Microwave Background anisotropy and lensing data tell the difference between a phenomenological modification in the growth of the large-scale structure, introduced at the linear level of the perturbation equations in General Relativity, and the suppression in structure formation caused by non-vanishing neutrino mass?

Speaker: Enrico Specogna (University of Sheffield)

Wednesday 11 June

pre-CMB

32 Introduction to the very early Universe

Speaker: Arttu Rajantie

rajantie_cosmofondue_2025.pdf

33 Time-reversed Stochastic Inflation

Based on ArXiv 2504.17680, B. Blachier, C. Ringeval (2025)

Cosmic inflation may exhibit stochastic periods during which quantum fluctuations dominate over the semi-classical evolution. Extracting observables in these regimes is a notoriously difficult program as quantum randomness makes them fully probabilistic. However, among all the possible quantum histories, the ones which are relevant for Cosmology are conditioned by the requirement that stochastic inflation ended. From an observational point of view, it would be more convenient to model stochastic periods as starting from the time at which they ended and evolving backwards in times. In some cases, it is even compulsory to compute observables with respect to local observers in order to avoid gauge artefacts.

We present a time-reversed approach to stochastic inflation, based on a reverse Fokker-Planck equation, which allows us to derive non-perturbatively the probability distribution of the field values at a given time before the end of the quantum regime. As a motivated example, we solve the flat semi-infinite potential and express the probability distribution of the quantum-generated curvature fluctuations. We show that even when allowing eternal inflation to occur, the reverse-time scheme cure divergences: the probability distribution is finite, exhibits heavy tails and some features of Gaussian statistics are recovered for small curvature perturbations. This reverse-time stochastic $\delta N$-formalism enables a path-by-path conditioning by the lifetimes of the stochastic trajectories, and express probabilities in number of $e$-folds in reference to the end of inflation hypersurface, hence it could be applied to any inflationary potentials and quantum diffusion eras, including the ones leading to the formation of primordial black holes.

Speaker: Baptiste Blachier (UCLouvain (CURL) and LPENS)

SEMINAR_Time_reversed_Stochastic_Inflation.pdf

34 Thermal neutrino interaction rates at NLO and for decoupling temperatures

The dynamics of neutrinos and antineutrinos within a QED plasma, around MeV temperatures (just prior to their decoupling), influences several key cosmological observables. Precision studies have become timely, and we recently computed the NLO interaction rate as a function of the neutrino momentum and flavour, finding relative corrections on the few percent level [1]. I will summarise this calculation, and explain how the same object can be obtained from an integral of more differential rates describing production, annihilation, and scattering [2]. The latter "double-differential" rates can be used to obtain other quantities, like the energy transfer rates which are needed to estimate the parameter $N_{\rm eff}$ in the Standard Model. To conclude, I will comment on how such rate coefficients serve as input for non-equilibrium neutrino kinetic equations.

[1] G. Jackson, M. Laine, JHEP 05 (2024), 089 [arXiv:2312.07015]
[2] G. Jackson, M. Laine [arXiv:2412.03958]

Speaker: Greg Jackson (Subatech (CNRS/IN2P3))

GregJackson_CosmoFondue.pdf

35 Multi-Messenger Signals from Memory Burden: Detectable Small Primordial Black Holes as Dark Matter

Mounting theoretical evidence suggests that the information stored in black holes suppresses their evaporation rate – a quantum effect known as memory burden. This phenomenon opens up a new window for small primordial black holes (PBHs) below $10^{15}\, \text{g}$ as viable dark matter candidates. In this talk, I will discuss observational signals from such small PBHs. Beyond constraints from BBN and CMB, the strongest bounds arise from present-day fluxes of astrophysical particles. Intriguingly, small PBHs that are currently transitioning from semi-classical evaporation to the memory-burdened phase are detectable through high-energetic neutrino events. Finally, I will highlight how inflationary production of small PBHs gives rise to high-frequency gravitational waves that can be observable in the future.

