Local Universe tests of gravity and dark energy

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One of the most startling discoveries of 21st century astrophysics is that the expansion of the universe is accelerating. Cosmologists have zeroed in on two possible explanations for this challenge: the widely accepted paradigmatic hypothesis is that a cosmological constant (vacuum energy) drives the universe towards increasing its rate of expansion. Despite wide acceptance of this theory in the community, no one knows the true physical nature of the so called “dark energy".A more unorthodox notion is that the law of gravity — general relativity — may be modified on large scales. In this project we aim to test the latter by studying in great detail the formation of the local universe and to use the local universe as a cosmological laboratory to study the nature of gravity and dark energy.Specifically we aim to run constrained simulations of the local universe assuming both hypotheses but focusing on the effect of modifications to gravity, in an effort to provide new independent ways of ruling one in or out. Constrained simulations are cosmological simulations of structure formation where the initial conditions have been constrained by observational data, such that the final result is destined to reproduce the observed cosmographic landscape. Such simulations allow cosmologists to directly model the environment of the Milky Way and Local Group, instead of simulating random statistically similar structures. The German side is a world leader in running and analysing such simulations, while the Polish side is a world leader in testing theories of modified gravity. By combining our expertise we will, for the first time, be able to produce models of the Local Universe under the assumption that the laws of general relativity, one of the foundations of modern physics that has stood prime for over a century, are only an approximation to nature, revealed on small scales. Our project will have a major impact in both the fields of galaxy and structure formation (addressing question of how the local group and local universe formed), as well as in the field of modified gravity ruling in or out specific models that have been put forth in the literature.The value of our cooperation is straight forward: only by combing our complementary expertise is such a study possible. Furthermore because our expertise is so complementary we envision lasting knowledge transfer between the Polish and German side with the German side gaining permanent expertise in modified gravity and the Polish side becoming a player in constrained simulations.In sum, this project - LUSTRE (Local UniverSe Tests of gRavity and dark Energy) - aims at using the Local Universe as a cosmological laboratory to probe the fundamental physics of general relativity and cosmic acceleration.