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Contenuto archiviato il 2024-05-28

Simulating the Dark Universe

Final Report Summary - SIDUN (Simulating the Dark Universe)

The SIDUN project has been conceived with the aim of developing, optimising, and exploiting highly efficient numerical tools specifically designed to perform large simulations of the formation and evolution of cosmic structures in the context of a variety of competing cosmological scenarios beyond the standard model. In this respect, SIDUN is a project lying at the boundary between theoretical and observational cosmology, and makes use of state-of-the-art high-performance computing techniques in order to provide a direct link between these two fields in the view of the advent of the "Precision Cosmology" era that will be inaugurated in the next decade by a number of ambitious observational campaigns at the international level.
In fact, while the standard model of cosmology has been extremely successful over the past 15 years in describing most of the available cosmological data, its theoretical foundations remain poorly understood and highly debated as they present significant inconsistencies with the standard model of particle physics. In particular, the origin of the observed accelerated expansion of the universe, often called "Dark Energy", represents one of the most fundamental open issues in modern physics, and its investigation represents a channel to discover new physics beyond the standard model. The SIDUN project focused specifically on this problem and on whether the observations of cosmological structures at large scales might provide a direct way to test possible alternative models of Dark Energy. In particular, SIDUN concentrated on models of interacting Dark Energy and Modified Gravity, which represent an attempt to explain the observed accelerated expansion as a consequence of a new fundamental force beyond the four known interactions described by standard physics. However, possible extensions of the standard cosmological model might involve also other aspects of physics completely independent of the nature of the accelerated expansion, as e.g. the physical properties of Dark Matter particles or the details of the inflationary expansion in the early universe. These might be characterised by observational effects similar or opposite to the ones expected from Dark Energy or Modified Gravity models, and could therefore significantly complicate the interpretation of observational data. The main objectives of the SIDUN project were therefore identified in two distinct goals, aiming at addressing some of the problems described above:
- The first objective of the project was to develop and optimize suitable numerical techniques to run large and accurate N-body simulations for a wide variety of cosmological scenarios beyond the standard model, both in the context of Dark Energy and Modified Gravity and of other independent extensions of the model as non-standard Dark Matter particle candidates, massive neutrinos, or non-standard realisations of cosmic inflation;
- The second objective of the project was to exploit these specifically-designed and highly-efficient algorithms to perform a large number of simulations using cutting-edge high-performance computing facilities across Europe to investigate the main observational effects of the corresponding cosmological scenarios and to test their possible observational degeneracies in order to assess the true discriminating power of present and future surveys in the era of Precision Cosmology.

In order to achieve these main objectives, a significant amount of work has been devoted in the first year of the SIDUN project towards the implementation of the algorithms, while the second year of the project has been mainly devoted to performing and analysing the simulations. More specifically, the development phase has been focused on implementing several non-standard cosmological scenarios as independent modules of the N-body code GADGET-3 (one of the most widely used cosmological N-body codes in the community) either as an individual enterprise, or in collaboration with other scientists in Europe, thereby starting new collaborations or strengthening the existing ones. The simulations and data analysis phase have then mostly focused on individual and highly competitive simulations of interacting Dark Energy and Modified Gravity models on one side, and on combined simulations of each of these two different scenarios of the cosmic acceleration with other independent extensions of the standard model, including massive neutrinos, Warm Dark Matter, and inflationary models providing a significant level of non-Gaussianity in the primordial density field. The individual simulations have allowed to identify and investigate in detail the main observational signatures of interacting Dark Energy and Modified Gravity models, significantly extending the results obtained in previous works (e.g. by significantly extending the CoDECS project, see www.marcobaldi.it/CoDECS/ and see the CoDECS logo attached to this document) and providing a large amount of publicly available simulations data that have been used by the community also for independent scientific investigations. The combined simulations, instead, have highlighted for the first time the issue of the strong observational degeneracy between f(R) Modified Gravity models and the cosmological background of massive neutrinos, showing how a combination of these two extensions of the standard cosmological model might appear indistinguishable from the standard model itself through a large number of observational probes (see the attached figure). The latter result represents a highly relevant piece of information for a correct interpretation of the large amount of cosmological data that will become available in the next decade.

The path that was started with the SIDUN project is expected to deliver further results in the next years, extending the benefits of the performed research tasks well beyond the end of the SIDUN funding period. In particular, the results obtained during the SIDUN project will be exploited in several different ways by the involved parties (the Researcher and the Host Institution) as well as by the European scientific community at large.
First of all, a large amount of simulations data, with different levels of post-processing making the data suitable for a wide range of scientific applications and analysis, have been stored for a long-term exploitation on a dedicated server (purchased on the SIDUN overhead fundings) that has been equipped with a simple web-interface allowing a general access to all the data through any web browser (see http://apps.difa.unibo.it/files/people/Str957-cluster/). Secondly, the basic idea of investigating the impact of Cosmic Degeneracies on the discriminating power of future large galaxy surveys (which was pioneered in the SIDUN project by considering the joint effects of Modified Gravity and massive neutrino models) will represent a driving concept in the development of the next generation of large N-body simulations aimed at cosmological model selection. In particular, the results obtained by the SIDUN project have been included in the planning of the simulations programme of the Euclid collaboration of the European Space Agency. Finally, the numerical techniques developed during the project will be applied to a larger range of non-standard cosmological models in the future, making the non-linear regime of structure formation as a reliable probe for such alternative scenarios.