Based on:
G. Dvali, M. Zantedeschi, S. Z., Transitioning to Memory Burden: Detectable Small Primordial Black Holes as Dark Matter, arXiv:2503.21740.
W. Barker, B. Gladwyn, S. Z., Inflationary and Gravitational Wave Signatures of Small Primordial Black Holes as Dark Matter, arXiv:2410.11948, accepted for publication in Phys. Rev. D.

Speaker: Dr Sebastian Zell (Ludwig Maximilian University & Max Planck Institute for Physics, Munich)

CosmoFondue_3.pdf

11:00 Coffee break

GWs emitted before the CMB

36 Introduction to gravitational wave probes of the early Universe

Speaker: Daniel Figueroa

2025_06_10th_to_13th_UNIGE_CosmoFondue_FIGUEROA_Short.pdf

37 Using Deep Learning to disentangle the cosmological and astrophysical Stochastic Gravitational Wave Backgrounds

We apply a Convolutional Neural Network (CNN) to Pulsar Timing Array residuals to identify the cosmological contribution to the Stochastic Gravitational wave Background for a variety of different cosmological models.

We find the CNN can accurately identify the cosmological contributions, and reconstruct injected signals with at least as much success as current Bayesian methods, but with considerably greater model coverage.

We apply this to generate bounds on the amplitudes and spectral indexes for different cosmological models required to disentangle the two contributions.

Speakers: Mr Bill Atkins (Swansea University), Matthew Di Ferrante (Independent Researcher)

ML SGWB.pdf

38 Gravitational waves detection from PTA and Astrometry

Pulsar Timing Array (PTA) observations provide strong evidence for a stochastic gravitational wave background (SGWB), potentially originating from astrophysical sources or early universe phenomena. If the SGWB is cosmological, our relative motion with respect to the SGWB rest frame induces a kinematic anisotropy, which could dominate over intrinsic anisotropies, similar to the cosmic microwave background dipole. We studied PTA sensitivity to this dipole and forecasted its detectability with future experiments like SKA. Additionally, astrometry is a complementary method to PTA observations, and by cross-correlating astrometric and PTA data, constraints on SGWB properties can be improved, aiding in determining its origin.

Speaker: Nayeli Marisol Jimenez Cruz (Swansea University)

PTA-Astrometry_NMJC.pdf

39 Gravitational waves from phase transitions

Gravitational waves (GWs) can be produced by a first-order phase transition in the early Universe via the fluid perturbations induced in the primordial plasma by the expansion and collision of broken-phase bubbles. I will review the production of GWs by the anisotropic stresses of velocity and magnetic fields induced in a first-order phase transition and present analytical estimates and numerical simulations that address the stochastic GW background produced by acoustic motion and magnetohydrodynamic (MHD) turbulence that could be detectable by GW observatories like LISA or PTA, allowing us, for example, to shed light on the nature of the electroweak phase transition and to probe beyond the Standard Model physics. In addition, the presence or production of magnetic fields at a phase transition will yield to highly coupled non-linear fluid perturbations, described by MHD, that will impact the stochastic GW background. The study of these effects can be used to put constraints on primordial magnetic fields that could evolve until present time and be observable in the cosmic voids of the large-scale structure of the Universe, allowing us to study sources of GWs in the early Universe in a multi-messenger approach.

Speaker: Alberto Roper Pol (University of Geneva)

RoperPol_CosmoFondue.pdf

13:00 Lunch break

Neutrino and astroparticle cosmology

40 Introduction to particle cosmology

I will start with a brief review on current neutrino mass bounds, which are inevitably related to the current debate on tensions between observables and departures from the LambdaCDM model. I will mention additional properties of dark matter and dark radiation that will be exciting to test with forthcoming experiments. These models are difficult to integrate into a precise theoretical framework for predicting the non-linear statistics of large scale structure. I will hint at some techniques related to gauge transformations that can ease this problem considerably.

