Hans Arnold Winther

• Fremstad, Dennis & Winther, Hans Arnold (2025). Emulating the non-linear matter power spectrum in mixed axion dark matter models. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 702. doi: 10.1051/0004-6361/202554621. Full text in Research Archive Show summary

  In order to constrain ultra light dark matter models with current and near future weak lensing surveys, we need the predictions for the non-linear dark matter power spectrum. This is commonly extracted from numerical simulations or from semi-analytical methods. For ultra light dark matter models, such numerical simulations are often very expensive due to the need of having a very high force resolution, often limiting the simulations to very small simulation boxes that do not contain very large scales. In this work, we take a different approach by relying on fast approximate N -body simulations. In these simulations, axion physics is only included in the initial conditions, allowing us to run a large number of simulations with varying axion and cosmological parameters. From our simulation suite, we used machine learning tools to create an emulator for the ratio of the dark matter power spectrum in mixed axion models – models where dark matter is a combination of CDM and axion – to that of ΛCDM. The resulting emulator only needs to be combined with existing emulators for ΛCDM to be able to be used in parameter constraints. We compared the emulator to semi-analytical methods, but a more thorough test to full simulations to verify the true accuracy of this approach is not possible at the present time and is left for future work.

• Hoyland, Sophie; Winther, Hans Arnold; Saadeh, Daniela; Koyama, Kazuya & Izard, Albert (2025). Fast generation of weak lensing maps in modified gravity with COLA. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 541(4), p. 3167–3183. doi: 10.1093/mnras/staf1071. Full text in Research Archive Show summary

  Accurate predictions of weak lensing observables are essential for understanding the large-scale structure of the Universe and probing the nature of gravity. In this work, we present a light-cone implementation to generate maps of the weak lensing convergence field using the COmoving Lagrangian Acceleration (COLA) method. The light-cone is constructed in spherical shells from the source to the observer following an onion representation of the Universe. We validate the COLA-generated convergence maps in general relativity by comparing five statistics to those of maps obtained with publically available high-resolution N-body simulations: the power spectrum, bispectrum, probability distribution function, peak counts, and Minkowski functionals. The convergence power spectrum is accurate to within 5 per cent up to $\ell \sim 500$ and to within 10 per cent up to $\ell \sim 750$, confirming the accuracy of this method on both linear and non-linear scales. For the probability distribution function, peak counts, and Minkowski functionals, we determine the map pixel resolution required for COLA to capture the statistical features of the N-body convergence maps. Our validation tests provide a baseline for the convergence map specifications at which we can trust COLA for each statistic considered. Using these map specifications, we extend our analyses to two representative theories of modified gravity, and demonstrate their imprints on the five convergence statistics considered. This work represents a step towards precise weak lensing predictions under both general relativity and modified gravity with reduced computational cost, providing a robust framework to explore the nature of gravity using field-level inference.

• Fraser, Tristan S.; Paillas, Enrique; Percival, Will J.; Nadathur, Seshadri; Radinovic, Sladana & Winther, Hans Arnold (2025). Modelling the BOSS void-galaxy cross-correlation function using a neural-network emulator. Journal of Cosmology and Astroparticle Physics (JCAP). 2025(06). doi: 10.1088/1475-7516/2025/06/001. Full text in Research Archive Show summary

  We introduce an emulator-based method to model the cross-correlation between cosmological voids and galaxies. This allows us to model the effect of cosmology on void finding and on the shape of the void-galaxy cross-correlation function, improving on previous template-based methods. We train a neural network using the AbacusSummit simulation suite and fit to data from the Sloan Digital Sky Survey Baryon Oscillation Spectroscopic Survey sample. We recover information on the growth of structure through redshift-space distortions (RSD), and the geometry of the Universe through the Alcock-Paczyński (AP) effect, measuring Ω m = 0.330 ± 0.020 and σ 8 = 0.777 +0.047 -0.062 for a ΛCDM cosmology. Comparing to results from a template-based method, we find that fitting the shape of the void-galaxy cross-correlation function provides more information and leads to an improvement in constraining power. In contrast, we find that errors on the AP measurements were previously underestimated if void centres were assumed to have the same response to the AP effect as galaxies — a common simplification. Overall, we recover a 28% reduction in errors for Ω 8 and similar errors on σ 8 with our new method. Given the statistical power of future surveys including DESI and Euclid, we expect the method presented to become the new baseline for the analysis of voids in these data.

• Koyama, K.; Pamuk, S.; Casas, S.; Bose, B.; Carrilho, P. & Sáez-Casares, I. [Show all 277 contributors for this article] (2025). Euclid preparation: LXXI. Simulations and nonlinearities beyond ΛCDM. 3. Constraints on f(R) models from the photometric primary probes. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 698. doi: 10.1051/0004-6361/202452184. Full text in Research Archive Show summary

  We study the constraint on f ( R ) gravity that can be obtained by photometric primary probes of the Euclid mission. Our focus is the dependence of the constraint on the theoretical modelling of the nonlinear matter power spectrum. In the Hu–Sawicki f ( R ) gravity model, we consider four different predictions for the ratio between the power spectrum in f ( R ) and that in Λ cold dark matter (ΛCDM): a fitting formula, the halo model reaction approach, ReACT , and two emulators based on dark matter only N -body simulations, FORGE and e-Mantis . These predictions are added to the MontePython implementation to predict the angular power spectra for weak lensing (WL), photometric galaxy clustering, and their cross-correlation. By running Markov chain Monte Carlo, we compare constraints on parameters and investigate the bias of the recovered f ( R ) parameter if the data are created by a different model. For the pessimistic setting of WL, one-dimensional bias for the f ( R ) parameter, log 10 | f R 0 |, is found to be 0.5 σ when FORGE is used to create the synthetic data with log 10 | f R 0 | = −5.301 and fitted by e-Mantis . The impact of baryonic physics on WL is studied by using a baryonification emulator, BCemu . For the optimistic setting, the f ( R ) parameter and two main baryonic parameters are well constrained despite the degeneracies among these parameters. However, the difference in the nonlinear dark matter prediction can be compensated for the adjustment of baryonic parameters, and the one-dimensional marginalised constraint on log 10 | f R 0 | is biased. This bias can be avoided in the pessimistic setting at the expense of weaker constraints. For the pessimistic setting, using the ΛCDM synthetic data for WL, we obtain the prior-independent upper limit of log 10 | f R 0 | < −5.6. Finally, we implement a method to include theoretical errors to avoid the bias due to inaccuracies in the nonlinear matter power spectrum prediction.

• Rácz, G.; Breton, M.-A.; Fiorini, B.; Brun, A. M. C. Le; Winther, Hans Arnold & Sakr, Z. [Show all 277 contributors for this article] (2025). Euclid preparation: LXIII. Simulations and non-linearities beyond Lambda cold dark matter. 2. Results from non-standard simulations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 695. doi: 10.1051/0004-6361/202452185. Full text in Research Archive Show summary

  The Euclid mission will measure cosmological parameters with unprecedented precision. To distinguish between cosmological models, it is essential to generate realistic mock observables from cosmological simulations that were run in both the standard Λ-cold-dark-matter (ΛCDM) paradigm and in many non-standard models beyond ΛCDM. We present the scientific results from a suite of cosmological N -body simulations using non-standard models including dynamical dark energy, k -essence, interacting dark energy, modified gravity, massive neutrinos, and primordial non-Gaussianities. We investigate how these models affect the large-scale-structure formation and evolution in addition to providing synthetic observables that can be used to test and constrain these models with Euclid data. We developed a custom pipeline based on the Rockstar halo finder and the nbodykit large-scale structure toolkit to analyse the particle output of non-standard simulations and generate mock observables such as halo and void catalogues, mass density fields, and power spectra in a consistent way. We compare these observables with those from the standard ΛCDM model and quantify the deviations. We find that non-standard cosmological models can leave large imprints on the synthetic observables that we have generated. Our results demonstrate that non-standard cosmological N -body simulations provide valuable insights into the physics of dark energy and dark matter, which is essential to maximising the scientific return of Euclid .

• Mellier, Y.; Abdurro’uf, .; Barroso, J.A. Acevedo; Achúcarro, A.; Adamek, J. & Adam, R. [Show all 1,138 contributors for this article] (2025). Euclid I. Overview of the Euclid mission. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202450810. Full text in Research Archive Show summary

  The current standard model of cosmology successfully describes a variety of measurements, but the nature of its main ingredients, dark matter and dark energy, remains unknown. Euclid is a medium-class mission in the Cosmic Vision 2015–2025 programme of the European Space Agency (ESA) that will provide high-resolution optical imaging, as well as near-infrared imaging and spectroscopy, over about 14 000 deg2 of extragalactic sky. In addition to accurate weak lensing and clustering measurements that probe structure formation over half of the age of the Universe, its primary probes for cosmology, these exquisite data will enable a wide range of science. This paper provides a high-level overview of the mission, summarising the survey characteristics, the various data-processing steps, and data products. We also highlight the main science objectives and expected performance.

