Swiss Cosmology Days 2025 - ETH Zurich

Jun 5, 2025, 8:00 AM → Jun 6, 2025, 5:00 PM UTC

Siemens Auditorium (HIT E 51) (ETH Zurich)

The ETH Cosmology Research Group is delighted to welcome you to the 2025 Swiss Cosmology Days!

The Swiss Cosmology Days are yearly meetings aimed at promoting communication and exchanges amongst cosmologists working in Switzerland. They offer a national platform for scientists to present their research, lead exciting discussions and enable closer collaborations and networking. Young scientists are particularly encouraged to participate. The first meeting took place in February 2013 at the University of Bern, and further editions took place at ETH Zurich, University of Geneva, EPFL, University of Basel, and at CERN. The 2025 edition will be hosted by the Cosmology Group, ETH Zurich.

Participants have to make their own arrangements for their accommodation (this link may be useful).

Social Dinner (Grill): Join us for the social dinner on Thursday, June 05, at 19:00 at Frau Gerolds Garten, Geroldstrasse 23, 8005 Zürich. Please register and pay in advance by Wednesday, 28.08.2025, using the following link: https://ethzurich.eventsair.com/cosmology/regi/Site/Register.

Thu, June 5

9:00 AM Welcome Coffee

Welcome & Introduction: Alexandre Refregier

LSS Measurements

1 Impact of Line-of-Sight Structure on Galaxy Cluster Weak Lensing Mass and Concentration

The all sky telescope, Euclid, will use weak lensing measurements of the distribution of matter in galaxy clusters to constrain cosmology. However, lensing is sensitive to all mass projected along the line-of-sight, meaning that intervening structures introduce systematic uncertainties. In this talk, I will show how we use the highest-resolution FLAMINGO hydrodynamical simulation to construct Euclid-like mock weak lensing maps, enabling us to quantify the extent of this systematic uncertainty. I will show that the uncertainties related to line-of-sight structure are significant, and if not taken into account can bias inferred properties of clusters and ultimately the estimates of cosmological parameters.

Speaker: Felix Vecchi (EPFL)

2 A Webb View of the COSMOS Field

The 2 square degree COSMOS field has been responsible for a host of exciting scientific discoveries, including some of the first studies of tomographic weak lensing. Now with the next generation James Webb Telescope we are able to see the COSMOS in a whole new light. With unprecedented depth, JWST enables weak lensing galaxy densities of over 120 galaxies per square arc-minute and unparalleled sensitivity. I will present recent weak lensing study of this field including the highest resolution mass map ever made.

Speaker: David Harvey (EPFL)

3 Toward Robust Measurements of Relativistic Effects in Galaxy Clustering with DESI and Euclid

Gravitational redshift and Doppler effects give rise to an antisymmetric component of the galaxy correlation function when cross-correlating two galaxy populations or different tracers. Relativistic effects can be isolated from density and redshift space distortions (RSDs) clustering signals by splitting the galaxy population into two catalogs and using adapted estimators, such as the dipole of the cross-correlation function. Spectroscopic galaxy surveys, specifically the Dark Energy Spectroscopic Instrument (DESI) at low redshift and Euclid at higher redshift, will allow us to extract the signal with high significance across a wide range of scales. In this talk, I will discuss the prospects of measuring this observable in the near future. I will present forecasts for DESI and Euclid based on N-body simulations and discuss the potential challenges in modelling this signal on large and small scales.

Speaker: Francesca Lepori (University of Zurich)

4 Multi-Probe Large-Scale Structure Cosmology with Simulation-Based Inference and Deep Learning

Progress in computing and machine learning has enabled an efficient extraction of information from cosmological fields beyond the Gaussian regime. In our work, we investigate the potential of combining non-Gaussian information from weak lensing and galaxy clustering observations to improve constraints on cosmological parameters. We develop a forward model based on the CosmoGrid simulation suite, allowing us to generate up to 1'000'000 independent simulated survey maps. We combine lensing and clustering statistics for a stage III-like survey and showcase the gain of reaching beyond 2-point statistics.