Speaker: Julien Lesgourgues

25.06.11_Cosmofondue.pdf

41 Impact of light sterile neutrinos on cosmic structure formation

Sterile neutrinos with masses on the $\mathrm{eV}$ scale are promising candidates to account for the origin of neutrino mass and the reactor neutrino anomalies. The mixing between sterile and active neutrinos in the early universe could result in a large abundance of relic sterile neutrinos, which depends on not only their physical mass $m_{\rm phy}$ but also their degree of thermalization, characterized by the extra effective number of relativistic degrees of freedom $\Delta N_{\rm eff}$. Using neutrino-involved N-body simulations, we investigate the effects of sterile neutrinos on the matter power spectrum, halo pairwise velocity, and halo mass and velocity functions. We find that the presence of sterile neutrinos suppress the matter power spectrum and halo mass and velocity functions, but enhance the halo pairwise velocity. We also provide fitting formulae to quantify these effects.

Speaker: Rui Hu (Chinese University of Hong Kong)

talk_geneva25.pdf

42 Hot axions and Neff: towards an understanding of hot QCD effects

The axion can address the strong CP problem and also provide a promising dark matter candidate in the form of a condensate of zero-momentum modes. In addition, axion models feature an unavoidable “hot” ensemble of thermally-produced axions acting as dark radiation and thus subject to present and future constraints from the effective number of neutrinos $N_\mathrm{eff}$.
I will concentrate on this latter population and show how the contribution to $N_\mathrm{eff}$ and its uncertainty can be quantified from thermal QCD dynamics above its crossover transition. In more detail, I will show how the leading-order thermal axion production rate has been computed in the past for axion momenta of the order of the temperature and I will show how different schemes for the incorporation of the collective dynamics of soft gluons extrapolate differently into the regime of softer axion production, thus giving us a first quantitative handle on the theory uncertainty of the rate. I will also show how popular gauge-dependent resummations in the thermal gravitino and axion literature are actually plagued by IR divergences. Finally, I will show how, upon solving the Boltzmann equation, the theory uncertainty on the axion rate translates to an uncertainty on $N_\mathrm{eff}$ in models with a direct axion-gluon coupling.
Talk based on 2404.06113

Speaker: Jacopo Ghiglieri (SUBATECH, Nantes)

axion_geneva25.pdf

43 Decaying Dark Matter from CMB to LSS

The two-body Decaying Dark Matter (DDM) model extends the standard cold dark matter paradigm by allowing dark matter particles to decay into a massive daughter particle and a relativistic species. This scenario arises naturally in particle physics and has testable implications for cosmological observables. In this talk, I will introduce the model and present updated constraints on the dark matter lifetime using data from various cosmological surveys, including Planck. Through a profile likelihood analysis, we highlight intriguing features in different datasets and assess the model’s consistency across a wide range of scales.

Speaker: Mr Thomas Montandon (Laboratoire Univers et Particule de Montpellier (LUPM))

DDM_Geneva.pdf

16:00 Coffee break

Discussion: Early Universe

Early_panel.pdf

Late_panel.pdf

Thursday 12 June

Large Scale Structure

44 Introduction to Large-Scale Structure probes

Speaker: Fabian Schmidt

FSchmidt CosmoFondue Geneva June 2025 slides.pdf

45 DESI Part 1: Overview of the DESI survey and clustering measurments from DR1&DR2

The Dark Energy Spectroscopic Instrument (DESI) collaboration is conducting a five-year redshift survey of over 40 million galaxies. By targeting four different galaxy tracers, bright galaxies, luminous red galaxies, emission line galaxies and quasars across a large redshift range 0.1<z<3.5, DESI is designed to measure the expansion history of the Universe using Baryon Acoustic Oscillations (BAO) and the growth of structure using Redshift Space Distortions (RSD). In this talk, I will provide an overview of the DESI instrument, its survey design and present the latest data release DR1/DR2. I will present the recently unblinded Year-2 BAO measurements from galaxy and quasar clustering over 0.1<z<2, which collectively surpass the precision of all pre-DESI BAO results. I will also highlight the systematics mitigation, covariance modeling, and survey validation efforts that underpin these high-precision results. These measurements represent a major milestone toward DESI’s goal of delivering sub-percent constraints on cosmic distances and structure growth and revealing a new trend for evolving dark energy.