• Adamek, J.; Fiorini, B.; Baldi, M.; Brando, G.; Breton, M.-A. & Hassani, Farbod [Show all 270 contributors for this article] (2025). Euclid preparation: LXII. Simulations and non-linearities beyond Lambda cold dark matter. 1. Numerical methods and validation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 695. doi: 10.1051/0004-6361/202452180. Full text in Research Archive Show summary

  To constrain cosmological models beyond ACDM, the development of the Euclid analysis pipeline requires simulations that capture the non-linear phenomenology of such models. We present an overview of numerical methods and N-body simulation codes developed to study the non-linear regime of structure formation in alternative dark energy and modified gravity theories. We review a variety of numerical techniques and approximations employed in cosmological N-body simulations to model the complex phenomenology of scenarios beyond ACDM. This includes discussions on solving non-linear field equations, accounting for fifth forces, and implementing screening mechanisms. Furthermore, we conduct a code comparison exercise to assess the reliability and convergence of different simulation codes across a range of models. Our analysis demonstrates a high degree of agreement among the outputs of different simulation codes, typically within 2% for the predicted modification of the matter power spectrum and within 4% for the predicted modification of the halo mass function, although some approximations degrade accuracy a bit further. This provides confidence in current numerical methods of modelling cosmic structure formation beyond ACDM. We highlight recent advances made in simulating the non-linear scales of structure formation, which are essential for leveraging the full scientific potential of the forthcoming observational data from the Euclid mission.

• Zimmermann, Tim; Marsh, D.J.E.; Rogers, K.K.; Winther, Hans Arnold & Shen, Sijing (2025). Interference in fuzzy dark matter filaments: Idealized models and statistics. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 696. doi: 10.1051/0004-6361/202554232. Full text in Research Archive Show summary

  Fuzzy (wave) dark matter (FDM), the dynamical model underlying an ultralight bosonic dark matter species, produces a rich set of nongravitational signatures that distinguishes it markedly from the phenomenologically related warm (particle) dark matter (WDM) scenario. The emergence of extended interference fringes hosted by cosmic filaments is one such phenomenon reported by cosmological simulations, and a detailed understanding of such may strengthen existing limits on the boson mass but also break the degeneracy with WDM, and provide a unique fingerprint of interference in cosmology. In this paper, we provide initial steps toward this goal. In particular, we show in a bottom-up approach, how the presence of interference in an idealized filament population can lead to a non-suppressive feature in the matter power spectrum – an observation supported by cosmological FDM simulations. To this end, we build on a theoretically motivated and numerically observed steady-state approximation for filaments and express the equilibrium dynamics of such in an expansion of FDM eigenstates. We optimize the size of the expansion by incorporating classical phase-space information. Ellipsoidal collapse considerations were used to construct a fuzzy filament mass function which, together with the reconstructed FDM wave function, allowed us to efficiently compute the one-filament power spectrum. We showcase our non-perturbative interference model for a selection of boson masses and confirm our approach is able to produce the matter power boost observed in fully cosmological FDM simulations. More precisely, we find an excess in correlation between the spatial scale associated with the FDM ground state and the quantum pressure scale. We speculate about applications of this effect in data analysis.

• Mauland-Hus, Renate; Winther, Hans Arnold & Ruan, Cheng-Zong (2024). Sesame: A power spectrum emulator pipeline for beyond-ΛCDM models. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 685. doi: 10.1051/0004-6361/202347892. Full text in Research Archive Show summary

  The mysterious nature of the dark sector of the Λ-cold-dark-matter (ΛCDM) model is one of the main motivators behind the study of alternative cosmological models. A central quantity of interest for these models is the matter power spectrum, which quantifies structure formation on various scales and can be cross-validated through theory, simulations, and observations. Here, we present a tool that can be used to create emulators for the non-linear matter power spectrum, and similar global clustering statistics, for models beyond ΛCDM with very little computation effort and without the need for supercomputers. We use fast approximate N-body simulations to emulate the boost, B(k, z) = Pbeyond − ΛCDM(k, z)/PΛCDM(k, z), and then rely on existing high-quality emulators made for ΛCDM to isolate Pbeyond − ΛCDM(k, z). Since both the ΛCDM and beyond-ΛCDM models are simulated in our approach, some of the lack of power on small scales due to the low force-resolution in the simulations is factored out, allowing us to extend the emulator to k ∼ 3 − 5 h Mpc−1 and still maintain good accuracy. In addition, errors from the simulation and emulation process can easily be estimated and factored into the covariance when using the emulator on data. As an example of using the pipeline, we create an emulator for the well-studied f(R) model with massive neutrinos, using approximately 3000 CPU hours of computation time. Provided with the paper is a fully functioning pipeline that generates parameter samples, runs a Boltzmann solver to produce initial conditions, runs the simulations, and then gathers all the data and runs it through a machine learning module to develop the emulator. This tool, named Sesame, can be used by anyone to generate a power spectrum emulator for the cosmological model of their choice.

• Radinovic, Sladana; Winther, Hans Arnold; Nadathur, Seshadri; Percival, Will J.; Paillas, Enrique & Fraser, Tristan Sohrab [Show all 8 contributors for this article] (2024). Alcock–Paczyński effect on void-finding: Implications for void-galaxy cross-correlation modelling. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 691. doi: 10.1051/0004-6361/202451358. Full text in Research Archive Show summary

  Under the assumption of statistical isotropy, and in the absence of directional selection effects, a stack of voids is expected to be spherically symmetric, which makes it an excellent object to use for an Alcock–Paczyński (AP) test. This test is commonly carried out using the void-galaxy cross-correlation function (CCF), which has emerged as a competitive probe, especially in combination with the galaxy-galaxy auto-correlation function. Current studies of the AP effect around voids assume that void-centre positions are influenced by the choice of fiducial cosmology in the same way as galaxy positions. We show that this assumption, though prevalent in the literature, is complicated by the response of void-finding algorithms to shifts in tracer positions. Using stretched simulation boxes to emulate the AP effect, we investigate how the void-galaxy CCF changes due to its presence, revealing an additional effect imprinted in the CCF that must be accounted for. The effect originates from the response of void finders to the distorted tracer field – which leads to reduction of the amplitude of the AP signal in the CCF – and thus depends on the specific void-finding algorithm used. We present results for four different void-finding packages, namely REVOLVER, VIDE, voxel, and the spherical void finder in the Pylians3 library, demonstrating how incorrect treatment of the AP effect results in biases in the recovered parameters, regardless of the technique used. Finally, we propose a method to alleviate this issue without resorting to complex and finder-specific modelling of the void-finder response to AP.

• Sletmoen, Herman & Winther, Hans Arnold (2024). A simple prediction of the nonlinear matter power spectrum in Brans-Dicke gravity from linear theory. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 689. doi: 10.1051/0004-6361/202450050. Full text in Research Archive Show summary

  Brans–Dicke (BD), one of the first proposed scalar-tensor theories of gravity, effectively makes the gravitational constant of general relativity (GR) time-dependent. Constraints on the BD parameter ω serve as a benchmark for testing GR, which is recovered in the limit ω → ∞. Current small-scale astrophysical constraints ω ≳ 105 are much tighter than large-scale cosmological constraints ω ≳ 103, but the two decouple if the true theory of gravity features screening. On the largest cosmological scales, BD approximates the most general second-order scalar–tensor (Horndeski) theory, so constraints here have wider implications. These constraints will improve with upcoming large-scale structure and cosmic microwave background surveys. To constrain BD with weak gravitational lensing, one needs its nonlinear matter power spectrum PBD. By comparing the boost B = PBD/PGR from linear theory and nonlinear N-body simulations, we show that the nonlinear boost can simply be predicted from linear theory if the BD and GR universes are parameterized in a way that makes their early cosmological evolution and quasilinear power today similar. In particular, they need the same H0/√Geff(a = 0) and σ8, where Geff is the (effective) gravitational strength. Our prediction is 1% accurate for ω ≥ 100, z ≤ 3, and k ≤ 1 h/Mpc; and 2% up to k ≤ 5 h/Mpc. It also holds for GBD that do not match Newton’s constant today, so one can study GR with different gravitational constants GGR by sending ω → ∞. We provide a code that computes B with the linear Einstein-Boltzmann solver HI_CLASS and multiplies it by the nonlinear PGR from EUCLIDEMULATOR2 to predict PBD.

• Gordon, Jonathan; Aguiar, Bernardo F. de; Rebouças, João; Brando, Guilherme; Falciano, Felipe & Miranda, Vivian [Show all 8 contributors for this article] (2024). Modeling nonlinear scales with the comoving Lagrangian acceleration method: Preparing for LSST Y1. Physical Review D. ISSN 2470-0010. 110(8). doi: 10.1103/PhysRevD.110.083529. Full text in Research Archive
• Frusciante, N.; Pace, F.; Cardone, V.F.; Casas, S.; Tutusaus, I. & Ballardini, M. [Show all 130 contributors for this article] (2024). Euclid: Constraining linearly scale-independent modifications of gravity with the spectroscopic and photometric primary probes. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 690. doi: 10.1051/0004-6361/202347526. Full text in Research Archive Show summary

