19. Kosmologietag

Apr 24, 2025, 12:30 PM → Apr 25, 2025, 1:25 PM Europe/Berlin

Plenarsaal (ZiF | Bielefeld University)

Dietrich Bödeker (Bielefeld University), Isabel Oldengott, Dominik Schwarz (Bielefeld University - Faculty of Physics), Cora Uhlemann (Bielefeld University)

April 24 - 25, 2025 at ZiF | Bielefeld University 

We kindly invite you to participate in the 19th Kosmologietag!
The workshop consists of mainly contributed talks, complemented by two invited 
overview talks and dinner on Thursday night.

You are welcome to submit a title for a contributed talk about any aspect of cosmology, astroparticle physics and gravity including theory, observation and experiment.

Registration is open.

Invited Talks: 
Luisa Lucie-Smith / University of Hamburg
"Interpretable Machine Learning in Cosmology"

Stefan Vogl / University of Freiburg 
"Probing the Nature of Dark Matter"

Image credit: Claire Lamman/DESI collaboration; custom colormap package by cmastro
The key figure used in the press release is from a narrow band on the sky, spanning 190° in right ascension and 14° in declination. The magnified section consists of galaxies from our Bright Galaxy Sample and extends to a redshift of 0.2. This represents less than 0.1% of our full survey volume. 

 

General registration is 50 EUR.

Submission of talks by April 06, 2025

Registration by April 21, 2025

 

Supported by 

   

Poster_19Kosmologietag.pdf

Thu, April 24

12:30 PM → 1:00 PM Registration + Coffee

1:00 PM → 2:00 PM Plenary: Interpretable Machine Learning in Cosmology (Luisa Lucie-Smith)

Lucie-Smith_Kosmologietag April 2025.pdf

2:00 PM → 2:20 PM Session

2:00 PM Development of CLApp: The AI Assistant for CLASS

In the pursuit of enhancing user interaction with the CLASS (Cosmic Linear Anisotropy Solving System) code, we developed CLApp (CLAss AI-Processing apP), an AI-driven assistant designed to support users in navigating the complexities of cosmological Einstein-Boltzmann solvers.

CLApp assists users in locating parameter names, function names, and modifying code effectively. It also facilitates the creation of documentation and the generation of plots, streamlining the workflow for researchers working with CLASS. By employing RAG (Retrieval-Augmented Generation) data, CLApp provides contextually relevant information and guidance, making it easier for users to engage with the code.

Additionally, the assistant utilizes multi-agent techniques to enhance collaborative problem-solving and responsiveness. Tools like Langchain are integrated to facilitate seamless interactions and efficient access to CLASS functionalities.

This presentation will detail the development process of CLApp, the innovative technologies employed, and its impact on making cosmological Einstein-Boltzmann solvers more accessible. By bridging complex computational methods with user-friendly interfaces, CLApp represents a significant advancement in the usability of cosmological modeling tools, fostering a more inclusive research environment.

Speaker: Santiago Casas (RWTH Aachen University, TTK)

CLAPP Agent- Full examples.pdf

CLAPP: CLASS LLM Agent for Pair Programming

CLAPP.pdf

2:05 PM OLÉ - Online Learning Emulation in Cosmology

OLÉ is a newly released emulator for use in cosmological inference. It features automatic accuracy estimation and online learning with on-the-fly training, allowing it to speed up inference for any cosmological model with no pre-training required. It relies on Gaussian Processes and Principal Component Analysis for efficient data compression and fast evaluation, yielding a speed up of up to O(100) showing excellent agreement with traditional theory codes, with the evaluation of the likelihood code becoming the computational bottleneck. It is released with interfaces for the popular cosmological samplers MontePython and Cobaya, the Einstein-Boltzmann solvers CLASS and CAMB, and its own built-in sampler.

Speaker: Markus Mosbech (RWTH Aachen)

OLE presentation Bielefeld 2025-1.pdf

2:10 PM PyUltralight 2 and Beyond: Advancing Ultralight Dark Matter Simulations at the Galactic Scale

Ultralight dark matter (ULDM, also sometimes referred to as Fuzzy DM) is a class of dark matter candidates with masses around $10^{-22}$ eV, leading to wave effects on kiloparsec scales. These wave phenomena offer alternative explanations for small-scale structure and galactic dynamics beyond standard cold dark matter.