Speaker: Antoine Rocher (EPFL)

CosmoFONDUE_2025.pdf

DESI_presentation_Gsponer_RocherCosmoFONDUE_2025.pptx

46 DESI Part 2: Cosmological Implication of DR1&DR2 measurements

The Dark Energy Spectroscopic Instrument (DESI) is conducting a five-year spectroscopic survey of 40 million galaxies and quasars, designed to map the cosmic expansion history and the growth of large-scale structure across the redshift range 0.1<z<3.5. Over the past year, the collaboration has released measurements of the baryon acoustic oscillation (BAO) scale from galaxies, quasars, and the Lyman-alpha forest, along with its first full-shape analyses of the power spectrum multipoles.
In this talk, I will present key cosmological results derived from DESI DR1 and DR2 data, with a focus on emerging indication for an evolving dark energy equation of state when DESI measurements are combined with other cosmological datasets. I will discuss the latest cosmological interpretations of these results, highlighting their implications for dark energy models, possible modifications to general relativity, and persistent tensions among current cosmological datasets.

Speaker: Rafaela Gsponer (EPFL)

47 Applications of Effective Field Theory in cosmology

I will discuss the applications of the Effective Field Theory (EFT) in cosmology.

I will review the main concepts of the EFT of Large-Scale Structure, a theoretical framework that provides a systematic analytic description of cosmological observables on large scales. I will present a general perturbative model for a tracer of matter that depends on the line-of-sight selection effects, and argue that it applies to the Lyman alpha forest. Then, I will formulate the one-loop EFT model for the cross-spectrum of the Lyman-alpha forest and a generic biased tracer of matter. I will demonstrate that including cross-correlations significantly improves constraints on EFT parameters compared to those obtained from individual auto-power spectra.

I will also discuss implications of the EFT of Dark Energy, which provides a systematic description of linear perturbations in general scalar-tensor theories. Focusing on general Horndeski theories, I will present constraints on modified gravity from the CMB, CMB lensing, CMB ISW-lensing, DESI BAO DR1, and SN Ia datasets. I will show that the inclusion of CMB ISW-lensing cross-correlations improves constraints on modified gravity, reducing the viable parameter space by 40-80%.

Speaker: Anton Chudaykin (University of Geneva)

CosmoFONDU_Chudaykin_pdf.pdf

48 Cosmology at small scales: combining probes with the baryonification method

Baryonic feedback processes significantly impact weak-lensing observations at small scales, introducing uncertainties and potential biases in cosmological parameter estimates. These challenges can be mitigated by combining weak-lensing data with complementary observations, such as X-ray and Sunyaev-Zeldovich (SZ) effect measurements. In this talk, I will introduce a new baryonification method that transforms gravity-only N-body simulations into individual density fields for dark matter, stars, and gas, alongside temperature and pressure fields. This versatile approach enables the joint analysis of cosmological observables either through power spectra and summary statistics or via simulation-based inference. By simultaneously constraining cosmological and baryonic feedback parameters, we will stress-test the LCDM model at all scales and learn more about the intergalactic medium on the way.

Speaker: Aurel Schneider (University of Zurich)

Geneva2025.key

Geneva2025pp.pptx

11:05 Coffee break

Dark Energy and Modified Gravity

49 Geometry and Gravity with non-linear clustering

Introduction to Dark Energy and Modified Gravity

A new avenue was recently developed for analyzing large-scale structure data which does not depend on assumptions about the power spectrum shape, the specific background
expansion, or the growth function.
In this talk I discuss how this model-independent methodology can be applied to answer three fundamental questions:
a) is space curve?; b) is gravity Einsteinian?; c) is the equivalence
principle violated?