  Context. The future Euclid space satellite mission will offer an invaluable opportunity to constrain modifications to Einstein’s general relativity at cosmic scales. In this paper, we focus on modified gravity models characterised, at linear scales, by a scale-independent growth of perturbations while featuring different testable types of derivative screening mechanisms at smaller non-linear scales. Aims. We considered three specific models, namely Jordan-Brans-Dicke, a scalar-tensor theory with a flat potential, the normal branch of Dvali-Gabadadze-Porrati (nDGP) gravity, a braneworld model in which our Universe is a four-dimensional brane embedded in a five-dimensional Minkowski space-time, and k-mouflage gravity, an extension of k-essence scenarios with a universal coupling of the scalar field to matter. In preparation for real data, we provide forecasts from spectroscopic and photometric primary probes by Euclid on the cosmological parameters and the additional parameters of the models, respectively, ωBD, Ωгc and ϵ2,0, which quantify the deviations from general relativity. This analysis will improve our knowledge of the cosmology of these modified gravity models. Methods. The forecast analysis employs the Fisher matrix method applied to weak lensing (WL); photometric galaxy clustering (GCph), spectroscopic galaxy clustering (GCsp) and the cross-correlation (XC) between GCph and WL. For the Euclid survey specifications, we define three scenarios that are characterised by different cuts in the maximum multipole and wave number, to assess the constraining power of non-linear scales. For each model we considered two fiducial values for the corresponding model parameter. Results. In an optimistic setting at 68.3% confidence interval, we find the following percentage relative errors with Euclid alone: for log10 ωBD, with a fiducial value of ωBD = 800, 27.1% using GCsp alone, 3.6% using GCph+WL+XC and 3.2% using GCph+WL+XC+GCsp; for log10 Ωгc, with a fiducial value of Ωгc = 0.25, we find 93.4, 20 and 15% respectively; and finally, for ϵ2,0 = −0.04, we find 3.4%, 0.15%, and 0.14%. From the relative errors for fiducial values closer to their ΛCDM limits, we find that most of the constraining power is lost. Our results highlight the importance of the constraining power from non-linear scales.

• Ballardini, M.; Akrami, Y.; Finelli, F.; Karagiannis, D.; Li, Baojiu & Li, Y. [Show all 129 contributors for this article] (2024). Euclid: The search for primordial features. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 683. doi: 10.1051/0004-6361/202348162. Full text in Research Archive
• Mauland-Hus, Renate; Elgarøy, Øystein; Mota, David & Winther, Hans Arnold (2023). The void-galaxy cross-correlation function with massive neutrinos and modified gravity. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 674. doi: 10.1051/0004-6361/202346287. Full text in Research Archive Show summary

  Massive neutrinos and f ( R )-modified gravity have degenerate observational signatures that can impact the interpretation of results in galaxy survey experiments, such as cosmological parameter estimations and gravity model tests. Because of this, it is important to investigate astrophysical observables that can break these degeneracies. Cosmic voids are sensitive to both massive neutrinos and modifications of gravity and provide a promising ground for disentangling the above-mentioned degeneracies. In order to analyse cosmic voids in the context of non-ΛCDM cosmologies, we must first understand how well the current theoretical framework operates in these settings. We performed a suite of simulations with the RAMSES -based N -body code ANUBISIS , including massive neutrinos and f ( R )-modified gravity both individually and simultaneously. The data from the simulations were compared to models of the void velocity profile and the void-halo cross-correlation function (CCF). This was done both with the real space simulation data as model input and by applying a reconstruction method to the redshift space data. In addition, we ran Markov chain Monte Carlo (MCMC) fits on the data sets to assess the capability of the models to reproduce the fiducial simulation values of fσ 8 ( z ) and the Alcock-Paczyǹski parameter, ϵ . The void modelling applied performs similarly for all simulated cosmologies, indicating that more accurate models and higher resolution simulations are needed in order to directly observe the effects of massive neutrinos and f ( R )-modified gravity through studies of the void-galaxy CCF. The MCMC fits show that the choice for the void definition plays an important role in the recovery of the correct cosmological parameters, but otherwise, there is no clear distinction between the ability to reproduce fσ 8 and ϵ for the various simulations.

• Woodfinden, Alex; Percival, Will J.; Nadathur, Seshadri; Winther, Hans Arnold; Fraser, T.S. & Massara, Elena [Show all 8 contributors for this article] (2023). Cosmological measurements from void-galaxy and galaxy-galaxy clustering in the Sloan Digital Sky Survey. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 523(4), p. 6360–6370. doi: 10.1093/mnras/stad1725. Full text in Research Archive
• Brando, Guilherme; Koyama, Kazuya & Winther, Hans Arnold (2023). Revisiting Vainshtein screening for fast N-body simulations. Journal of Cosmology and Astroparticle Physics (JCAP). 2023(6). doi: 10.1088/1475-7516/2023/06/045. Full text in Research Archive Show summary

  We revisit a method to incorporate the Vainshtein screening mechanism in N-body simulations proposed by R. Scoccimarro in [1]. We further extend this method to cover a subset of Horndeski theories that evade the bound on the speed of gravitational waves set by the binary neutron star merger GW170817. The procedure consists of the computation of an effective gravitational coupling that is time and scale dependent, Geff (k,z), where the scale dependence will incorporate the screening of the fifth-force. This is a fast procedure that when contrasted to the alternative of solving the full equation of motion for the scalar field inside N-body codes, reduces considerably the computational time and complexity required to run simulations. To test the validity of this approach in the non-linear regime, we have implemented it in a COmoving Lagrangian Approximation (COLA) N-body code, and ran simulations for two gravity models that have full N-body simulation outputs available in the literature, nDGP and Cubic Galileon. We validate the combination of the COLA method with this implementation of the Vainshtein mechanism with full N-body simulations for predicting the boost function: the ratio between the modified gravity non-linear matter power spectrum and its General Relativity counterpart. This quantity is of great importance for building emulators in beyond-ΛCDM models, and we find that the method described in this work has an agreement of below 2% for scales down to k ≈ 3h/Mpc with respect to full N-body simulations.

• Adamek, J.; Angulo, R. E.; Arnold, C.; Baldi, Marco; Biagetti, M. & Bose, B. [Show all 133 contributors for this article] (2023). Euclid: modelling massive neutrinos in cosmology - a code comparison. Journal of Cosmology and Astroparticle Physics (JCAP). 2023(6). doi: 10.1088/1475-7516/2023/06/035. Full text in Research Archive Show summary

  The measurement of the absolute neutrino mass scale from cosmological large-scale clustering data is one of the key science goals of the Euclid mission. Such a measurement relies on precise modelling of the impact of neutrinos on structure formation, which can be studied with N-body simulations. Here we present the results from a major code comparison effort to establish the maturity and reliability of numerical methods for treating massive neutrinos. The comparison includes eleven full N-body implementations (not all of them independent), two N-body schemes with approximate time integration, and four additional codes that directly predict or emulate the matter power spectrum. Using a common set of initial data we quantify the relative agreement on the nonlinear power spectrum of cold dark matter and baryons and, for the N-body codes, also the relative agreement on the bispectrum, halo mass function, and halo bias. We find that the different numerical implementations produce fully consistent results. We can therefore be confident that we can model the impact of massive neutrinos at the sub-percent level in the most common summary statistics. We also provide a code validation pipeline for future reference.

• Radinovic, Sladana; Nadathur, S.; Winther, Hans Arnold; Percival, W.J.; Woodfinden, A. & Massara, E. [Show all 121 contributors for this article] (2023). Euclid: Cosmology forecasts from the void-galaxy cross-correlation function with reconstruction. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 677. doi: 10.1051/0004-6361/202346121. Full text in Research Archive Show summary

  We have investigated the cosmological constraints that can be expected from measurement of the cross-correlation of galaxies with cosmic voids identified in the Euclid spectroscopic survey, which will include spectroscopic information for tens of millions of galaxies over 15 000 deg 2 of the sky in the redshift range 0.9 ≤  z  &lt; 1.8. We have done this using simulated measurements obtained from the Flagship mock catalogue, the official Euclid mock that closely matches the expected properties of the spectroscopic dataset. To mitigate anisotropic selection-bias effects, we have used a velocity field reconstruction method to remove large-scale redshift-space distortions from the galaxy field before void-finding. This allowed us to accurately model contributions to the observed anisotropy of the cross-correlation function arising from galaxy velocities around voids as well as from the Alcock–Paczynski effect, and we studied the dependence of constraints on the efficiency of reconstruction. We find that Euclid voids will be able to constrain the ratio of the transverse comoving distance D M and Hubble distance D H to a relative precision of about 0.3%, and the growth rate fσ 8 to a precision of between 5% and 8% in each of the four redshift bins covering the full redshift range. In the standard cosmological model, this translates to a statistical uncertainty ΔΩ m  = ±0.0028 on the matter density parameter from voids, which is better than what can be achieved from either Euclid galaxy clustering and weak lensing individually. We also find that voids alone can measure the dark energy equation of state to a 6% precision.

• Brando, Guilherme; Fiorini, Bartolomeo; Koyama, Kazuya & Winther, Hans Arnold (2022). Enabling matter power spectrum emulation in beyond-ΛCDM cosmologies with COLA. Journal of Cosmology and Astroparticle Physics (JCAP). 2022(9). doi: 10.1088/1475-7516/2022/09/051. Full text in Research Archive
• Hartman, Stian Thomas Heimdal; Winther, Hans Arnold & Mota, David (2022). Cosmological simulations of self-interacting Bose-Einstein condensate dark matter. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 666. doi: 10.1051/0004-6361/202243496. Full text in Research Archive Show summary

  Fully 3D cosmological simulations of scalar field dark matter with self-interactions, also known as Bose-Einstein condensate dark matter, are performed using a set of effective hydrodynamic equations. These are derived from the non-linear Schrödinger equation by performing a smoothing operation over scales larger than the de Broglie wavelength, but smaller than the self-interaction Jeans’ length. The dynamics on the de Broglie scale become an effective thermal energy in the hydrodynamic approximation, which is assumed to be subdominant in the initial conditions, but become important as structures collapse and the fluid is shock-heated. The halos that form have Navarro-Frenk-White envelopes, while the centers are cored due to the fluid pressures (thermal + self-interaction), confirming the features found by Dawoodbhoy et al. (2021, MNRAS, 506, 2418) using 1D simulations under the assumption of spherical symmetry. The core radii are largely determined by the self-interaction Jeans’ length, even though the effective thermal energy eventually dominates over the self-interaction energy everywhere, a result that is insensitive to the initial ratio of thermal energy to interaction energy, provided it is sufficiently small to not affect the linear and weakly non-linear regimes. Scaling relations for the simulated population of halos are compared to Milky Way dwarf spheroidals and nearby galaxies, assuming a Burkert halo profile, and are found to not match, although they conform better with observations compared to fuzzy dark matter-only simulations.

