Farbod Hassani

• Rácz, G.; Breton, M.-A.; Fiorini, B.; Brun, A. M. C. Le; Winther, Hans Arnold & Sakr, Z. [Vis alle 277 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv Vis sammendrag

  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. [Vis alle 1 138 forfattere av denne artikkelen] (2025). Euclid I. Overview of the Euclid mission. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202450810. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Etezad-Razavi, Saba; Abbasgholinejad, Erfan; Sotoudeh, Mohammad-Hadi; Hassani, Farbod; Raeisi, Sadegh & Baghram, Shant (2025). Cosmic velocity, density, and halo mass function: Insights from deep learning. Physical Review D. ISSN 2470-0010. 111(4). doi: 10.1103/PhysRevD.111.043521. Fulltekst i vitenarkiv Vis sammendrag

  We discuss an implementation of a deep learning framework to gain insight into dark matter (DM) structure formation. We investigate the contribution of velocity and density field information to the construction of the halo mass function (HMF) in cosmological 𝑁-body simulations. We train a convolutional neural network (CNN) on the initial snapshot of a DM-only simulation to predict the halo mass that individual particles fall into at 𝑧 =0, in the halo mass range of 10.5 <log⁡(𝑀/𝑀⊙) <14. We show that for the standard Λ⁢CDM cosmology with amplitude of initial perturbations 𝐴𝑠 =2 ×10−9, the initial velocity and density fields have equivalent information, as expected in the linear regime, and manifest the power of our CNN to diagnose the redundant information. To investigate the nonlinear effects, we increase the initial power spectrum. In the linear regime, this is equivalent to decreasing the initial redshift. The CNN model trained on the simulation snapshots with large 𝐴𝑠 shows a considerable improvement in the HMF prediction when adding the velocity field information. Using our CNN map without further physical assumptions, we precisely evaluate when these nonlinear effects become vital. Eventually, for the simulation with 𝐴𝑠 =8 ×10−8, the model trained with only density information shows at least an 80% increase in the mean squared error relative to the model with both velocity and density information. Our work shows the interpretability and ability of CNNs to read higher-order information from simple images, making them an excellent tool for cosmological studies.

• Adamek, J.; Fiorini, B.; Baldi, M.; Brando, G.; Breton, M.-A. & Hassani, Farbod [Vis alle 270 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Christiansen, Øyvind; Hassani, Farbod & Mota, David Fonseca (2025). Environmental cosmic acceleration from a phase transition in the dark sector. Journal of Cosmology and Astroparticle Physics (JCAP). 2025. doi: 10.1088/1475-7516/2025/01/043. Fulltekst i vitenarkiv Vis sammendrag

  A new degravitation mechanism within the framework of scalar tensor gravity is postulated and included by prescription. The mechanism eliminates all constant contributions from the potential to the Friedmann equation, leaving only the kinematic and the dynamic terms of the potential to drive cosmic acceleration. We explore a scenario involving a density-triggered phase transition in the late-time universe, and argue that the resulting effective energy density and equation of state parameter can explain late-time cosmology when extrapolated to a region of the parameter space.

• Christiansen, Øyvind; Adamek, J.; Hassani, Farbod & Mota, David (2025). Gravitational waves from dark domain walls. Journal of Cosmology and Astroparticle Physics (JCAP). 2025(1). doi: 10.1088/1475-7516/2025/01/149. Fulltekst i vitenarkiv Vis sammendrag

  For most of cosmic history, the evolution of our Universe has been governed by the physics of a `dark sector', consisting of dark matter and dark energy, whose properties are only understood in a schematic way. The influence of these constituents is mediated exclusively by the force of gravity, meaning that insight into their nature must be gleaned from gravitational phenomena. The advent of gravitational-wave astronomy has revolutionised the field of black hole astrophysics, and opens a new window of discovery for cosmological sources. Relevant examples include topological defects, such as domain walls or cosmic strings, which are remnants of a phase transition. Here we present the first simulations of cosmic structure formation in which the dynamics of the dark sector introduces domain walls as a source of stochastic gravitational waves in the late Universe. We study in detail how the spectrum of gravitational waves is affected by the properties of the model, and extrapolate the results to scales relevant to the recent evidence for a stochastic gravitational wave background. Our relativistic implementation of the field dynamics paves the way for optimal use of the next generation of gravitational experiments to unravel the dark sector.