Speaker: Jozef Bucko (ETH Zurich)

5 The Dark Energy Survey Year 6 3x2pt: Multi-Probe Modeling Strategy and Validation

The Dark Energy Survey (DES) is preparing to release its Legacy cosmological analysis based on six years of observations, comprising shape and position measurements for over 130 million galaxies across 5,000 square degrees. This unprecedented dataset enables a powerful joint study of weak gravitational lensing and galaxy clustering—commonly referred to as the 3×2pt analysis—to robustly constrain the ΛCDM model and its extensions. In this talk, I will present the development of our modeling pipeline, designed to fully exploit the statistical power of the DES Y6 data. I will highlight key components of the theoretical framework, including the modeling of baryonic feedback effects on cosmic shear, the treatment of galaxy bias in galaxy clustering and galaxy-galaxy lensing, and the mitigation of other dominant systematics. Finally, I will show validation results and discuss forecasted cosmological constraints, underscoring the expected impact of the DES Y6 analysis on our understanding of the Universe.

Speaker: David Sanchez Cid (University of Zurich)

6 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)

7 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)

8 Reconstructing the dark energy density in light of DESI BAO observations

In light of the evidence for dynamical dark energy (DE) found from the Dark Energy
Spectroscopic Instrument (DESI) baryon acoustic oscillation (BAO) measurements, we perform
a non-parametric, model-independent reconstruction of the DE density evolution. To do so, we
develop and validate a new framework that reconstructs the DE density through a third-degree
piece-wise polynomial interpolation, allowing for direct constraints on its redshift evolution without
assuming any specific functional form. The strength of our approach resides in the choice of directly
reconstructing the DE density, which provides a more straightforward relation to the distances
measured by BAO than the equation of state parameter. We investigate the constraining power
of cosmic microwave background (CMB) observations combined with supernovae (SNe) and BAO
measurements. In agreement with results from other works, we find a preference for deviations
from ΛCDM, with a significance of 2.4σ when using the Dark Energy Survey Year 5 (DESY5) SNe
data, and 1.3σ with PantheonPlus. The pipeline developed in the work I present is ready to be used with future high-precision data to further investigate the evidence for a non-standard background evolution.

Speaker: Maria Berti

9 Robust cosmology through combined probes

As wide-field surveys yield increasingly precise data, multiprobe analyses offer significant advantages. In this talk, I will discuss our study in which we use a previously developed framework to analyze combinations of three CMB (Planck PR3, Planck PR4, and ACT+WMAP) datasets, DESI Y1 Baryon Acoustic Oscillation (BAO) data, and a 9 × 2pt low-z dataset comprising KiDS-1000, BOSS DR12, and Planck CMB lensing/Integrated Sachs Wolfe (including all cross-correlations). I will discuss the internal consistency of and the hints of possible systematic effects in these data. Then I will show our associated constraints in ΛCDM and, motivated by recent DESI results, dynamical dark energy (w0waCDM) and free neutrino mass extensions. Finally, I will discuss the implications of these results in the context of recent cosmological hints of a dynamical dark energy scenario and (unphysical) negative neutrino masses. Based on https://arxiv.org/abs/2502.01722.

Speaker: Alexander Reeves (ETHZ)

12:15 PM Lunch

H0 Measurement

10 A consensus measurement of the local expansion rate of the Universe

The Hubble constant (H0) is crucial to the understanding of the Universe and its evolution. Hence, one would expect agreement on its value in an era of precision cosmology. However, improvements to direct H0 measurements have resulted in a growing discord with the cosmology-dependent predictions of H0 based on the Cosmic Microwave Background observed by the ESA Planck mission. Importantly, strong agreement between direct H0 measurements susceptible to different systematics has been established. Yet, correlations and interdependencies between these measurements have precluded simple averaging of partially complementary constraints.