• Joudaki, Shahab; Ferreira, Pedro G.; Lima, Nelson A. & Winther, Hans Arnold (2022). Testing gravity on cosmic scales: A case study of Jordan-Brans-Dicke theory. Physical Review D. ISSN 2470-0010. 105(4). doi: 10.1103/PhysRevD.105.043522. Full text in Research Archive Show summary

  We provide an end-to-end exploration of a distinct modified gravitational theory in Jordan-Brans-Dicke (JBD) gravity, from an analytical and numerical description of the background expansion and linear perturbations, to the nonlinear regime captured with a hybrid suite of N-body simulations, to the cosmological constraints from existing probes of the expansion history, the large-scale structure, and the cosmic microwave background (CMB). We have focused on JBD gravity as it both approximates a wider class of Horndeski scalar-tensor theories on cosmological scales and allows us to adequately model the nonlinear corrections to the matter power spectrum. In a combined analysis of the Planck 2018 CMB temperature, polarization, and lensing reconstruction, together with Pantheon supernova distances and the Baryon Oscillation Spectroscopic Survey (BOSS) measurements of baryon acoustic oscillation distances, the Alcock-Paczynski effect, and the growth rate, we constrain the JBD coupling constant to ωBD>970 (95% confidence level; C.L.) in agreement with the General Relativistic expectation given by ωBD→∞. In the unrestricted JBD model, where the effective gravitational constant at present, Gmatter/G, is additionally varied, increased dataset concordance (e.g., within 1σ agreement in S8=σ8√Ωm/0.3) enables us to further include the combined (“3×2pt”) dataset of cosmic shear, galaxy-galaxy lensing, and overlapping redshift-space galaxy clustering from the Kilo Degree Survey and the 2-degree Field Lensing Survey (KiDS×2dFLenS). In analyzing the weak lensing measurements, the nonlinear corrections due to baryons, massive neutrinos, and modified gravity are simultaneously modeled and propagated in the cosmological analysis for the first time. In the joint analysis of all datasets, we constrain ωBD>1540 (95% C.L.), Gmatter/G=0.997±0.029, the sum of neutrino masses, ∑mν<0.12  eV (95% C.L.), and the baryonic feedback amplitude, B<2.8 (95% CL), all in agreement with the standard model expectation. In fixing the sum of neutrino masses, the lower bound on the coupling constant strengthens to ωBD>1460 and ωBD>2230 (both at 95% C.L.) in the restricted and unrestricted JBD models, respectively. We explore the impact of the JBD modeling choices, and show that a more restrictive parametrization of the coupling constant degrades the neutrino mass bound by up to a factor of three. In addition to the improved concordance between KiDS×2dFLenS and Planck, the tension in the Hubble constant between Planck and the direct measurement of Riess et al. (2019) is reduced to ∼3σ; however, we find no substantial model selection preference for JBD gravity relative to ΛCDM. We further show that a positive shift in the effective gravitational constant suppresses the CMB damping tail, which might complicate future inferences of small-scale physics, given its degeneracy with the primordial helium abundance, the effective number of neutrinos, and the running of the spectral index.

• Woodfinden, Alex; Nadathur, Seshadri; Percival, Will J; Radinovic, Sladana; Massara, Elena & Winther, Hans Arnold (2022). Measurements of cosmic expansion and growth rate of structure from voids in the Sloan Digital Sky Survey between redshift 0.07 and 1.0. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 516(3), p. 4307–4323. doi: 10.1093/mnras/stac2475. Full text in Research Archive Show summary

  We present measurements of the anisotropic cross-correlation of galaxies and cosmic voids in data from the Sloan Digital Sky Survey (SDSS) Main Galaxy Sample, Baryon Oscillation Spectroscopic Survey (BOSS), and extended BOSS luminous red galaxy catalogues from SDSS Data Releases 7, 12, and 16, covering the redshift range 0.07 < z < 1.0. As in our previous work analysing voids in subsets of these data, we use a reconstruction method applied to the galaxy data before void finding in order to remove selection biases when constructing the void samples. We report results of a joint fit to the multipole moments of the measured cross-correlation for the growth rate of structure, fσ8(z), and the ratio DM(z)/DH(z) of the comoving angular diameter distance to the Hubble distance, in six redshift bins. For DM/DH, we are able to achieve a significantly higher precision than that obtained from analyses of the baryon acoustic oscillations and galaxy clustering in the same data sets. Our growth rate measurements are of lower precision but still comparable with galaxy clustering results. For both quantities, the results agree well with the expectations for a Λ cold dark matter model. Assuming a flat Universe, our results correspond to a measurement of the matter density parameter Ωm=0.337+0.026−0.029⁠. For more general models, the degeneracy directions obtained are consistent with and complementary to those from other cosmological probes. These results consolidate void–galaxy cross-correlation measurements as a pillar of modern observational cosmology.

• Lyall, Stuart; Blake, Chris; Turner, Ryan; Ruggeri, Rossana & Winther, Hans Arnold (2022). Testing modified gravity scenarios with direct peculiar velocities. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 518(4), p. 5929–5941. doi: 10.1093/mnras/stac3323. Full text in Research Archive Show summary

  The theoretical basis of dark energy remains unknown and could signify a need to modify the laws of gravity on cosmological scales. In this study, we investigate how the clustering and motions of galaxies can be used as probes of modified gravity theories, using galaxy and direct peculiar velocity auto- and cross-correlation functions. We measure and fit these correlation functions in simulations of ΛCDM, DGP, and f(R) cosmologies and, by extracting the characteristic parameters of each model, we show that these theories can be distinguished from General Relativity (GR) using these measurements. We present forecasts showing that with sufficiently large data samples, this analysis technique is a competitive probe that can help place limits on allowed deviations from GR. For example, a peculiar velocity survey reaching to z = 0.5 with 20 per cent distance accuracy would constrain model parameters to 3-σ confidence limits log10|fR0| < −6.45 for f(R) gravity and rc>2.88c/H0 for nDGP, assuming a fiducial GR model.

• Mina, Mattia; Mota, David & Winther, Hans Arnold (2022). Solitons in the dark: First approach to non-linear structure formation with fuzzy dark matter. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 662. doi: 10.1051/0004-6361/202038876. Full text in Research Archive Show summary

  We present the results of a full cosmological simulation with the new code SCALAR , where dark matter is in the form of fuzzy dark matter (FDM), described by a light scalar field with a mass of m B  = 2.5 × 10 −22 eV and evolving according to the Schrödinger-Poisson system of equations. In comoving units, the simulation volume is 2.5  h −1 Mpc on a side, with a resolution of 20  h −1 pc at the highest refinement level. While the resulting large-scale resolution prevents us from studying the general properties of the FDM structure formation, the extremely high small-scale resolution allows a detailed analysis of the formation and evolution of central solitonic cores, which are found to leave their imprints on dark matter density profiles, resulting in shallower central densities, and on rotation curves, producing an additional circular velocity peak at small radii from the centre. Despite the limitations on the large-scale resolution, we find that the suppression of structures due to the quantum nature of the scalar field reveals indications of a shallower halo mass function in the low-mass end compared to the case of a ΛCDM simulation, in which dark matter is expected to cluster at all mass scales even if it was evolved with the same initial conditions as used for FDM. Furthermore, we verify the scaling relations characterising the solution to the Schrödinger–Poisson system for both isolated and merging haloes, and we find that they are preserved by merging processes. We characterise each FDM halo in terms of the dimensionless quantity Ξ ∝  E halo / M halo 3 , and we show that the core mass is tightly linked to the halo mass by the core–halo mass relation M core / M halo  ∝ Ξ 1/3 . We also show that the core surface density of the simulated FDM haloes does not follow the scaling with the core radius, as observed for dwarf galaxies. This is a challenge for the FDM model as the sole explanation of core formation.

• Massara, Elena; Percival, Will J.; Dalal, Neal; Nadathur, Seshadri; Radinovic, Sladana & Winther, Hans Arnold [Show all 7 contributors for this article] (2022). Velocity profiles of matter and biased tracers around voids. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 517(3), p. 4458–4471. doi: 10.1093/mnras/stac2892. Full text in Research Archive Show summary

  ABSTRACT The velocity profile of galaxies around voids is a key ingredient for redshift space distortion (RSD) measurements made using the void–galaxy correlation function. In this paper, we use simulations to test whether the velocity profile of the tracers used to find the voids matches the velocity profile of the dark matter around these voids. A mismatch is expected and found in the inner part of voids, where tracers are very sparse. We discuss how this difference is caused by a selection effect where the void centre positions are correlated to the particular realization of the sparse tracers and their spatial distribution. In turn, this then affects the RSD void–galaxy correlation analysis. We show this by evaluating the Jacobian of the real to redshift space mapping using the tracer or matter velocity profile. Differences of the order of 20 per cent in the velocity profile translate into differences of the order of few per cent in the Jacobian. This small discrepancy propagates to the monopole and quadrupole of the void–tracer correlation function, producing modifications of comparable magnitude to those from changes in fσ8 at the level of the statistical uncertainties from current analyses.