• Christiansen, Øyvind; Hassani, Farbod & Mota, David (2024). Asimulation: Domain formation and impact on observables in resolved cosmological simulations of the (a)symmetron. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 689. doi: 10.1051/0004-6361/202449188. Fulltekst i vitenarkiv Vis sammendrag

  The symmetron is a dark energy and dark matter candidate that forms topological defects in the late-time universe and holds the promise of resolving some of the cosmological tensions. We performed high-resolution simulations of the dynamical and non-linear (a)symmetron using the recently developed relativistic N-body code asevolution. By extensively testing the temporal and spatial convergence of domain decompositioning and domain wall stability, we determined criteria and physical intuition for the convergence. We applied the resolution criteria to run five high-resolution simulations with 12803 grids and a box size of 500 Mpc h−1 of the (a)symmetron. We considered the behaviour of the scalar field and the domain walls in each scenario. We find the effect on the matter power spectra, the HMFs, and observables computed over the past light cone of an observer, such as the integrated Sachs-Wolfe and non-linear Rees-Sciama effect and the lensing, compared to ΛCDM. We show local oscillations of the fifth force strength and the formation of planar structures in the density field. The dynamics of the field was visualised in animations with high resolution in time. The simulation code is made publicly available.

• Khoshtinat, Mohadese; Ansarifard, Mohammad; Hassani, Farbod & Baghram, Shant (2024). Clustering of dark matter in the cosmic web as a probe of massive neutrinos. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 531(1), s. 575–584. doi: 10.1093/mnras/stae1195. Fulltekst i vitenarkiv Vis sammendrag

  The large-scale structure of the Universe is distributed in a cosmic web. Studying the distribution and clustering of dark matter particles and haloes may open up a new horizon for studying the physics of the dark Universe. In this work, we investigate the nearest neighbour statistics and spherical contact function in cosmological models with massive neutrinos. For this task, we use the relativistic N-body code, gevolution, and study particle snapshots at three different redshifts. In each snapshot, we find the haloes and evaluate the letter functions for them. We show that a generic behaviour can be found in the nearest neighbour, G(r), and spherical contact functions, F(r), which makes these statistics promising tools to constrain the total neutrino mass.

• Chegeni, Amirmohammad; Hassani, Farbod; Sadr, Alireza Vafaei; Khosravi, Nima & Kunz, Martin (2024). Clusternets: a deep learning approach to probe clustering dark energy. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 531(1), s. 1534–1545. doi: 10.1093/mnras/stae1075. Fulltekst i vitenarkiv
• Nouri-Zonoz, Ahmad; Hassani, Farbod & Kunz, Martin (2024). k-eμlator: emulating clustering effects of the k-essence dark energy. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 536(4), s. 3445–3465. doi: 10.1093/mnras/stae2775. Fulltekst i vitenarkiv
• Bose, B.; Gupta, A. Sen; Fiorini, B.; Brando, G.; Hassani, Farbod & Baker, T. [Vis alle 12 forfattere av denne artikkelen] (2024). Matter power spectra in modified gravity: a comparative study of approximations and N-body simulations. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 536(1), s. 664–683. doi: 10.1093/mnras/stae2562. Fulltekst i vitenarkiv
• Eckmann, Jean-Pierre; Hassani, Farbod & Zaag, Hatem (2023). Instabilities appearing in cosmological effective field theories: when and how? Nonlinearity. ISSN 0951-7715. 36(9), s. 4844–4861. doi: 10.1088/1361-6544/ace769. Fulltekst i vitenarkiv
• Adamek, J.; Angulo, R. E.; Arnold, C.; Baldi, Marco; Biagetti, M. & Bose, B. [Vis alle 133 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Hassani, Farbod; Adamek, Julian; Kunz, Martin; Shi, Pan & Wittwer, Peter (2023). A generic instability in clustering dark energy? Classical and Quantum Gravity. ISSN 0264-9381. 40(15). doi: 10.1088/1361-6382/ace148. Fulltekst i vitenarkiv Vis sammendrag

  In this paper, we study the effective field theory (EFT) of dark energy (DE) for the k-essence model beyond linear order. Using particle-mesh N-body simulations that consistently solve the DE evolution on a grid, we find that the next-to-leading order in the EFT expansion, which comprises the terms of the equations of motion that are quadratic in the field variables, gives rise to a generic instability in the regime of low speed of sound (high Mach number). We rule out the possibility of a numerical artefact by considering simplified cases in spherically and plane symmetric situations analytically. If the speed of sound vanishes exactly, the non-linear instability makes the evolution singular in finite time, signalling a breakdown of the EFT framework. The case of finite (but small) speed of sound is subtle, and the local singularity could be replaced by some other type of behaviour with strong non-linearities. While an ultraviolet completion may cure the problem in principle, there is no reason why this should be the case in general. As a result, for a large range of the effective speed of sound cs, a linear treatment is not adequate.