Speaker: Luca Amendola

50 Thawing gravity and the cosmological tensions

It is found that the DESI observation can be explained by non-minimal coupled gravity. In this talk I will show that the resulting non-minimally coupled scalar-tensor gravity theory, Thawing Gravity (TG), also presents a possible resolution to the cosmological tensions. Using the standard Bayes model comparison method, TG has moderate evidence over $\Lambda$CDM with a Bayes factor $\ln B=+1.5$ in the baseline analysis CMB+BAO+SNIa, and strong evidence $\ln B=+11.8$ if the Cepheids calibration of the SNIa distance ladder from SH0ES is taken into consideration in the baseline+SH0ES analysis, yielding $H_0=71.78\pm0.86 \ {\rm km/s/Mpc}$ and $S_8=0.793\pm0.012$. Interestingly, TG predicts a prerecombination Newtonian constant $G_{\rm CMB}$ larger than that of today $G_N$ at $\sim2\sigma$ in the baseline CMB+BAO+SNIa and $4\sigma$ with baseline+$H_0$. The obtained $G_{\rm CMB}/G_N$ is consistent with current BBN constraint and can be tested by future observations. I will also discuss the possible screening mechanism to make TG compatible with solar system constraints.

Speaker: Gen Ye (Leiden University)

thawing_gravity.pdf

51 Field-level cosmological model selection: field-level simulation-based inference for Stage IV cosmic shear can distinguish dynamical dark energy

We present a framework that for the first time allows Bayesian model comparison to be performed for field-level inference of cosmological models. We achieve this by taking a simulation-based inference (SBI) approach using neural likelihood estimation, which we couple with the learned harmonic mean estimator in order to compute the Bayesian evidence for model comparison. We apply our framework to mock Stage IV cosmic shear observations to assess its effectiveness at distinguishing between various models of dark energy. If the recent DESI results that provided exciting hints of dynamical dark energy were indeed the true underlying model, our analysis shows Stage IV cosmic shear surveys could definitively detect dynamical dark energy. We also perform traditional power spectrum likelihood-based inference for comparison, which we find is not able to distinguish between dark energy models, highlighting the enhanced constraining power for model comparison of our field-level SBI approach.

Speaker: Alessio Spurio Mancini (Royal Holloway, University of London)

Alessio_SpurioMancini_COSMOFONDUE.pdf

52 Om and Statefinder Diagnostics of Anisotropic \( f(R, \mathcal{L}_m) \) Gravity

A locally rotationally symmetric Bianchi type-I model has been analyzed with a perfect fluid within the framework of ( f(R, \mathcal{L}m) ) gravity. The exact field equations were derived, the variable deceleration parameter ( q(t) = \alpha - \frac{\beta}{H} ) has been used here. The cosmological parameters such as energy density, pressure, equation of state, spatial volume, the Hubble parameter, expansion scalar, deceleration parameter, anisotropy parameter, and shear scalar were evaluated. The best-fit curve for ( H(z) ) was determined using 57 observational data points and evaluated through the ( R^2 )-test, achieving an ( R^2 ) value of 0.9321. The best-fit parameters obtained were ( \alpha = 0.542^{+0.019}{-0.022} ), ( \beta = 52.9^{+2.3}{-2.7} ), and ( c_1 = -0.877^{+0.055}{-0.058} ), with a Hubble constant ( H_0 = 64.39^{+0.04}_{-0.47} \, \text{km/s/Mpc} ). Our results show that the model closely matches the (\Lambda)CDM model.
Additionally, we calculated and plotted the evolution of energy density ( \rho(z) ), pressure ( p(z) ), and the equation of state parameter ( \omega(z) ). The results indicate a rapid increase in density at higher redshifts and negative pressure, consistent with dark energy driving the accelerated expansion of the universe. The statefinder diagnostics with ( (r, s) = (1, 0) ) confirm alignment with the (\Lambda)CDM model, while ( r < 1 ) and ( s > 0 ) suggest a quintessence-like behavior. These findings underscore the model's compatibility with current cosmological observations and the need for precise parameter determination to further enhance our understanding of cosmic evolution. The ( \Omega(z) ) plot shows close alignment with the (\Lambda)CDM model at higher redshifts, with rapid expansion changes near ( z = 0 ). As redshift increases, the model stabilizes, reflecting uniform expansion and reduced uncertainties. The evolution of the Strong, Weak, Null, and Dominant Energy Conditions has been discussed.