• Hamaus, Nico; Aubert, M.; Pisani, Alice; Contarini, S.; Verza, G. & Cousinou, M.-C. [Show all 130 contributors for this article] (2022). Euclid: Forecasts from redshift-space distortions and the Alcock-Paczynski test with cosmic voids. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 658. doi: 10.1051/0004-6361/202142073. Full text in Research Archive Show summary

  Euclid is poised to survey galaxies across a cosmological volume of unprecedented size, providing observations of more than a billion objects distributed over a third of the full sky. Approximately 20 million of these galaxies will have their spectroscopy available, allowing us to map the three-dimensional large-scale structure of the Universe in great detail. This paper investigates prospects for the detection of cosmic voids therein and the unique benefit they provide for cosmological studies. In particular, we study the imprints of dynamic (redshift-space) and geometric (Alcock–Paczynski) distortions of average void shapes and their constraining power on the growth of structure and cosmological distance ratios. To this end, we made use of the Flagship mock catalog, a state-of-the-art simulation of the data expected to be observed with Euclid . We arranged the data into four adjacent redshift bins, each of which contains about 11 000 voids and we estimated the stacked void-galaxy cross-correlation function in every bin. Fitting a linear-theory model to the data, we obtained constraints on f / b and D M H , where f is the linear growth rate of density fluctuations, b the galaxy bias, D M the comoving angular diameter distance, and H the Hubble rate. In addition, we marginalized over two nuisance parameters included in our model to account for unknown systematic effects in the analysis. With this approach, Euclid will be able to reach a relative precision of about 4% on measurements of f / b and 0.5% on D M H in each redshift bin. Better modeling or calibration of the nuisance parameters may further increase this precision to 1% and 0.4%, respectively. Our results show that the exploitation of cosmic voids in Euclid will provide competitive constraints on cosmology even as a stand-alone probe. For example, the equation-of-state parameter, w , for dark energy will be measured with a precision of about 10%, consistent with previous more approximate forecasts.

• Hartman, Stian Thomas Heimdal; Winther, Hans Arnold & Mota, David (2022). Constraints on self-interacting Bose-Einstein condensate dark matter using large-scale observables. Journal of Cosmology and Astroparticle Physics (JCAP). 2022(2). doi: 10.1088/1475-7516/2022/02/005. Full text in Research Archive
• Hartman, Stian Thomas Heimdal; Winther, Hans Arnold & Mota, David (2021). Dynamical friction in Bose-Einstein condensed self-interacting dark matter at finite temperatures, and the Fornax dwarf spheroidal. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 647. doi: 10.1051/0004-6361/202039865. Full text in Research Archive
• Alam, Shadab; Arnold, Christian; Aviles, Alejandro; Bean, Rachel; Cai, Yan-Chuan & Cautun, Marius [Show all 36 contributors for this article] (2021). Towards testing the theory of gravity with DESI: summary statistics, model predictions and future simulation requirements. Journal of Cosmology and Astroparticle Physics (JCAP). 2021(11). doi: 10.1088/1475-7516/2021/11/050. Full text in Research Archive Show summary

  Shortly after its discovery, General Relativity (GR) was applied to predict the behavior of our Universe on the largest scales, and later became the foundation of modern cosmology. Its validity has been verified on a range of scales and environments from the Solar system to merging black holes. However, experimental confirmations of GR on cosmological scales have so far lacked the accuracy one would hope for — its applications on those scales being largely based on extrapolation and its validity there sometimes questioned in the shadow of the discovery of the unexpected cosmic acceleration. Future astronomical instruments surveying the distribution and evolution of galaxies over substantial portions of the observable Universe, such as the Dark Energy Spectroscopic Instrument (DESI), will be able to measure the fingerprints of gravity and their statistical power will allow strong constraints on alternatives to GR. In this paper, based on a set of N -body simulations and mock galaxy catalogs, we study the predictions of a number of traditional and novel summary statistics beyond linear redshift distortions in two well-studied modified gravity models — chameleon f ( R ) gravity and a braneworld model — and the potential of testing these deviations from GR using DESI. These summary statistics employ a wide array of statistical properties of the galaxy and the underlying dark matter field, including two-point and higher-order statistics, environmental dependence, redshift space distortions and weak lensing. We find that they hold promising power for testing GR to unprecedented precision. The major future challenge is to make realistic, simulation-based mock galaxy catalogs for both GR and alternative models to fully exploit the statistic power of the DESI survey (by matching the volumes and galaxy number densities of the mocks to those in the real survey) and to better understand the impact of key systematic effects. Using these, we identify future simulation and analysis needs for gravity tests using DESI.

• Tamosiunas, Andrius; Winther, Hans Arnold; Koyama, Kazuya; Bacon, David; Nichol, Robert C. & Mawdsley, Ben (2021). Investigating cosmological GAN emulators using latent space interpolation. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 506(2), p. 3049–3067. doi: 10.1093/mnras/stab1879. Full text in Research Archive Show summary

  ABSTRACT Generative adversarial networks (GANs) have been recently applied as a novel emulation technique for large-scale structure simulations. Recent results show that GANs can be used as a fast and efficient emulator for producing novel weak lensing convergence maps as well as cosmic web data in 2D and 3D. However, like any algorithm, the GAN approach comes with a set of limitations, such as an unstable training procedure, inherent randomness of the produced outputs, and difficulties when training the algorithm on multiple data sets. In this work, we employ a number of techniques commonly used in the machine learning literature to address the mentioned limitations. Specifically, we train a GAN to produce weak lensing convergence maps and dark matter overdensity field data for multiple redshifts, cosmological parameters, and modified gravity models. In addition, we train a GAN using the newest Illustris data to emulate dark matter, gas, and internal energy distribution data simultaneously. Finally, we apply the technique of latent space interpolation as a tool for understanding the feature space of the GAN algorithm. We show that the latent space interpolation procedure allows the generation of outputs with intermediate cosmological parameters that were not included in the training data. Our results indicate a 1–20 per cent difference between the power spectra of the GAN-produced and the test data samples depending on the data set used and whether Gaussian smoothing was applied. Similarly, the Minkowski functional analysis indicates a good agreement between the emulated and the real images for most of the studied data sets.

• Fiorini, Bartolomeo; Koyama, Kazuya; Izard, Albert; Winther, Hans Arnold; Wright, Bill S. & Li, Baojiu (2021). Fast generation of mock galaxy catalogues in modified gravity models with COLA. Journal of Cosmology and Astroparticle Physics (JCAP). 2021(09). doi: 10.1088/1475-7516/2021/09/021. Full text in Research Archive
• Mina, Mattia; Mota, David & Winther, Hans Arnold (2020). SCALAR: an AMR code to simulate axion-like dark matter models. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 641. doi: 10.1051/0004-6361/201936272. Full text in Research Archive Show summary

  We present a new code,SCALAR, based on the high-resolution hydrodynamics andN-body codeRAMSES, to solve the Schrödingerequation on adaptive refined meshes. The code is intended to be used to simulate axion or fuzzy dark matter models where theevolution of the dark matter component is determined by a coupled Schrödinger-Poisson equation, but it can also be used as a stand-alone solver for both linear and non-linear Schrödinger equations with any given external potential. This paper describes the numericalimplementation of our solver and presents tests to demonstrate how accurately it operates.

• Hartman, Stian; Winther, Hans Arnold & Mota, David (2020). Collapse of spherical overdensities in superfluid models of dark matter. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 639. doi: 10.1051/0004-6361/201937263. Full text in Research Archive Show summary

  Aims. We intend to understand cosmological structure formation within the framework of superfluid models of dark matter with finite temperatures. Of particular interest is the evolution of small-scale structures where the pressure and superfluid properties of the dark matter fluid are prominent. We compare the growth of structures in these models with the standard cold dark matter paradigm and non-superfluid dark matter. Methods. The equations for superfluid hydrodynamics were computed numerically in an expanding ΛCDM background with spherical symmetry; the effect of various superfluid fractions, temperatures, interactions, and masses on the collapse of structures was taken into consideration. We derived the linear perturbation of the superfluid equations, giving further insights into the dynamics of the superfluid collapse. Results. We found that while a conventional dark matter fluid with self-interactions and finite temperatures experiences a suppression in the growth of structures on smaller scales, as expected due to the presence of pressure terms, a superfluid can collapse much more efficiently than was naively expected due to its ability to suppress the growth of entropy perturbations and thus gradients in the thermal pressure. We also found that the cores of the dark matter halos initially become more superfluid during the collapse, but eventually reach a point where the superfluid fraction falls sharply. The formation of superfluid dark matter halos surrounded by a normal fluid dark matter background is therefore disfavored by the present work.