• Christiansen, Øyvind; Hassani, Farbod; Jalilvand, Mona & Mota, David (2023). Asevolution: a relativistic N-body implementation of the (a)symmetron. Journal of Cosmology and Astroparticle Physics (JCAP). 2023(5). doi: 10.1088/1475-7516/2023/05/009. Fulltekst i vitenarkiv Vis sammendrag

  We present asevolution, a cosmological N-body code developed based on gevolution, which consistently solves for the (a)symmetron scalar field and metric potentials within the weak-field approximation. In asevolution, the scalar field is dynamic and can form non-linear structures. A cubic term is added in the symmetron potential to make the symmetry-broken vacuum expectation values different, which is motivated by observational tensions in the late-time universe. To study the effects of the scalar field dynamics, we also implement a constraint solver making use of the quasi-static approximation, and provide options for evaluating the background evolution, including using the full energy density averaged over the simulation box within the Friedmann equation. The asevolution code is validated by comparison with the Newtonian N-body code ISIS that makes use of the quasi-static approximation. There is found a very small effect of including relativistic and weak-field corrections in our small test simulations; it is seen that for small masses, the field is dynamic and can not be accurately solved for using the quasi-static approximation; and we observe the formation of unstable domain walls and demonstrate a useful way to identify them within the code. A first consideration indicates that the domain walls are more unstable in the asymmetron scenario.

• Hassani, Farbod; Adamek, Julian; Durrer, Ruth & Kunz, Martin (2022). Biased tracers as a probe of beyond-ΛCDM cosmologies. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 668. doi: 10.1051/0004-6361/202244405. Fulltekst i vitenarkiv Vis sammendrag

  Cosmological models beyond ΛCDM, such as those featuring massive neutrinos or modifications of gravity, often display a characteristic change (scale-dependent suppression or enhancement) in the matter power spectrum when compared to a six-parameter ΛCDM baseline. It is therefore a widely held view that constraints on those models can be obtained by searching for such features in the clustering statistics of large-scale structure. However, when using biased tracers of matter in the analysis, the situation is complicated by the fact that the bias also depends on cosmology. Here we investigate how the selection of tracers affects the observed signatures for two examples of beyond-ΛCDM cosmologies: massive neutrinos and clustering dark energy ( k -essence). We study the signatures in the monopole, quadrupole, and hexadecapole of the redshift-space power spectra for halo catalogues from large N -body simulations and argue that a fixed selection criterion based on local attributes, such as tracer mass, leads to a near loss of signal in most cases. Instead, the full signal is recovered only if the selection of tracers is done at fixed bias. This emphasises the need to model or measure the bias parameters accurately in order to get meaningful constraints on the cosmological model.

• Fard, Mohammad Ansari; Baghkhani, Zahra; Ghodsi, Laya; Taamoli, Sina; Hassani, Farbod & Baghram, Shant (2022). Structure of cosmic web in non-linear regime: the nearest neighbour and spherical contact distributions. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 512(4), s. 5165–5182. doi: 10.1093/mnras/stac256. Fulltekst i vitenarkiv Vis sammendrag

  ABSTRACT In non-linear scales, the matter density distribution is not Gaussian. Consequently, the widely used two-point correlation function is not adequate anymore to capture the matter density field’s entire behaviour. Among all statistics beyond correlation functions, the spherical contact (or equivalently void function), and nearest neighbour distribution function seem promising tools to probe matter distribution in non-linear regime. In this work, we use haloes from cosmological N-body simulations, galaxy groups from the volume-limited galaxy group and central galaxies from mock galaxy catalogues, to compare the spherical contact with the nearest neighbour distribution functions. We also calculate the J-function (or equivalently the first conditional correlation function), for different samples. Moreover, we consider the redshift evolution and mass-scale dependence of statistics in the simulations and dependence on the magnitude of volume-limited samples in group catalogues as well as the mock central galaxies. The shape of the spherical contact probability distribution function is nearly skew-normal, with skewness and kurtosis being approximately 0.5 and 3, respectively. On the other hand, the nearest neighbour probability distribution function is nearly lognormal, with logarithmic skewness and kurtosis being approximately 0.1 and 2.5, respectively. Accordingly, the spherical contact distribution function probes larger scales compared to the nearest neighbour distribution function, which is influenced by details of structures. We also find a linear relation between the mean and variance of the spherical contact probability distribution function in simulations and mock galaxies, which could be used as a distinguishing probe of cosmological models.

• Hassani, Farbod; Shi, Pan; Adamek, Julian; Kunz, Martin & Wittwer, Peter (2022). New nonlinear instability for scalar fields. Physical Review D. ISSN 2470-0010. 105(2). doi: 10.1103/PhysRevD.105.L021304. Fulltekst i vitenarkiv Vis sammendrag

  In this Letter, we introduce the nonlinear partial differential equation ∂2τπ∝(→∇π)2 showing a new type of instability. Such equations appear in the effective field theory (EFT) of dark energy for the k-essence model as well as in many other theories based on the EFT formalism. We demonstrate the occurrence of instability in the cosmological context using a relativistic N-body code, and we study it mathematically in 3+1 dimensions within spherical symmetry. We show that this term dominates for the low speed of sound limit where some important linear terms are suppressed.

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