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Dance of galaxies: testing General Relativity and alternatives using galaxy velocity fields

Periodic Reporting for period 1 - GalaxyDance (Dance of galaxies: testing General Relativity and alternatives using galaxy velocity fields)

Reporting period: 2017-09-04 to 2019-09-03

Cosmology has recently become one of the most important and successful branches of physical sciences. This status was sanctioned by three Nobel Prizes in physics awarded in the recent years for studies in this field (including this year prize for Jim Peebles). The standard cosmological model - LCDM (Lambda Cold Dark Matter) - is very successful in explaining the emergence of large-scale structures and the evolution of the Universe as a whole. Nonetheless, currently it is troubled by several fundamental puzzles regarding the physical nature of our Universe. One of the most pressing issues is to find and understand the physical origin of the observed late-time cosmic acceleration.

One of the most fundamental problems of contemporary physics is to explain the physical mechanism behind the observed acceleration of the Universal expansion. The LCDM model employs General Relativity (GR) as the underlying theory of gravity, assumed to be valid on all scales. When interpreted within GR, the universal acceleration is caused by a cosmological constant of a very small value. However, this is very difficult to reconcile with particle physics which predicts it to be some 50 orders of magnitude larger than measured. This mismatch has motivated vivid research activity on the cosmological constant problem. On the theoretical side we have in particular seen the emergence of a plethora of modified gravity (MG) theories, which attribute the observed acceleration to a breakdown of GR on cosmological scales. On the observational side, many new ambitious astronomical surveys are being deployed to both test GR and Dark Energy and to study the cosmic acceleration in exquisite details; a prime example is the flagship European space mission Euclid to be launched already in 2021. Testing GR and verifying its validity on large scales is one of the main goals of the 21st-century extragalactic astronomy. However, so far Einstein’s theory of gravity has only been rigorously probed on astrophysical scales, i.e. in the Solar System or for compact objects like black holes and neutron stars. At present, by using Einstein's theory to describe the whole observable Universe we make an extrapolation of 15 orders of magnitude.

“GalaxyDance” takes at heart the urgent need to design possible tests of Dark Energy and Dark Matter models. To envisage such tests a new state-of-the-art computer simulations of standard and alternative cosmological models are used. Such “virtual universes” created with a help of supercomputers are then a subject of a high-level analysis that indicate which properties of the observed Universe can be used to test different models and to shed the light on the dark side of the Cosmos.
During execution of the Fellowship a number of new state-of-the-art very large computer simulations were conducted. These simulations follow the formation of the structures in the expanding Cosmos as it cooled down and expanded over its nearly 14 billion of years of evolution. These simulations were the further post-processed to obtain artificial galaxy catalogues. In such a way, the researchers could test efficiency of various astronomical observations for testing the Einstein’s theory of gravity on the cosmological and intergalactic scales. It has been discovered that our location within the grand-scheme of the Cosmic Web has a particular strong impact on the observed properties we infer about the galaxy motions. Such a specific “breaking of the Copernican principle” needs to be address in a delicate manner. It has been found that in order to use the measured local galaxy flows, this new special effects needs to be taken into account. Only then, the data can be used to distinguished between the standard and modified gravity and dark energy models. This is very important as the other results of the conducted research indicate that the galaxy velocity field should contain a strong signal of the potential new physics related to the accelerated expansion of the Universe.

Another important finding consisted in recognising that dedicated artificial galaxy catalogues need to be created in a more consistent with current observations fashion in order to faithfully represent any potential for discovering new gravitational clustering physics. Once a specific anchoring data from astronomical galaxy survey is used many previously considered promising observables turned-out to be less suited for performing tests of theory of gravity. The researchers however, had found that a specific clustering statistics of higher-order nature can be constructed for galaxy data. Such clustering statistics should contain a strong signal of modified physics even when the artificial galaxy catalogue had been designed to met additional observational constraints. This show a great promise to foster new exciting and stringent tests of gravity to be conducted with the astronomical data incoming from the new grand surveys.
The detail assessment and census of all resulting applications and socio-economic impact is a very difficult task due to fundamental research nature of the project. Nonetheless, below I give an itemized list of results and impact this project contributed to:
• The development and build-up of the field of computational cosmology in Poland and in Central Europe. The developments and research in this field is ultimately connected with reduction, analysis and interpretation of all BigData stemming from extra-galactic astronomy observations. Thanks to “GalaxyDance”, the fellow of this action was able to create, a new, internationally competitive research group in computational cosmology. Developing and tightening the links of country’s local astronomical research community with the European and global one.
• Increase of the potential and perspective of active participation of local research and industry in planned ESA and ESO missions. The potential to actively participate in numerous missions and campaigns of ESA & ESO by Polish researchers and local industry companies is yet to be fully exploited. The research and development task that were conducted in “GalaxyDance” will contribute towards increasing chances of Poland’s participation on technological and science levels in these projects. This undoubtedly contribute towards increasing innovative potential of Polish and European Science and Economy in the global level.
• Increase of awareness and knowledge of the General Public in the are of modern cosmology. The ambitious goals of this proposal resulted in several research papers published in high impact factor journals. The fellow had invested a major effort to educate the general audience of the importance and merit of “GalaxyDance” and modern cosmology in general research topics and results. This was achieved not only via use of modern communications channels like the social media (YouTube and Facebook pages), but also using using more traditional media like radio, TV and public science festivals The long-term impact on society that popularisation of science, and especially on research and discoveries made by local scientists, is well recognized and valued in all modern developed countries. This offers excellent training giving fundamental research flagship role in attracting children to science and technology.
The signal of various modified gravity theories as expected in galaxy hierarchical clustering.