• Nadathur, Seshadri; Percival, Will J.; Beutler, Florian & Winther, Hans Arnold (2020). Testing Low-Redshift Cosmic Acceleration with Large-Scale Structure. Physical Review Letters. ISSN 0031-9007. 124(22). doi: 10.1103/PhysRevLett.124.221301. Full text in Research Archive
• Bose, Benjamin; Winther, Hans Arnold; Pourtsidou, Alkistis; Casas, Santiago; Lombriser, Lucas & Xia, Qianli [Show all 7 contributors for this article] (2020). Hybrid Pℓ(k): general, unified, non-linear matter power spectrum in redshift space. Journal of Cosmology and Astroparticle Physics (JCAP). 2020(09). doi: 10.1088/1475-7516/2020/09/001. Full text in Research Archive Show summary

  Constraints on gravity and cosmology will greatly benefit from performing joint clustering and weak lensing analyses on large-scale structure data sets. Utilising non-linear information coming from small physical scales can greatly enhance these constraints. At the heart of these analyses is the matter power spectrum. Here we employ a simple method, dubbed "Hybrid Pℓ(k)", based on the Gaussian Streaming Model (GSM), to calculate the quasi non-linear redshift space matter power spectrum multipoles. This employs a fully non-linear and theoretically general prescription for the matter power spectrum. We test this approach against comoving Lagrangian acceleration simulation measurements performed in GR, DGP and f(R) gravity and find that our method performs comparably or better to the dark matter TNS redshift space power spectrum model {for dark matter. When comparing the redshift space multipoles for halos, we find that the Gaussian approximation of the GSM with a linear bias and a free stochastic term, N, is competitive to the TNS model.} Our approach offers many avenues for improvement in accuracy as well as further unification under the halo model.

• Aviles, Alejandro; Koyama, Kazuya; Cervantes-Cota, Jorge L; Winther, Hans Arnold & Li, Baojiu (2020). Marked correlation functions in perturbation theory. Journal of Cosmology and Astroparticle Physics (JCAP). 2020(01). doi: 10.1088/1475-7516/2020/01/006. Full text in Research Archive Show summary

  We develop perturbation theory approaches to model the marked correlation function constructed to up-weight low density regions of the Universe, which might help distinguish modified gravity models from the standard cosmology model based on general relativity. Working within Convolution Lagrangian Perturbation Theory, we obtain that weighted correlation functions are expressible as double convolution integrals that we approximate using a combination of Eulerian and Lagrangian schemes. We find that different approaches agree within 1% on quasi non-linear scales. Compared with N-body simulations, the perturbation theory is found to provide accurate predictions for the marked correlation function of dark matter fields, dark matter halos as well as Halo Occupation Distribution galaxies down to 30 Mpc/h. These analytic approaches help to understand the degeneracy between the mark and the galaxy bias and find a way to maximize the differences among various cosmological models.

• Winther, Hans Arnold; Casas, Santiago; Baldi, Marco; Koyama, Kazuya; Li, Baojiu & Lombriser, Lucas [Show all 7 contributors for this article] (2019). Emulators for the nonlinear matter power spectrum beyond ΛCDM. Physical Review D. ISSN 2470-0010. 100(12), p. 123540-1–123540-13. doi: 10.1103/PhysRevD.100.123540. Full text in Research Archive Show summary

  Accurate predictions for the nonlinear matter power spectrum are needed to confront theory with observations in current and near future weak-lensing and galaxy clustering surveys. We propose a computationally cheap method to create an emulator for modified gravity models by utilizing existing emulators for Λ CDM . Using a suite of N-body simulations, we construct a fitting function for the enhancement of both the linear and nonlinear matter power spectrum in the commonly studied Hu-Sawicki f(R) gravity model valid for wave numbers k≲ 5 – 10 h     Mpc − 1 and redshifts z≲ 3 . We show that the cosmology dependence of this enhancement is relatively weak so that our fit, using simulations coming from only one cosmology, can be used to get accurate predictions for other cosmological parameters. We also show that the cosmology dependence can, if needed, be included by using linear theory, approximate N-body simulations (such as comoving lagrangian acceleration) and semianalytical tools like the halo model. Our final fit can easily be combined with any emulator or semianalytical models for the nonlinear Λ CDM power spectrum to accurately, and quickly, produce a nonlinear power spectrum for this particular modified gravity model. The method we use can be applied to fairly cheaply construct an emulator for other modified gravity models. As an application of our fitting formula, we use it to compute Fisher forecasts for how well galaxy clustering and weak lensing in a Euclid-like survey will be at constraining modifications of gravity.

• Bose, Benjamin; Koyama, Kazuya & Winther, Hans Arnold (2019). Assessing non-linear models for galaxy clustering III: theoretical accuracy for stage IV surveys. Journal of Cosmology and Astroparticle Physics (JCAP). 2019(10). doi: 10.1088/1475-7516/2019/10/021. Full text in Research Archive
• L'Huillier, Benjamin; Winther, Hans Arnold; Mota, David; Park, Changbom & Kim, Juhan (2017). Dark matter haloes in modified gravity and dark energy: interaction rate, small- and large-scale alignment. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 468(3), p. 3174–3183. doi: 10.1093/mnras/stx700. Full text in Research Archive Show summary

  We study the properties of dark matter haloes in a wide range of modified gravity models, namely, f(R), DGP and interacting dark energy models. We study the effects of modified gravity and dark energy on the internal properties of haloes, such as the spin and the structural parameters. We find that f(R) gravity enhances the median value of the Bullock spin parameter, but could not detect such effects for DGP and coupled dark energy. f(R) also yields a lower median sphericity and oblateness, while coupled dark energy has the opposite effect. However, these effects are very small. We then study the interaction rate of haloes in different gravity and find that only strongly coupled dark energy models enhance the interaction rate. We then quantify the enhancement of the alignment of the spins of interacting halo pairs by modified gravity. Finally, we study the alignment of the major axes of haloes with the large-scale structures. The alignment of the spins of interacting pairs of haloes in DGP and coupled dark energy models show no discrepancy with GR, while f(R) shows a weaker alignment. Strongly coupled dark energy shows a stronger alignment of the halo shape with the large-scale structures. © 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society

• Carlesi, Edoardo; Mota, David & Winther, Hans Arnold (2017). The dynamics of the Local Group as a probe of dark energy and modified gravity. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 466(4), p. 4813–4825. doi: 10.1093/mnras/stx056. Full text in Research Archive Show summary

  In this work, we study the dynamics of the Local Group (LG) within the context of cosmological models beyond General Relativity (GR). Using observable kinematic quantities to identify candidate pairs, we build up samples of simulated LG-like objects drawing from f(R), symmetron, Dvali, Gabadadze & Porrati and quintessence N-body simulations together with their Λ cold dark matter (ΛCDM) counterparts featuring the same initial random phase realizations. The variables and intervals used to define LG-like objects are referred to as LG model; different models are used throughout this work and adapted to study their dynamical and kinematic properties. The aim is to determine how well the observed LG dynamics can be reproduced within cosmological theories beyond GR, We compute kinematic properties of samples drawn from alternative theories and ΛCDM and compare them to actual observations of the LG mass, velocity and position. As a consequence of the additional pull, pairwise tangential and radial velocities are enhanced in modified gravity and coupled dark energy with respect to ΛCDM inducing significant changes to the total angular momentum and energy of the LG. For example, in models such as f(R) and the symmetron this increase can be as large as 60 per cent, peaking well outside of the 95 per cent confidence region allowed by the data. This shows how simple considerations about the LG dynamics can lead to clear small-scale observational signatures for alternative scenarios, without the need of expensive high-resolution simulations. © 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society

• santos, Marcelo Vargas dos; Winther, Hans Arnold; Mota, David & Waga, Ioav (2016). N-body simulations of γ gravity. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 587. doi: 10.1051/0004-6361/201527645. Full text in Research Archive
• Bull, Philip James; Cheghasiahi, Yashar Akrami; Adamek, Julian; Baker, Tessa; Bellini, Emilio & Jiménez, Jose Beltrán [Show all 40 contributors for this article] (2016). Beyond ΛCDM: Problems, solutions, and the road ahead. Physics of the Dark Universe. 12, p. 56–99. doi: 10.1016/j.dark.2016.02.001. Full text in Research Archive
• Gronke, Max Balthasar; Hammami, Amir; Mota, David & Winther, Hans Arnold (2016). Estimates of cluster masses in screened modified gravity. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 595. doi: 10.1051/0004-6361/201628644. Full text in Research Archive
• Grönke, Max Balthasar; Llinares, Claudio; Mota, David & Winther, Hans Arnold (2015). Halo velocity profiles in screened modified gravity theories. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 449(3), p. 2837–2844. doi: 10.1093/mnras/stv496. Full text in Research Archive Show summary

  Screened modified gravity predicts potentially large signatures in the peculiar velocity field that makes it an interesting probe to test gravity on cosmological scales. We investigate the signatures induced by the Symmetron and a Chameleon f(R) model in the peculiar velocity field using N-body simulations. By studying fifth force and halo velocity profiles, we identify three general categories of effects found in screened modified gravity models: a fully screened regime where we recover Λ cold dark matter to high precision, an unscreened regime where the fifth force is in full operation, and, a partially screened regime where screening occurs in the inner part of a halo, but the fifth force is active at larger radii. These three regimes can be pointed out very clearly by analysing the deviation in the maximum cluster velocity. Observationally, the partially screened regime is of particular interest since an uniform increase of the gravitational force – as present in the unscreened regime – is degenerate with the (dynamical) halo mass estimate, and, thus, hard to detect. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