In my PhD work, I developed PyUltralight 2 [FWPhys.com/PyUL], a fast and lightweight pseudo-spectral solver for ultralight dark matter (ULDM) dynamics, along with a suite of auxiliary tools to facilitate simulation and analysis. In this talk, I will discuss on several research directions made possible by this codebase, such as an effort to better model ULDM-matter particle interactions (with Russell Boey, University of Auckland) [arxiv:2403.09038].

I will also outline early independent explorations, including a variable-mass black hole accretion model and ongoing numerical refactoring using JAX to prepare for differentiable and large-scale simulations.

Together, these projects aim to expand the modeling toolkit for wave-like dark matter in dynamically rich regimes, while inviting feedback on their direction and implementation.

Speaker: Yourong Frank Wang (University of Göttingen)

Link to Full Talk and Relevant Visualisations

2:15 PM Analysing Intensity Mapping surveys with 2- and 1-point statistics

Galaxies and neutral hydrogen trace the dark matter distribution. To maximize the extraction of cosmological information from these observables we need to take into account that each survey has its systematic errors that need to be taken into account to not bias our cosmological conclusions. In this work we focus on HI Intensity Mapping surveys. We discuss if the Baryonic Acoustic Oscillations (BAO) signal could be recovered modelling an Square Kilometre Array (SKA) Neutral Hydrogen (HI) Intensity Mapping survey, taking into account the telescope beam and foreground removal observational effects. We delelop for this purpose several two-point statistics to bring out the BAO signal. We also analyze how the Counts-in-Cells are affected by the aforementioned systematics.

Speaker: Bernhard Vos Ginés

Kosmologie Tag BVG.pdf

2:20 PM → 2:50 PM Coffee

2:50 PM → 4:30 PM Session

2:50 PM Higher dimensional operators in thermal EFTs impact gravitational wave predictions

Thermal effective theories are an important tool for describing the thermodynamics of cosmological phase transitions. Using the Abelian Higgs model as a toy setup, we examine the impact of marginal, higher-dimensional operators that arise at higher orders in the high-temperature expansion used to construct such theories. We demonstrate consistent matching, using field redefinitions to construct a complete, minimal, and gauge-invariant basis for the leading sextic operators. Marginal operators are found to weaken the transition strength and induce large uncertainties for strong transitions, signalling slow convergence of the high-temperature expansion. For even stronger transitions that could lead to gravitational wave backgrounds strong enough to be detected by LISA, the validity of the high-temperature expansion becomes questionable, which limits the regime of applicability of thermal effective theories, including for non-perturbative lattice studies.

Speaker: Philipp Klose (Nikhef)

klose_Bielefeld.pdf

3:10 PM To log or not to log: NANOGrav bounds on the tension of stable cosmic strings

In the NANOGrav 15-year New Physics analysis (arXiv:2306.16219), a log-uniform prior on $G\mu$ was imposed to determine upper limits on the tension of stable cosmic strings. Here, we examine the prior dependence of this bound through comparison with new upper limits obtained using a uniform prior on $G\mu$. New posterior distributions and upper limits on stable cosmic string tension were calculated using semi-analytic and numerical (MCMC) methods, and show that the 95% bounds are robust against the prior choice, up to variations within a factor of 1.5 or so. We also compare the cosmic string model with a log-uniform prior and the same model with a uniform prior in terms of the associated Bayes factor. At face value, this Bayes factor seems to indicate a preference for a uniform prior choice; however, upon closer inspection, it serves as an illustration of the underlying sensitivity to prior volume effects.

Speaker: Olivia Bitcon (Institute for Theoretical Physics - University of Münster)

Kosmologietag slides.pdf

3:30 PM Gravitational waves from supercooled audible axions

In the audible axion mechanism, axion-like particles source primordial gravitational waves via their coupling to a dark Abelian gauge field. The original setup, however, relies on a large axion decay constant and coupling to produce sizable signals. In this talk, I will demonstrate that delaying the onset of axion oscillations opens up the testable parameter space and reduces the required coupling to $\alpha \sim \mathcal{O}(1)$. Furthermore, I will show that the generation of gravitational waves via the axion coupling to the Standard Model photon becomes possible, even in the presence of Schwinger pair production.

Speaker: Daniel Schmitt

AudibleAxions_Kosmologietag.pdf

3:50 PM Velocity statistics and stream crossing in a particle approach to cosmic structure

In the past few years, a field-theoretical formulation of non-equilibrium kinetic theory has been developed, based on the path-integral formulation for the Hamiltonian dynamics of an ensemble of classical particles. By specializing it to the self-gravitating case on an expanding background, it can be applied to the problem of cosmic large-scale structure formation.

