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Astrophysics for the Dark Universe: Cosmological simulations in the context of dark matter and dark energy research

Periodic Reporting for period 3 - COSMO_SIMS (Astrophysics for the Dark Universe: Cosmological simulations in the context of dark matter and dark energy research)

Reporting period: 2019-09-01 to 2020-09-30

Cosmological simulations take the pivotal role when linking cosmological and astrophysical models to the reality of our Universe and there is no doubt that they will be of continually growing importance with the increasing area and depth of upcoming surveys. So far, observations continually seem to confirm that the concordance cosmological ΛCDM model very well describes the cosmological structure of our Universe. While its phenomenological successes are thus impressive, its physical foundation – explaining the nature of dark matter and the mystery of cosmic acceleration – is not yet understood and among the most pressing and profound questions of contemporary physics. Any discovery opens the door to new physics.

Cosmological observations will test this model with increasing rigour, searching for cracks at the seams of concordance cosmology, and complement laboratory efforts of particle physics experiments at this “cosmic frontier”. Some tantalising inconsistencies between different observations have recently emerged, but they have still to stand the test of time and more data. Current and upcoming surveys push the limits of traditional astronomy, covering nearly the entire sky, imaging most galaxies out to depths when the Universe was only about half of its current age, and detecting gas out to much further distances. The European Union is heavily invested in these efforts through various space missions (e.g. e-Rosita, Euclid, …) as well as ground based observatories (e.g. VLT with its multitude of instruments, LOFAR, SKA). The data accumulated by these multi-wavelength surveys will have to be confronted with the predictions of increasingly better astrophysical models in cosmological context. This will enable us to leverage their full power and to obtain robust constraints on the properties of the dark constituents of our Universe and the physics that underlies them.

This project tackles crucial issues in the broad context of relating theoretical models on the one hand, with large-scale observations on the other hand, through numerical simulations. Specifically, this project will improve each of three main pillars: (1) the modelling of dark matter through novel techniques, (2) the modelling of baryonic processes related to astronomical observables important for cosmology, as well as (3) the way we generate and disseminate initial conditions for cosmological simulations, as a crucial step to a reproducible numerical universe.
Dark matter modelling:

We have developed and implemented a hybrid phase-space tessellation/N-body scheme that is able to combine the advantages of tessellation methods (Hahn+2013, Hahn&Angulo 2016) to provide a continuous density field in lower density regions, with the advantages of N-body methods in regions of rapid phase mixing in high density regions (see Figure 1)

We have conducted a thorough study of the premature disruption of subhaloes in N-body simulations and developed a library of idealised simulations of subhalo disruption that can serve as a foundation to develop physical models for the survival rates of subhaloes/satellites in cold dark matter haloes such as our Milky. The simulation data together with a non-parametric machine learned model is available through this website:

Large-scale structure dynamics: We have studied the dynamics of large-scale structure and measured, to our knowledge for the first time using N-body simulations the anisotropic stress that arises after anisotropic shell-crossing in multi-stream regions of the evolving dark matter fluid.

Initial conditions:

We have developed a web-based database and interface called “cosmICweb” to explore numerical Universes, select objects for re-simulations and provide unique identifiers to objects. This already allows to e.g. easily select a set of Milky Way like object at high redshift, and resimulate it with a simulation code of your choice, and then pass on the ‘ID’ of that object (e.g. in a publication) for other researchers to study and simulate the same object. The database is already functional, we are currently integrating also the dataset from the Eagle simulation (Schaye et al.) and will release it publicly for beta-testing in the coming months. See attached Figure 2a/b. The development version is hosted at

We have begun the overhaul of the MUSIC initial conditions generator. This will be done in two steps: The first step will be a single resolution version of MUSIC 2.0 that includes higher order perturbation theory (up to 3LPT) and directly integrates the CLASS code. It will be released publicly by the end of 2019.

Regarding initial conditions for baryons in Eulerian (grid) codes, we have made major progress through the development of semiclassical techniques that practically allow to perform Lagrangian perturbation theory for a Eulerian field (cf. Uhlemann et al. 2019). This can dramatically increase the accuracy of baryon initial conditions for codes like RAMSES, ART or ENZO.

Baryon modelling:

We have just begun studying the impact of anisotropic thermal conduction at variance with AGN feedback on cosmological hydrodynamical simulations of galaxy cluster formation and evolution with a particular focus on cosmological observables from clusters.
All results outlined above, regarding the DM modelling, disruption modelling of substructures, semiclassical techniques for IC generation have been beyond the state of the art of the field. We expect to exploit them in numerical simulations and make all the tools publicly available by the end of the project.
the halo/galaxy sample selection in cosmICweb
the halo property explorer in cosmICweb
the hybrid LPSE/N-body method