Here, I present the Distance Network, jointly developed by 36 experts across all relevant disciplines in the context of a hands-on workshop at the International Space Science Institute (ISSI) in Bern in March 2025. The Distance Network generalizes the idea of the well-known Distance Ladder to jointly consider constraints on H0 provided by different methods in pursuit of the best consensus measurement of the local expansion rate of the Universe. The presentation will describe the philosophy underlying the Distance Network and the process by which scientific consensus was reached. The resulting Consensus Report on the Hubble constant features the most accurate direct H0 measurement today, assessment of systematics by variants, and recommendations for next steps required for further improvements. Implications for the Hubble tension will be highlighted.

Speaker: Richard Anderson (EPFL)

11 Cosmology with Strong Lensing: measuring the Hubble Constant with Gravitationally Lensed Quasars

Time-delay cosmography with lensed quasars is a one-step method for estimating the Hubble constant in the local Universe independently of the cosmic distance ladder. It does not require any intermediate calibration and relies on measuring the time delays between multiple images of strongly lensed quasars, which are inversely proportional to the Hubble constant.

I will present our team's recent progress in obtaining accurate time-delay estimates within a single observing season and explain how these time delays can be used to derive cosmological constraints under different assumptions about the mass distribution of the lens galaxies. I will also discuss how recent and upcoming high-resolution stellar kinematics measurements from Keck and JWST can help break remaining lensing degeneracies, enabling a precise and accurate measurement of the Hubble constant.

Speaker: Martin Millon (ETHZ)

Simulations

12 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 small scales and learn more about the intergalactic medium on the way.

Speaker: Aurel Schneider (University of Zurich)

13 Fast Simulation of Cosmological Neutral Hydrogen based on the Halo Model

Cosmological neutral hydrogen (HI) surveys offer a powerful tomographic
probe of the post-reionization universe to test the standard model of cosmology. Efficiently simulating this signal is essential for optimizing the outcome of upcoming surveys. We present a fast simulation method of the cosmological HI distribution employing the halo model framework. Our approach uses the approximate but fast PINOCCHIO code to generate the past light cone of dark matter halos, which are then populated with HI based on an empirical HI-halo mass relation. Given the large volumes and high mass resolution required for 21 cm intensity mapping, we simulate a past light cone covering declinations between -15° and -35° and frequencies from 700 to 800 MHz, matching HIRAX (the Hydrogen Intensity and Real-time Analysis eXperiment). To achieve this, we simulate a 1 h^{−3}Gpc^3 box with 6700^3 simulation particles, which is replicated to cover the entire lightcone, resolving halos masses down to 4.3 × 10^9 M_⊙ and capturing over 97 % of the expected HI density. From the past light cone, we construct HI maps with 5 MHz bandwidths. We compare key statistical properties, such as the mass function, mass density, power spectrum, and angular power spectrum against an analytical dark matter and HI halo model, which we implemented in PyCosmo, a Python package for theoretical cosmological predictions. Strong agreement is found between the analytical model and the simulation, with discrepancies within a few percent for the one-point statistics and under 10 % for the two-point statistics. Our approach enables efficient forecasting and forward-modeling for dedicated intensity mapping experiments like HIRAX, as well as future cosmological surveys with the SKAO, with strong potential for cross-correlation studies.

Speaker: Pascal Hitz (ETH Zurich)

14 Interpretable Machine Learning for Constraining Self-Interacting Dark Matter in Galaxy Clusters

Cold dark matter, the standard cosmological model, faces several challenges on small scales that self-interacting dark matter (SIDM) may help resolve. Traditional methods to constrain the SIDM cross-section often rely on summary statistics, which discard much of the available information, or require complex and computationally expensive lensing models. Machine learning (ML) has gained traction in astronomy for its ability to extract features from high-dimensional data, but its black box nature raises concerns for scientific inference.
We present an interpretable ML algorithm for constraining the SIDM cross-section from cosmological simulations of galaxy clusters. Our algorithm embeds weak lensing maps into a low-dimensional feature space based on their similarity, allowing simulations to cluster based on their physical differences. This feature space provides a way to assess whether a test dataset, such as observations, lies within the training domain, indicating whether any SIDM constraint is reliable or requires extrapolation.
Our ML algorithm provides accurate parameter recovery alongside a measure of prediction confidence for improved ML trustworthiness.