We present the key steps required to compute $n$-point statistics of the cold dark matter distribution in this framework and show results for the momentum statistics and density-momentum cross-correlations. We will discuss how the stream-crossing problem of Eulerian standard perturbation theory is avoided by construction (without introducing effective parameters) and in what way the perturbative scheme differs from more established approaches.

Speaker: Marvin Sipp (Institut für Theoretische Physik, Universität Heidelberg)

4:10 PM Modeling the interlopers using higher-order statistics for Euclid

The Euclid spectroscopic galaxy survey will identify emission-line galaxies (ELGs) via H$\alpha$ emission within the redshift range $0.9 < z < 1.8$. However, measurement precision is contingent upon both the signal-to-noise ratio of line intensity and the spectrograph's energy resolution. Furthermore, potential line misidentification can occur due to genuine emission lines from different redshifts, particularly [S III] and [O III]. These phenomena result in noise and line interlopers, which collectively contaminate the observed galaxy distribution, potentially biasing cosmological parameter inference if not properly addressed.

In this study, we utilize $50$ synthetic spectroscopic catalogs (EuclidLargeMocks) that replicate the expected area and selection function of the forthcoming Euclid observations to conduct a comprehensive analysis of interloper contamination effects on galaxy two-point and three-point correlation functions. A dedicated computational pipeline, BIPOLARS, is developed to measure these statistics and assess the contributions from each of the interloper species. We further explore different models based on the relative impact of these contaminants on clustering statistics, seeking to identify the most efficient modeling approach for respective redshift bins that accurately reproduces the measured data, considering the anticipated redshift uncertainties for Euclid Data Release 1 (DR1), DR2, and DR3.

Our findings indicate that different interlopers exert an unignorable influence in different redshift bins, with a consistent correspondence between the species most affecting the power spectrum and those impacting the bispectrum. Moreover, we determine that contribution from interlopers in modeling the power spectrum is crucial for cosmological analysis at DR1, while for bispectrum analysis, modeling exclusively the correctly identified galaxies proves adequate under the assumption that the simulated catalogs are representative of the future observational data.

Speaker: Asit Dave (University of Bonn)

Bielefeld-Presentation_new.pdf

Bielefeld-Presentation.pdf

4:30 PM → 5:00 PM Coffee

5:00 PM → 6:35 PM Session

5:00 PM Relativistic Extension of Kinetic Field Theory

Understanding observations of the early and late universe requires studying the evolution of large-scale structures (LSS) in analytic models. A lot of research is concerned with cold dark matter (CDM), but the influence of massive/massless neutrinos, and photons is not insignificant. Therefore, in this talk, a generalized version of kinetic field theory (KFT) is presented by going from a classical approach based on Newtonian gravity on an expanding background to a perturbative approach in general relativity without restricting to non-relativistic motion.

This extended theory is based on the motion of massive and massless point particles on an arbitrary spacetime, including special relativistic effects, which was impossible within (KFT) before. To investigate the LSS, a perturbed FLRW spacetime is then considered. Here, it is shown that the extended theory is effective in studying the clustering behavior of ultra-relativistic particles and reduces to classical KFT in the non-relativistic limit. In the non-interacting or free theory, the evolution of power spectra for non- and ultra-relativistic particles are presented, and the asymptotic evolution is discussed. Moreover, the free theory is shown to be invariant under the time-slicing of the underlying spacetime.

A consistent treatment of relativistic motion and general relativity within (KFT) is achieved, which enables future research to extend beyond $\Lambda$CDM.

Speaker: Tim-Leon Klocke (Institute for Theoretical Physics, Heidelberg University)

klocke-rel-ext-kft-flash-talk.pdf

5:05 PM Cosmology with weak lensing higher-order statistics

The higher-order statistics comprise a set of statistical tools that are designed to capture the non-Gaussianities encoded in the cosmic density field. In weak lensing cosmology, they are implemented to unveil complementary information to the two-point statistics, and to calibrate systematic effects such as baryons, photo-z, etc. In this talk, I will introduce weak lensing higher-order statistics, their modeling and the challenge of training accurate emulators for Stage-III and Stage-IV galaxy surveys. My talk will focus on the impact of baryonic feedback on these various statistics, and how to combine them to obtain tighter and unbiased cosmological constraints.