Speaker: Dnyaneshwar Pawar (School of Mathematical Sciences Swami Ramanand Teerth Marathwada University, Vishnupuri, Nanded, India)

CosmoFondue_PPT_DDP (2).pdf

13:00 Luch break

21cm Intensity Mapping

53 Fundamental physics with HI in emission and absorption

Introduction to 21cm physics

Speaker: Matteo Viel

COSMOFONDUE_VIEL.pdf

54 MeerKLASS: A single-dish HI Intensity Mapping survey

Intensity mapping surveys of neutral hydrogen (HI) are a new way to measure the large-scale matter distribution of our Universe at low spatial resolution over a wide range of redshifts, and thus constrain cosmological parameters such as the Universal expansion. MeerKAT can be used in a “single-dish” mode to access large cosmic scales above 1 degree that are not accessible to the interferometer, while achieving massive survey speeds and depth compared to actual single dish telescopes. Measurements on these ultra-large scales are immensely important to probe deviations from Einstein’s gravity and the Physics of the early Universe. In this talk, I will review the progress of MeerKLASS (MeerKAT Large Area Synoptic Survey), which has pioneered the approach of using an entire array in a fast-scanning multi-dish autocorrelation mode. In this talk, I will give an overview of the progress with MeerKLASS which includes two independent detections of the HI-galay cross power spectrum around z~0.44. I will also give an update on our progress in surveying the Southern Sky at high redshift $0.5

Speaker: Laura Wolz (University of Manchester)

cosmofondue_LwolzJun25.pdf

55 Foreground removal and angular power spectrum estimation of 21cm signal using harmonic space ILC

Mapping the distribution of neutral atomic hydrogen (HI) in the Universe through its 21 cm emission line provides a powerful cosmological probe to map the large-scale structures and shed light on various cosmological phenomena. The Baryon Acoustic Oscillations at low redshifts can potentially be probed by sensitive HI intensity mapping experiments and constrain the properties of dark energy. However, the 21 cm signal detection faces formidable challenges due to the dominance of various astrophysical foregrounds, which can be several orders of magnitude stronger. Our current work introduces a novel and model-independent Internal Linear Combination (ILC) method in harmonic space using the principal components of the 21 cm signal for accurate foreground removal and power spectrum estimation. We estimate the principal components by incorporating prior knowledge of the theoretical 21 cm covariance matrix. We test our methodology by detailed simulations of radio observations, incorporating synchrotron emission, free-free radiation, extragalactic point sources, and thermal noise. We estimate the full sky 21 cm angular power spectrum after application of a mask on the full sky cleaned 21 cm signal by using the mode-mode coupling matrix. These full sky estimates of angular spectra can be directly used to measure the cosmological parameters. For the first time, we demonstrate the effectiveness of a foreground model-independent ILC method in harmonic space to reconstruct the 21 cm signal.

Speaker: Albin Joseph (Arizona State University)

Albin_COSMOFONDUE.pdf

56 Weighing neutrinos with 21cm Intensity Mapping with the SKAO

The large-scale distribution of neutral hydrogen in the late Universe, mapped through the hydrogen 21cm line emission using radio telescopes, holds significant potential to emerge as a key cosmological probe in the coming years. In the work that I’m going to present, we developed a Gaussian likelihood code for the 21cm intensity mapping power spectrum and the 21cm-galaxy clustering cross-correlation power spectrum. We applied our analysis pipeline to forecast the constraining power of future 21cm intensity mapping observations at the SKA Observatory. I will present the latest forecasts of the constraining power of the SKAO on the sum of neutrino masses. We find that adding 21cm intensity mapping power spectrum multipoles data, assuming a fiducial value of the sum of neutrino masses of 0.06 eV, significantly enhances constraints from the Planck 2018 observations of <0.241 eV to <0.105 eV in the most ideal scenario.