Speaker: Ethan Tregidga (EPFL)

15 Speeding up galaxy evolution simulations with machine learning

Cosmological simulations are an essential part of understanding the large-scale structure of the universe and testing current cosmological theories. However, they can be computationally expensive. In particular, modelling the chemical evolution and gas cooling that occurs inside galaxies can take up significant amount of the computational budget. Solving the complex, coupled differential equations are typically handled by chemical evolution libraries such as Grackle, which iteratively evolve chemical abundances and temperature. In this talk I will show how we are utilizing advances in machine learning to accelerate these calculations and by learning approximations of complex physical models without explicitly solving the full set of equations we are able to speed up calculations whilst retaining accuracy. Specifically, I will present how we generate our data, the architecture of the model and ultimately its performance. Finally, I will show how this approach offers a method for reducing computational cost in cosmological simulations, ultimately making it more feasible to study a wider range of cosmological theories and to run larger simulations.

Speaker: Elif Ozlem Adiguzel (EPFL)

3:30 PM Coffee Break + Group Photo

Theory (no GW)

16 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: Davide Piras (University of Geneva)

Zurich_900s_presentation.pdf

Zurich_900s_presentation.pptx

17 Beyond Gaussian Likelihoods: The Exact Weak Lensing Correlation Function Likelihood and Its Implications for Stage-IV Parameter Inference

The two-point correlation function is a widely used summary statistic for extracting information from cosmological fields. Even when these fields are perfectly Gaussian, the likelihood of two-point function estimators is inherently non-Gaussian, particularly on the large angular scales that will be probed by upcoming stage-IV weak lensing surveys. Despite this, weak lensing analyses have so far relied on Gaussian likelihood approximations.
As stage-IV surveys push to larger scales with higher precision, these Gaussian approximations will become increasingly inaccurate—an issue already hinted at by indications of non-Gaussianity in correlation functions from stage-III surveys. Revisiting these statistical modeling choices is especially relevant given the ongoing "S8 tension," the discrepancy in clustering amplitudes inferred from high- and low-redshift probes.
We present a framework specifically designed to compute the exact likelihood for correlation functions of spin-0 and spin-2 fields, with the latter being directly relevant for cosmic shear. Our results reveal significant skewness in the likelihood, leading to a systematic shift in its mode toward lower values. Initial tests indicate that this effect can shift the posterior mean of S8 by up to two percent, comparable to the precision of current stage-III surveys.
To efficiently model non-Gaussian likelihoods in high dimensions, we implement a Gaussian copula approach, enabling feasible evaluations beyond the exact low-dimensional likelihood. First results from this method will be presented, along with comparisons to simulated distributions and standard Gaussian likelihoods for noisy weak lensing maps.

Speaker: Veronika Oehl (ETH Zurich)

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))

7:00 PM Social Dinner

Fri, June 6

Theory (no GW)

19 Testing the equivalence principle with the distortion of time

The weak equivalence principle is a fundamental pillar in our understanding of gravity, stating that any test particle falls in a gravitational potential in the same way, regardless of its composition. The validity of this principle for dark matter remains a key open question, which is however rarely addressed in cosmological analyses and often studied under the assumption of specific models. In my talk, I will introduce a model-independent test of this principle based on the cross-correlation of two populations of galaxies. In particular, I will highlight the crucial role played by large-scale relativistic effects, most notably the distortion of time, which will be measurable with upcoming galaxy surveys.

Speaker: Sveva Castello (University of Geneva)

20 Gravitational Redshift from Galaxy Clusters -- a Relativistic Approach

Gravitational redshift is one of Einstein’s first predictions. On cosmological scales, we observe that the light emitted by Bright Central Galaxies at the bottom of a cluster’s gravitational potential is more redshifted than the average member galaxy. A first detection of this signal was made in 2011, but later studies questioned its interpretation, highlighting additional kinematic contributions. In this talk, I will present a fully relativistic derivation of these effects and compare it with previous studies. With a proper theoretical understanding, this effect provides a unique opportunity to test the equivalence principle at Mpc scales.