Speaker: Daniela Grandon (Leiden University)

bielefeld.pdf

5:10 PM No scalar-induced GW signal from PBH evaporation

We discuss the scalar-induced-gravitational-wave (SIGW) signal from primordial black hole (PBH) evaporation. In the monochromatic scenario, a significant signal is generated by the poltergeist mechanism as a result of a sudden transition from matter to radiation dominance. Non-trivial mass distributions affect this mechanism by prolonging the transition period. We show that for realistic mass distributions, commonly assumed from superhorizon formation, no signal can be expected in any part of the PBH parameter space in any of the upcoming GW experiments. We pay particular attention to the validity of the linearity of the perturbation theory, the cutoff of the curvature power spectrum, and curvature sources from adiabatic and isocurvature initial conditions.

Speaker: Nicholas Leister (Johannes Gutenberg Universität)

Bielefeld_Leister.pdf

5:15 PM What is neutrino isocurvature?

While searches for neutrino isocurvature usually constrain a specific linear combination of isocurvature perturbations, this talk focusses on realistic cosmological scenarios giving rise to neutrino isocurvature - mostly in the form of dark radiation. In general both, neutrino and dark matter isocurvature perturbations are generated, whose ratio can be parameterised by a newly introduced mixing angle. The talk will discuss first limits on this new mixing angle from PLANCK data, as well as novel insights into the early universe that could be provided by future measurements.

Speaker: Christopher Gerlach (Johannes Gutenberg-Universität Mainz)

Gerlach_Isocurvature_Kosmologietag_2025_V2.pdf

5:35 PM Kinetic Gauge Friction in Natural Inflation

Abstract: In this talk I will present our recent work where we studied an extension of the natural inflation model comprising a non-Abelian gauge sector coupled to the axion-inflaton kinetic term. I will show how such non-minimal coupling serves as a source of friction for the rolling inflaton granting sixty or more $e$-folds of accelerated expansion for sub-Planckian values of the axion decay constant. After that, I will discuss the analysis of perturbations which reveals a negative sound speed squared, thus signaling an instability. Implementing a Chern-Simons-type coupling between the inflaton and gauge sectors cures the instability by delivering a positive speed. Finally, I will delve into the numerical study of scalar and tensor perturbations for a fiducial set of parameters finding that the corresponding observables are compatible with current CMB bounds.

Speaker: Rodrigo Gonzalez Quaglia (University of Groningen)

Bielefeld_RodrigoQuaglia.pdf

5:55 PM Towards Understanding Primordial Black Holes: Insights from Theory and Phenomenology

Primordial black holes (PBHs) are intriguing astrophysical objects that could shed light on the early universe and offer insights into cosmology and gravitational physics. This presentation provides an overview of PBHs, focusing on their abundance and the associated scalar-induced gravitational waves (SIGWs), which are detectable through pulsar timing arrays (PTAs). A key topic will be the methodologies and uncertainties in calculating PBH abundance and SIGWs, highlighting recent advancements and open challenges.

Speaker: Antonio Junior Iovino (New York University Abu Dhabi)

Bielefeld_Iovino.pptx

6:15 PM 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 Ela Rogelj (University of Bern, AEC, ITP)

Bielefeld_slides.pdf

7:30 PM → 9:30 PM Dinner at Brauhaus Joh. Albrecht

Fri, April 25

9:00 AM → 10:00 AM Session

9:00 AM Upper bound on thermal gravitational wave backgrounds from hidden sectors

Hot viscous plasmas unavoidably emit a gravitational wave background, similar to electromagnetic black body radiation.We study the contribution from hidden particles to the diffuse background emitted by the primordial plasma in the early universe.While this contribution can easily dominate over that from Standard Model particles, we find that both are capped by a generic upper bound that makes them difficult to detect with interferometers in the foreseeable future.We illustrate our results for axion-like particles and heavy neutral leptons.Finally, our results suggest that previous works overestimated the gravitational wave background from particle decays out of thermal equilibrium.

Speaker: Juraj Klarić (Univerziteit van Amsterdam, Nikhef, University of Zagreb)

klaric_GWbound_Bielefeld.pdf

9:20 AM ALP production from Abelian Gauge Bosons: Beyond HTL

Previous studies on the production of feebly interacting particles have encountered challenges due to unphysical (negative) interaction rates at soft momenta. We resolve this issue by investigating the thermal production of Axion-Like Particles (ALPs) through freeze-in mechanisms via feeble interactions with U(1) gauge fields, employing the full 1PI-resummed gauge boson propagator. This approach ensures a consistent treatment across all momentum scales, avoiding the need for matching or subtraction techniques.

