Farbod Hassani

Updated list of my publications, metrics, and citations available at: ADS Database
 

• 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.

• 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. Full text in Research Archive Show summary

  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 [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.

• 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. Full text in Research Archive Show summary

  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. Full text in Research Archive Show summary

  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. Full text in Research Archive Show summary

  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), p. 575–584. doi: 10.1093/mnras/stae1195. Full text in Research Archive Show summary

  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), p. 1534–1545. doi: 10.1093/mnras/stae1075. Full text in Research Archive
• 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), p. 3445–3465. doi: 10.1093/mnras/stae2775. Full text in Research Archive

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