Speaker: Enea Di Dio (University of Geneva)

21 Probing the Universe with Effective Field Theory

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 present the reanalysis of the DESI full-shape data. I will provide the updated constraints from the DESI FS+BAO analysis for dynamical dark energy model and massive neutrinos. I will discuss the impact of including the bispectrum on parameter constraints and potential implications for future DESI analyses.

Finally, I will 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)

22 Cosmology with the CMB and LSS: from probing gas to testing GR

We present ongoing work on a multi-probe analysis combining X-rays, large-scale structure, and the CMB. This joint analysis will uniquely probe both matter and gas distributions while accounting for baryonic feedback effects. A critical component of our analysis is the inclusion of CMB lensing, constructed using quadratic estimators. In the second part of the talk, we demonstrate how similar quadratic estimator techniques can be used to test General Relativity using large-scale structure observables. We will show forecasts, and an application to state-of-the-art N-body simulations.

Speaker: Omar Darwish (University of Geneva)

10:15 AM Coffee Break

Gravitational Waves (obs/theory)

23 Higher-dimensional operators at finite-temperature affect gravitational-wave predictions

We investigate the effect of higher-dimensional marginal operators on the thermodynamics of cosmological phase transitions. Focusing on the Abelian Higgs model, we systematically match these operators, which arise at higher orders in the underlying high-temperature expansion of thermal effective field theory, and use field redefinitions to construct a complete, minimal, and gauge-invariant operator basis. We argue that for strong transitions, temporal gauge modes, which enhance the transition strength, should be treated on equal footing with spatial gauge modes in perturbation theory. Marginal operators are found to weaken the transition strength and induce significant uncertainties for strong transitions. For even stronger transitions that could potentially produce a gravitational wave background detectable by LISA, the validity of the high temperature expansion is uncertain, which may impact the applicability of effective theory techniques, including their use in non-perturbative lattice studies.

Speaker: Philipp Schicho (University of Geneva)

24 What is the maximum temperature ever reached in the universe?

Gravitational waves are naturally sourced by hydrodynamical fluctuations in a thermal medium, as the one that filled the universe before recombination. Since the corresponding gravitational wave spectrum is expected to show rapid growth at high frequencies, unprecedented prospects to detect these signals may be offered by proposed interferometers. While the spectral shape is well understood, the peak amplitude is set by the plasma temperature at emission. To estimate a model-independent upper bound for the maximal temperature reached after inflation, the late stage of the reheating process may be captured by a two-component approach, in which a self-interacting plasma has already attained local equilibrium, while the inflaton field is still far from equilibrium. We describe the foundations of such an approach, and also discuss the implications for the curvature power spectrum, which can in principle be studied both in a linear and non-linear regime.

Speaker: Simona Procacci (University of Geneva)

25 Probing SUSY at Gravitational Wave Observatories

Under the assumption that the recent pulsar timing array evidence for a stochastic gravitational wave (GW) background at nanohertz frequencies is generated by metastable cosmic strings, we analyze the potential of present and future GW observatories for probing the change of particle degrees of freedom caused, e.g., by a supersymmetric (SUSY) extension of the Standard Model (SM). We find that signs of the characteristic doubling of degrees of freedom predicted by SUSY could be detected at Einstein Telescope and Cosmic Explorer even if the masses of the SUSY partner particles are as high as about 10^4 TeV, far above the reach of any currently envisioned particle collider. We also discuss the detection prospects for the case that some entropy production, e.g. from a late decaying modulus field inducing a temporary matter domination phase in the evolution of the universe, somewhat dilutes the GW spectrum, delaying discovery of the stochastic GW background at LIGO-Virgo-KAGRA. In our analysis we focus on SUSY, but any theory beyond the SM predicting a significant increase of particle degrees of freedom could be probed this way.