Our analysis confirms that the ALP production rate remains positive throughout and identifies the dominant production mechanisms. At soft ALP momenta (p≲g^2T), interactions involving two spacelike gauge bosons dominate, surpassing other processes by an order of magnitude. At even softer momenta (p≲g4T), we find that interactions with two timelike bosons become significant and may exceed other contributions by another order of magnitude.
These results refine the predicted thermal ALP abundance and momentum distribution, providing important input for structure formation constraints on keV-mass ALP dark matter. More broadly, our approach offers a systematic and physically consistent framework for addressing infrared effects in feebly interacting particle production.

Speaker: Cristina Puchades Ibáñez (JGU)

ALPs-talk.pdf

9:40 AM Probing Solar Heavy Neutrinos with Heliospheric Electrons

We search for an excess of electrons and positrons in the interplanetary space from the decays of heavy neutrinos produced in nuclear reactions in the Sun. Using measurements of the electron spectra in the MeV range from the Ulysses and SOHO satellites, we report the strongest direct upper bound to date on the mixing between heavy neutral leptons with MeV masses and electron neutrinos, reaching $U^2_e≃10^{−6}$ at $M_N=10 \text{MeV}$. Our sensitivity is predominantly constrained by the uncertainties in the propagation of electrons and positrons, particularly the diffusion coefficient in the inner Solar System, as well as the uncertainties in the astrophysical background. Enhancing our understanding of either of these factors could lead to a significant improvement in sensitivity.

ArXiv : 2412.14752

Speaker: Valentin Weber (UCLouvain - CP3)

Talk_on_HNL (23).pdf

10:00 AM → 10:30 AM Coffee

10:30 AM → 11:45 AM Session

10:30 AM Probing Axion Inflation via Gravitational-Wave Production

Axion inflation is an extension of the slow-roll paradigm featuring helical gauge-field production with possible consequences for inflationary magnetogenesis, leptogenesis, reheating, and gravitational wave production. I focus on the latter, studying the detectability of gauge-field-induced gravitational waves from axion inflation. For this, I consider two models: the first featuring a „sterile“gauge field, which does not couple to any scalars of fermions, and a more realistic model, where the gauge field in question is the standard-model hypercharge field. The coupling to standard model fermions allows for efficient fermion production via the Schwinger mechanism, drawing energy from the gauge fields and thus impacting the gravitational-wave spectrum.

Speaker: Richard von Eckardstein (Institute for Theoretical Physics, University of Münster)

Gravitational_Waves_from_Axion_Inflation__Flash_Talk____Kosmologietag.pdf

10:35 AM From Cosmic Strings to a Running Power Law and Back Again

In this talk, I will present results obtained with a new method to infer approximate Bayesian posterior distributions for spectral models of a stochastic gravitational wave background relevant for pulsar timming arrays. To this end we make use of a simple reference model but go beyond the commonly used power-law ansatz by considering the next logical step: a running-power-law (RPL) with a logarithmic frequency dependence of the spectral index. After performing a Bayesian fit of the RPL to the NANOGrav 15-year data set we are able to refit other spectra to the same data by mapping them onto the RPL. To construct this map we use a new fit-method based on a $\chi^2$-minimization. The results I will show in my talk were obtained by applying this approach to stable and metastable cosmic strings. In order to show which parts of this new approach are essential I will also show the corresponding results obtained by replacing the RPL with the usual (constant) power law and by replacing the $\chi^2$-minimization with a more naive map.

Speaker: David Esmyol (Institute for Theoretical Physics - University of Münster)

Esmyol-From Cosmic Strings to a Running Power Law and Back Again.pdf

10:40 AM Connecting Relativistic MOND Theories with Mimetic Gravity

We find a connection between relativistic Modified Newtonian Dynamics (MOND) theories and (scalar) mimetic gravity. We first demonstrate that any relativistic MOND model featuring a unit-timelike vector field, such as TeVeS or Aether-scalar-tensor theory, can be embedded within a conformal/disformal-invariant framework. Gauge fixing the conformal/disformal symmetry amounts to imposing a constraint on the norm of the vector, the scalar field or the cross contraction. Notably, we find that these constraints can be interchanged as long as the vector and scalar fields remain timelike. This means that relativistic MOND theories may be recasted as a mimetic gravity theory. Lastly, by constructing the fundamental building blocks of a conformal-invariant scalar-vector-tensor theory, we establish a new framework for developing relativistic MOND theories. This perspective offers deeper insight into how non-invertible disformal transformations and conformal/disformal symmetries serve as fundamental principles in constructing viable alternatives to dark matter.

