VErTIGO - VElocities TestIng Gravity and cOsmology

VErTIGO - VElocities TestIng Gravity and cOsmology Testing GR and verifying its validity in the large-scale regime became one of the main goals of the 21st century extragalactic astronomy. This is indeed a pressing issue since GR has only been rigorously tested on Solar System and smaller scales. At present, by using Einstein's theory to describe the whole observable Universe we make an extrapolation of 15 orders of magnitude. Thus in the last decade efforts aimed at delivering precise cosmological GR tests have significantly intensified. The importance and timeliness of this challenge was also recognised by the whole international community and is clearly underlined by multi- billion worth various observational campaigns, space missions and Earth-based laboratories. Research project objectives. In this proposal – VErTIGO (VElocities TestIng Gravity and cOsmology) – I introduce a novel approach that consists of the use of low-order statistics of galaxy velocities and clustering for testing GR and searching for Modified Gravity/Dark Energy signatures on intergalactic scales. This approach has several unique advantages: it is (gravity-) model independent, free of significant galaxy bias and largely unaffected by baryonic physics. The disadvantage is that peculiar velocities can only be measured with sufficient accuracy for a small sample of local galaxies and even then there are potentially significant systematic errors in the calibration of distance indicators and, consequently, in the estimate of the peculiar velocities. But this disadvantage is fully compensated by the fact that modified gravity (MG) signature stands out much more prominently in peculiar velocities rather than in the cosmic density field. Therefore the potential gain is tremendous as peculiar velocities can provide a novel MG/GR test that also constitute an independent consistency check for the GR framework. The second-leg of this project will aim at studying in detail baryonic processes in beyond-GR models, which will allow to use high-order moments of galaxy clustering as a secondary and complementary cosmological gravity probes. Research project methodology. We will employ the cutting-edge methods from computer science (i.e. task- based parallelism) to conduct the new state-of-the-art N-body and hydrodynamical simulations of large-scale structure and galaxy formation for beyond-GR models. The simulations will be used to generate mock galaxy catalogues, which will be analysed against all potential systematic and selection effects of galaxy velocity and redshift surveys. Next, optimal low-order galaxy pairwise and peculiar velocity statistics for testing theory of gravity and growth-rate of structures will be selected. Additionally, we will study the galaxy clustering patterns and the systematic effects induced by bias and baryonic feedback. This will allow for: i) optimal use of standard multipoles of 2-point redshift-space clustering statistics; ii) going beyond 2-point statistics for better accuracy. Finally, applying the developed methodology to existing and incoming velocity and redshift data will provide new stringent cosmological tests of GR. Further, combining measurements from different observables and datasets will provide new consistency checks for GR, that might reveal unknown systematics or new physics. Expected impact of the research project. In the past decade we have witnessed an advent of high-quality and rich galaxy peculiar velocity data, available as of now through many datasets (such as CosmicFlows-3, SFI+ +,6dfGRS). Although the new velocity data opens the window towards novel and independent gravity test, the data alone is not enough to make this test a reality. The key missing ingredients here are N-body and hydro simulations and galaxy mock catalogues of enough quality and size designed and delivered self- consistently for GR and MG models and tailored for specific survey data. This is because non-GR specific signatures are entangled with various systematic effects and the only way to find and underpin them is via use of novel cutting-edge quality MG simulations augmented with robust and realistic galaxy modelling. My holistic approach to MG cosmological studies implemented in this project will fully address this lacking and bridge the gap that is currently hindering the velocity-based gravity tests. The salient physics and methods that I propose to use in VErTIGO will provide independent tests of GR and its alternatives on intergalactic scales with the accuracy better than 10%. The new velocity data coming from the TAIPAN, WALLABY and GAIA surveys combined with the excellent novel framework and methods I will develop in VErTIGO will further improve the accuracy up to 5%.