Speaker: Kevin Hinze

26 Impact of detector orientation on compact binary coalescence and stochastic gravitational-wave background searches

The next generation of ground-based gravitational wave detectors, such as the Einstein Telescope and Cosmic Explorer, will enable us to probe the cosmos in unprecedented ways. The design, location, and arm orientation of these detectors significantly influence their sensitivity and performance for various gravitational wave sources. For cosmological studies, the localization and distance measurement of compact binary coalescences, as well as the search for stochastic gravitational wave backgrounds are of particular interest. These targets, however, favor different detector configurations. We present a method to optimize the arm orientations within the detector network and demonstrate how to achieve a balanced configuration. Using Bayesian parameter estimation, we explicitly compare the detection, sky localization, and distance estimation capabilities for specific network configurations, focusing on binary neutron star coalescences.

Speaker: Michael Ebersold (University of Zurich)

27 Gravitational wave cosmology with extreme mass-ratio inspirals

Extreme mass-ratio inspirals (EMRIs) are one of the major sources of gravitational waves (GWs) that will be detected by LISA. Similar to the compact binary mergers detected by current GW detectors, EMRIs can be used as cosmic rulers to probe the expansion of the Universe and our current cosmological paradigm, the LambdaCDM model. Interestingly, modified gravity theories can affect the propagation of GWs over cosmological distances, with modifications that can be phenomenologically parametrised and uniquely probed with GW observations.
In this talk, I will discuss how EMRIs can be used as “dark standard sirens” together with redshift information from galaxy catalogues. I will first show what constraints can be placed on the Hubble constant and the dark energy equation of state using EMRIs as dark sirens; then I will focus on the so-called $\Xi_0-n$ parametrisation for general relativity deviations and show what constraints can be placed on it using EMRIs.

Speaker: Danny Laghi (UZH)

28 Model Faint Blue Galaxy Depth Threshold Analysis for a Newly Proposed Sirenas Survey

The field of gravitational-wave cosmology has picked up steam in recent years due to important observations done by LIGO-VIRGO-KAGRA, such as GW150914 and GW170817. However, the absence of a galaxy catalog that is optimized for the purpose of resolving the positions of gravitational-wave sources has been a known issue. This work is part of the newly proposed survey, Sirenas, and aims to create galaxy samples uniformly with similar absolute luminosities to those 20-30 events of interest. To achieve this goal, we utilized a model faint blue galaxy and the luminosity distances to each event to determine our depth thresholds in absolute luminosity space. This work contributed to calculate and optimize the survey exposure times in each region to ensure that we achieved the target depths in all areas while optimizing the distribution of telescope time across all targets.

Speaker: Yavuz Can Gençel

12:15 PM Lunch

New Experiments

29 The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) - Overview and Metrology

The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) radio interferometer array aims to observe neutral hydrogen (HI) through intensity mapping (IM) in the redshift range of 0.775-2.55. It is currently being built at the South African Radio Astronomy Observatory (SARAO) Square Kilometer Array (SKA) site in South Africa. HI IM makes it possible to tomographically probe large, cosmological volumes, enabling constraints on, for example, the dark energy equation of state. In this talk, we will present an overview of the HIRAX instrument and its science goals. Systematics are a significant concern in deriving cosmological constraints from HI IM due to the presence of strong foreground signals, therefore we need to carefully control the systematics and calibration. We will discuss the main dish production and the commissioning of the test array with a focus on the metrology of the dishes.

Speaker: Jennifer Studer (ETH Zurich)

30 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)

31 The future Stage-V Spectroscopic Surveys

In this presentation, I will give a status of the next generation Stage-V spectroscopic surveys in preparation. This include in particular the 6.5m Multiplexed Survey Telescope from Tsinghua, the Spectroscopic Stage-V project from LBNL and the European Widefield Spectroscopic Telescope. I will describe the main science motivation of the different projects, and their timelime. I will also explain the robotic development at EPFL that will enable these massive spectroscopic surveys.

Speaker: Jean-Paul Kneib (EPFL)

2:45 PM Coffee Break

Discussion Panel