Speaker: Alexander Ganz

Connecting_MOND_Mimetic_Gravity.pdf

10:45 AM Simulating realistic radio continuum survey maps with diffusion models

The next generation pf radio surveys is going to be transformative for cosmology and other parts of astrophysics. Realistic simulations of radio observations are essential for the design and planning of radio surveys.
We implement a software for machine learning-assisted simulation of realistic surveys with the Low-Frequency Array (LOFAR), resulting in a synthetic radio sky model and a corresponding artificial telescope observation. We explore different ways to evaluate our resulting sky
We employ a diffusion model trained on observations from the LOFAR Two-Metre Sky Survey (LoTSS) to generate individual radio galaxy images with control over the angular size. Single sources are assembled into a radio sky model, using an input catalog from cosmological simulations. We then transform this sky model into visibilities corresponding to a typical LoTSS pointing. We add realistic noise to this synthetic measurement and obtain our final simulated sky maps through deconvolution.
We are able to simulate realistic LOFAR observations, covering a sky patch of 5x5 degrees at an effective resolution of 8.5 arcsec. Simulated sources have flux and size distributions that match real observations, and the resulting maps have sensitivities compatible with LoTSS observations. Our diffusion model is able to synthesize high-quality realistic radio galaxy images with precise control over the source sizes. This software can readily be applied to other instruments.

Speaker: Tobias Vičánek Martínez

Bielefeld Kosmologietag.key

Bielefeld Kosmologietag.pptx

11:05 AM Small-Scale Asymptotics of the Free Density Perturbation Bispectrum

We study the asymptotic behaviour of the free, cold-dark matter density fluctuation bispectrum in the limit of small separations. From an initially Gaussian random field, we draw phase-space positions of test particles which then propagate along Zel’dovich trajectories. A suitable expansion of the initial momentum auto-correlations of these particles leads to an asymptotic series whose lower-order power-law exponents we calculate. The dominant contribution has an exponent of −11/2. For triangle configurations with zero surface area, this exponent is even enhanced to −9/2. These power laws can only be revealed by non-perturbative calculation with respect to the initial power spectrum and go against the usually expected exponent of -6. They are valid for general class of initial power spectra with a cut-off function, required to enforce convergence of its moments. We confirm our analytic results numerically. Finally, we use this asymptotic behaviour to investigate the shape dependence of the bispectrum in the small-scale limit, and to show how different shapes grow over cosmic time. These confirm the usual model of gravitational collapse within the Zel’dovich picture.

Speaker: Ricardo Waibel (Institute for Theoretical Physics, Heidelberg University)

talk-bielefeld-25-waibel.pdf

11:25 AM A theoretical approach to density-split clustering

Density-split correlation functions, that probe galaxy clustering in different density environments, are an easily interpretable and promising statistics to extract non-Gaussian information from galaxy clustering, as highlighted for instance in the Beyond-2pt mock challenge. While previous analyses have relied on simulation-based models, here we develop a theoretical approach to model these statistics. Density-split correlation functions can be expressed in terms of the two-point probability density function (PDF) of the density field. We derive analytical predictions using three levels of approximation for the two-point PDF: a bivariate Gaussian distribution, a bivariate shifted log-normal distribution, and a prediction based on the Large Deviation Theory framework. Under spherical symmetry (e.g. for count-in-cell densities, with spherical top-hat smoothing) LDT predicts the density two-point PDF in the large-separation regime relative to the smoothing radius. We validate our model against dark matter N-body simulations in real space, incorporating Poisson shot noise. Our results show that the LDT predictions outperform the log-normal approximation, and agrees with simulations on large scales within the cosmic variance of a typical DESI DR1 sample, despite relying on only one degree of freedom. We additionally study the case of galaxies, by adding a Gaussian Lagrangian bias model with beyond Poisson shot noise, and compare it against N-body simulations populated with ELGs using a HOD model.

Speaker: Mathilde Pinon (CEA/Irfu/DPhP)

bielefeld_cosmoday_25-04-2025.pdf

11:45 AM → 12:15 PM Coffee

12:15 PM → 1:15 PM Plenary: Probing the Nature of Dark Matter (Stefan Vogl)

Lucie-Smith_Kosmologietag April 2025.pdf

1:15 PM → 1:20 PM Farewell

conference_picture.jpg