Speaker: Gabriele Autieri (SISSA)

Gabriele_Autieri_COSMOFONDUE.key

Gabriele_Autieri_COSMOFONDUE_pdf.pdf

16:00 Coffee break

Discussion: Late Universe

Early_panel.pdf

Late_panel.pdf

20:00 Social dinner

Friday 13 June

Tensions

57 Anomalies and Tensions in Cosmological Data

Speaker: Eleonora Di Valentino

divalentino.pdf

58 Anchors no more: using peculiar velocities to constrain H0 and the primordial Universe without calibrators

We develop a novel approach to constrain the Hubble parameter H0 and the primordial power spectrum amplitude As using supernovae type Ia (SNIa) data. By considering SNIa as tracers of the peculiar velocity field, we can model their distance and their covariance as a function of cosmological parameters without the need of calibrators like Cepheids; this yields a new independent probe of the large-scale structure based on SNIa data without distance anchors. Crucially, we implement a differentiable pipeline in JAX, including efficient emulators and affine sampling, reducing inference time from years to hours on a single GPU. We first validate our method on mock datasets, demonstrating that we can constrain H0 and As within ∼10% using ∼1000 SNIa. We then test our pipeline with SNIa from an N-body simulation, obtaining 7%-level unbiased constraints on H0 with a moderate noise level. We finally apply our method to Pantheon+ data, constraining H0 at the 10% level without Cepheids when fixing As to its Planck value. On the other hand, we obtain 15%-level constraints on As in agreement with Planck when including Cepheids in the analysis. Upcoming observations of low redshift SNIa from the Zwicky Transient Facility and the Vera Rubin Legacy Survey of Space and Time will allow us to fully exploit the potential of our method.

Speaker: Francesco Sorrenti (University of Geneva)

dipole_presentation_cosmofondue-1.pdf

59 Quantifying BAO tension and its implications for late-time solutions to the Hubble crisis

Several studies in the literature have found a discrepancy between Baryon Acoustic Oscillation (BAO) measurements derived from two distinct methodologies, i.e. the two-dimensional (2D, angular) and the three-dimensional (3D, anisotropic) BAO. Since these probes play a key role in building the inverse distance ladder, this inconsistency affects discussions on the Hubble tension and its theoretical solutions. With the aid of type Ia Supernovae (SNIa) and through a largely model-independent approach, we reinterpret this discrepancy in terms of a BAO tension and study the effects of replacing the angular components of the 3D BAO data from BOSS/eBOSS with the recent data from DESI Y1. The tension is found to be at ∼2𝜎 and ∼2.5𝜎, respectively, when the SNIa of the Pantheon+ compilation are used, rising to ∼4.6𝜎 with DESY5. In view of these results, we apply a calibrator-independent method to test the robustness of the distance duality relation, finding no evidence of its violation. Remarkably, we show how 2D and 3D BAO leave an imprint on completely different scales when studying the late-time phenomenology required to solve the Hubble crisis, assuming that standard physics holds before recombination.

Speaker: Arianna Favale

CosmoFondue2025_Favale.pdf

60 Breaking the “distance duality relation” to explain cosmic tensions

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, LUPM/CNRS)

CosmoFONDUE_Teixeira.key

CosmoFONDUE_Teixeira.pdf

11:00 Coffee break

Conclusions and Outlook

Caprini_cosmofundue.pdf

61 Perspectives of future Cosmology

Speakers: Camille Bonvin, Chiara Caprini (CERN and University of Geneva)

Caprini_cosmofundue.pdf

future.pdf

62 Discussion Session

closing.pdf

CERN visit (optional)