• Hammami, Amir; Llinares, Claudio; Mota, David & Winther, Hans Arnold (2015). Hydrodynamic effects in the symmetron and f(R)-gravity models. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 449(4), p. 3635–3644. doi: 10.1093/mnras/stv529. Full text in Research Archive Show summary

  In this paper, we present the first results from implementing two scalar–tensor modified gravity theories, the symmetron and the Hu–Sawicki f(R)-gravity model, into a hydrodynamic N-body code with dark matter particles and a baryonic ideal gas. The study is a continuation of previous work where the symmetron and f(R) have been successfully implemented in the ramses code, but for dark matter only. By running simulations, we show that the deviation from Λ cold dark matter (ΛCDM) in these models for the gas density profiles are significantly lower than the dark matter equivalents. When it comes to the matter power spectrum, we find that hydrodynamic simulations agree very well with dark matter only simulations as long as we consider scales larger than k ∼ 0.5 h Mpc−1. In general the effects of modified gravity on the baryonic gas is found to not always mirror the effects it has on the dark matter, but when it does, it does it to a lesser extent. The largest signature is found when considering temperature profiles. We find that the gas temperatures in the modified gravity model studied here show deviations, when compared to ΛCDM, that can be a factor of a few larger than the deviations found in density profiles and power spectra. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

• Winther, Hans Arnold; Schmidt, F.; Barreira, Alexandre; Arnold, Christian; Bose, S. & Llinares, Claudio [Show all 15 contributors for this article] (2015). Modified gravity N-body code comparison project. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 454(4), p. 4208–4234. doi: 10.1093/mnras/stv2253. Full text in Research Archive
• Gronke, Max Balthasar; Mota, David & Winther, Hans Arnold (2015). Universal predictions of screened modified gravity on cluster scales. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 583. doi: 10.1051/0004-6361/201526611. Full text in Research Archive Show summary

  Modified gravity models require a screening mechanism to be able to evade the stringent constraints from local gravity experiments and, at the same time, give rise to observable astrophysical and cosmological signatures. Such screened modified gravity models necessarily have dynamics determined by complex nonlinear equations that usually need to be solved on a model-by-model basis to produce predictions. This makes testing them a cumbersome process. In this paper, we investigate whether there is a common signature for all the different models that is suitable to testing them on cluster scales. To do this we propose an observable related to the fifth force, which can be observationally related to the ratio of dynamical-to-lensing mass of a halo, and then show that the predictions for this observable can be rescaled to a near universal form for a large class of modified gravity models. We demonstrate this using the Hu-Sawicki f(R), the Symmetron, the nDGP, and the Dilaton models, as well as unifying parametrizations. The universal form is determined by only three quantities: a strength, a mass, and a width parameter. We also show how these parameters can be derived from a specific theory. This self-similarity in the predictions can hopefully be used to search for signatures of modified gravity on cluster scales in a model-independent way. Reproduced with permission from Astronomy & Astrophysics, © ESO

• Silva, M. F.; Winther, Hans Arnold; Mota, David & Martins, Carlos Jose Amaro Parente (2014). Spatial variations of the fine-structure constant in symmetron models. Physical Review D. ISSN 1550-7998. 89. doi: 10.1103/PhysRevD.89.024025. Full text in Research Archive Show summary

  We investigate the variation of the fine-structure constant, α, in symmetron models using N-body simulations in which the full spatial distribution of α at different redshifts has been calculated. In particular, we obtain simulated sky maps for this variation, and determine its power spectrum. We find that in high-density regions of space (such as deep inside dark matter halos) the value of α approaches the value measured on Earth. In the low-density outskirts of halos the scalar field value can approach the symmetry breaking value and leads to significantly different values of α. If the scalar-photon coupling strength βγ is of order unity we find that the variation of α inside dark matter halos can be of the same magnitude as the recent claims by Webb et al. of a dipole variation. Importantly, our results also show that with low-redshift symmetry breaking these models exhibit some dependence of α on lookback time (as opposed to a pure spatial dipole) which could in principle be detected by sufficiently accurate spectroscopic measurements, such as those of ALMA and the ELT-HIRES. © 2014 American Physical Society

• Llinares, Claudio; Mota, David & Winther, Hans Arnold (2014). ISIS: a new N-body cosmological code with scalar fields based on RAMSES - Code presentation and application to the shapes of clusters. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 562. doi: 10.1051/0004-6361/201322412. Full text in Research Archive Show summary

  Several extensions of the standard cosmological model include scalar fields as new degrees of freedom in the underlying gravitational theory. A particular class of these scalar field theories include screening mechanisms intended to hide the scalar field below observational limits in the solar system, but not on galactic scales, where data still gives the freedom to find possible signatures of their presence. To make predictions to compare with observations coming from galactic and clusters scales (i.e. in the non-linear regime of cosmological evolution), cosmological N-body simulations are needed, for which codes that can solve for the scalar field must be developed. We present a new implementation of scalar-tensor theories of gravity that include screening mechanisms. The code is based on the already existing code RAMSES, to which we have added a non-linear multigrid solver that can treat a large class of scalar tensor theories of modified gravity. We present details of the implementation and the tests that we made to the code. As application of the new code, we studied the influence that two particular modified gravity theories, the symmetron and f(R) gravity, have on the shape of cluster sized dark matter haloes and found consistent results with previous estimations made with a static analysis. Reproduced with permission from Astronomy & Astrophysics, © ESO

• Winther, Hans Arnold (2013). Layzer-Irvine equation for scalar-tensor theories: A test of modified gravity N-body simulations. Physical Review D. Particles and fields. ISSN 0556-2821. 88(4). doi: 10.1103/PhysRevD.88.044057. Full text in Research Archive
• Brax, P; Davis, A.-C.; Li, Baojiu; Winther, Hans Arnold & Zhao, Gong-Bo (2013). Systematic simulations of modified gravity: chameleon models. Journal of Cosmology and Astroparticle Physics (JCAP). doi: 10.1088/1475-7516/2013/04/029. Full text in Research Archive
• Winther, Hans Arnold; Mota, David & Li, Baojiu (2012). ENVIRONMENT DEPENDENCE OF DARK MATTER HALOS IN SYMMETRON MODIFIED GRAVITY. The Astrophysical Journal (ApJ). ISSN 0004-637X. 756(2). doi: 10.1088/0004-637X/756/2/166. Full text in Research Archive
• Brax, P; Davis, A.-C. & Winther, Hans Arnold (2012). Cosmological supersymmetric model of dark energy. Physical Review D. Particles and fields. ISSN 0556-2821. 85(8). doi: 10.1103/PhysRevD.85.083512. Full text in Research Archive Show summary

  Recently, a supersymmetric model of dark energy coupled to cold dark matter, the supersymmetron, has been proposed. In the absence of cold dark matter, the supersymmetron field converges to a supersymmetric minimum with a vanishing cosmological constant. When cold dark matter is present, the supersymmetron evolves to a matter-dependent minimum where its energy density does not vanish and could lead to the present acceleration of the Universe. The supersymmetron generates a short-ranged fifth force which evades gravitational tests. It could lead to observable signatures on structure formation due to a very strong coupling to dark matter. We investigate the cosmological evolution of the field, focusing on the linear perturbations and the spherical collapse and find that observable modifications in structure formation can indeed exist. Unfortunately, we find that when the growth rate of perturbations is in agreement with observations, an additional cosmological constant is required to account for dark energy. In this case, effects on large-scale structures are still present at the nonlinear level which are investigated using the spherical collapse approach. © 2012 American Physical Society

• Brax, P; Davis, A.-C.; Li, Baojiu & Winther, Hans Arnold (2012). Unified description of screened modified gravity. Physical Review D. Particles and fields. ISSN 0556-2821. 86(4). doi: 10.1103/PhysRevD.86.044015. Full text in Research Archive Show summary

  We consider modified gravity models driven by a scalar field whose effects are screened in high density regions due to the presence of nonlinearities in its interaction potential and/or its coupling to matter. Our approach covers chameleon, f(R) gravity, dilaton and symmetron models and allows a unified description of all these theories. We find that the dynamics of modified gravity are entirely captured by the time variation of the scalar field mass and its coupling to matter evaluated at the cosmological minimum of its effective potential, where the scalar field has sat since an epoch prior to big bang nucleosynthesis. This new parametrization of modified gravity allows one to reconstruct the potential and coupling to matter and therefore to analyze the full dynamics of the models, from the scale dependent growth of structures at the linear level to nonlinear effects requiring N-body simulations. This procedure is illustrated with explicit examples of reconstruction for chameleon, dilaton, f(R) and symmetron models. © 2012 American Physical Society

• Brax, P; Davis, A.-C.; Li, Baojiu; Winther, Hans Arnold & Zhao, Gong-Bo (2012). Systematic simulations of modified gravity: symmetron and dilaton models. Journal of Cosmology and Astroparticle Physics (JCAP). 10. doi: 10.1088/1475-7516/2012/10/002. Full text in Research Archive
• Davis, A.-C.; Li, Baojiu; Mota, David & Winther, Hans Arnold (2012). STRUCTURE FORMATION IN THE SYMMETRON MODEL. The Astrophysical Journal (ApJ). ISSN 0004-637X. 748(1). doi: 10.1088/0004-637X/748/1/61. Full text in Research Archive Show summary

  Scalar fields, strongly coupled to matter, can be present in nature and still be invisible to local experiments if they are subject to a screening mechanism. The symmetron is one such mechanism that relies on restoration of a spontaneously broken symmetry in regions of high density to shield the scalar fifth force. We have investigated structure formation in the symmetron model by using N-body simulations and find observable signatures in both the linear and nonlinear matter power spectrum and on the halo mass function. The mechanism for suppressing the scalar fifth force in high-density regions is also found to work very well. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

• Mota, David & Winther, Hans Arnold (2011). COSMOLOGY OF CHAMELEONS WITH POWER-LAW COUPLINGS. The Astrophysical Journal (ApJ). ISSN 0004-637X. 733(1). doi: 10.1088/0004-637X/733/1/7. Full text in Research Archive
• Gannouji, Radouane; Moraes, Bruno; Mota, David Fonseca; Polarski, David; Tsujikawa, S & Winther, Hans Arnold (2010). Chameleon dark energy models with characteristic signatures. Physical Review D. Particles and fields. ISSN 0556-2821. 82. doi: 10.1103/PhysRevD.82.124006. Full text in Research Archive
• Brax, P; Bruck, C. van de; Mota, David Fonseca; Nunes, Nelson & Winther, Hans Arnold (2010). Chameleons with field-dependent couplings. Physical Review D. Particles and fields. ISSN 0556-2821. 82(8). doi: 10.1103/PhysRevD.82.083503. Full text in Research Archive

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• Winther, Hans Arnold (2023). The Cosmic Web. Full text in Research Archive
• Winther, Hans Arnold (2023). Nytt romteleskop skal kaste lys over de mørke sidene av universet. [Internet]. Forskning.no. Full text in Research Archive
• Zimmermann, Tim; Shen, Sijing & Winther, Hans Arnold (2025). On the wave-like nature of fuzzy dark matter. Modeling, phenomenology, and limits from the bottom up. Universitetet i Oslo. Full text in Research Archive Show summary

  Cold dark matter (CDM), a thermally produced, nonrelativistic form of matter, has established itself as a successful effective theory for the abundant nonluminous matter in the universe, accurately describing the large-scale structure of the universe. At the same time, null results of direct detection efforts emphasize that the microphysical nature of DM remains elusive. With hard detection limits approaching and a plethora of phenomenological degeneracies lurking on galactic scales, looking beyond CDM remains a vital effort in constraining the nature of DM. This thesis investigates ultralight or fuzzy dark matter (FDM) as a theoretically well-motivated alternative to CDM and focuses on its astrophysical implications due to its wave-like nature. To this end, we adopt a bottom-up methodology and examine the behavior of FDM dimension-by-dimension, state-by-state, and filament-by-filament. The results of this thesis are (i) a deepened understanding of its numerical modeling, (ii) a new, robust limit for its mass, and (iii) first steps towards a principled understanding of the emerging wave phenomena in cosmic filaments.

• Radinovic, Sladana; Winther, Hans Arnold & Nadathur, Seshadri (2024). Out of Nothing: Constraining cosmology and gravity with the void-galaxy cross-correlation function. Universitetet i Oslo. Full text in Research Archive Show summary

  Though a young probe in a young field, cosmic voids are proving to be a promising way to study cosmology. Over the past one hundred years, cosmologists have been trying to explain the observed expansion of the Universe. Whether this expansion is driven by a new form of energy (called dark energy) or a modification to the laws of gravity is still an open question. By measuring the spatial relation of voids and the galaxies that surround them, we can explore both possibilities at the same time. However, two observational effects complicate this measurement, creating geometrical and dynamical distortions in the observed cosmic web. Luckily, this is a feature, not a bug! Dynamical distortions, known as redshift-space distortions, arise due to the motion of galaxies under the effect of gravity and can help us study it. Geometrical distortions, on the other hand, arise from mistakes in our assumptions, and can relate the Universe as we see it to the way it really is. This effect, called the Alcock–Paczyński effect, can help us pin down the amount of dark energy in the Universe, as well as its evolution in time.

• Arroyave, Isabel Montoya; Cicone, Claudia; Shen, Sijing & Winther, Hans Arnold (2024). Multi-tracer molecular gas observations of local infrared luminous galaxies. Universitetet i Oslo. Full text in Research Archive Show summary

  In my thesis, I investigated local (ultra-)luminous infrared galaxies, typically identified as galaxy mergers. These galaxies are fascinating due to their intense star formation and active galactic nuclei, which can lead to powerful galactic outflows spreading gas across large distances. My research focused on the molecular gas phase, a critical component for star formation and the evolution of galaxies. Using high-sensitivity observations, I investigated carbon monoxide and atomic carbon emissions to gain insights into the properties of their molecular gas. Special attention was given to the extended and diffuse gas components, which could be linked to galactic outflows. My findings revealed a wide range of gas excitation states, pointing to extreme conditions throughout the interstellar medium of these galaxies. Moreover, I identified a potential connection between high gas excitation and the presence of powerful outflows. My work highlights the importance of sensitive observations for capturing the full picture of these extreme sources, especially in understanding how star formation and galactic nuclei activities impact their evolution.

• Christiansen, Øyvind; Mota, David & Winther, Hans Arnold (2024). Cosmological Simulations of Phase Transitions in Screened Scalar-Tensor Gravity: Cosmic acceleration, topological defects, gravitational waves and relativistic observables. Universitetet i Oslo. Full text in Research Archive Show summary

  Christiansen presents cosmological simulations of the (a)symmetron, a scalar-tensor theory of gravity undergoing a phase transition in the late-time universe. He demonstrates novel signatures in gravitational waves and other relativistic observables, and explores in detail the formation of domain walls and their imprint on cosmic structure formation, including dark matter halos and the cosmic web. Christiansen uncovers an interesting part of the parameter space where phase transitions occur inside the nonlinear underdensities, and proposes an application of the nonlinear and dynamical theory to explain late-time cosmology as well as to unify the dark sector.

• Mauland-Hus, Renate; Elgarøy, Øystein & Winther, Hans Arnold (2023). Large-scale structure with massive neutrinos and modified gravity: A simulation-based approach. Universitetet i Oslo. Full text in Research Archive Show summary

  Massive neutrinos and alternative theories of gravity are two seemingly different aspects of cosmological study. On one hand, we have actual particles known to exist, and on the other, a proposed change to the description of gravity. While the presence of massive neutrinos suppresses structure formation on certain scales, modified gravity theories tend to enhance it, resulting in a degenerate effect for some cosmological observables. To properly constrain models of modified gravity, this degeneracy must be considered. This is also true in reverse, when using cosmology to constrain the absolute mass scale of neutrinos, which is yet to be accurately determined. In this thesis, the effects of modified gravity and massive neutrinos are investigated both separately and combined. This is done through the development, use, and analysis of cosmological simulations. The above-mentioned degeneracy is further explored via the analysis of cosmic voids, which are vast underdense regions in the cosmic web. In addition, a pipeline for generating emulators of large-scale structure statistics for universe models beyond the standard one is created using approximate simulation techniques and machine learning. This pipeline, named Sesame, opens the opportunity of studying alternative universe models without the need for large computing resources.

• Hartman, Stian Thomas Heimdal; Mota, David & Winther, Hans Arnold (2022). Cosmology of Scalar Field Dark Matter with Self-Interactions. Universitetet i Oslo. Full text in Research Archive Show summary

  In the standard model of the universe, the presence of a cold and dark form of matter is essential for explaining a wide range of observables, from the very largest scales such as the microwave background radiation, to galaxies and sub-galactic scales. However, despite this dark matter being about five times more abundant than all of the visible matter in our universe, its true identity is a complete mystery. Unveiling the fundamental nature of dark matter would contribute to a deeper understanding of our universe and the physical laws that govern it, and is therefore one of the Holy Grails of modern physics. In this thesis I have explored how dark matter behaves (in sometimes surprising ways) in models where the hypothetical dark matter particle is very light and self-interacting, using both theoretical tools and numerical simulations. This effort has contributed to an improved understanding of these models in certain scenarios, as well as provided additional constraints on the free parameters of these models by investigating their effect on observables.

• Mina, Mattia; Mota, David & Winther, Hans Arnold (2020). Cosmology at small scales: ultra-light dark matter and baryon cycles in galaxies. Universitetet i Oslo. Full text in Research Archive Show summary

  The standard model of cosmology has been quite successful in describing how baryonic matter and dark matter cluster by means of gravity, forming the structures we can see today while looking at the sky, and it can reproduce many of the general properties of the observed Universe. However, on the scales of individual galaxies, this model has several complications due to the fact that the true nature of dark matter is still unknown and physical processes involving ordinary matter withing galaxies are not fully understood. By means of computer simulations, I first studied a specific class of dark matter models, called Ultra-Light Dark Matter. I showed that these models better describe some of the properties of dark matter halos, and they develop distinct features which can be tested against astronomical observations. Then, I studied how stars can affect the life cycle of galaxies and dark matter halos. I showed that using a realistic model for supernova explosions can potentially solve the discrepancies between the theoretical predictions of the standard model of cosmology and astronomical observations, reproducing dwarf galaxies and dark matter halos that closely resemble observed ones.

• Winther, Hans Arnold (2013). Cosmological Probes of Modified Gravity. Universitetet i Oslo. Full text in Research Archive

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