Search for dark energy and dark matter Hidden from human view, dark energy and dark matter fascinates and intrigues our imaginations. At the same time, EU-funded scientists prepare for an ambitious mission to map millions of galaxies to shine light on the invisible universe. Fundamental Research Space © sakkmesterke, Shutterstock The European Space Agency’s Euclid space telescope will be equipped with an infrared camera to track the distribution of galaxies and measure cosmic acceleration. Scientists think that the accelerating expansion of the universe is driven by dark energy. In the EU-funded project DRKFRCS (Disambiguating the invisible universe), scientists focused on testing whether the cosmological constant is apt to describe the expanding universe. Irrespective of acceleration, cosmology provides the opportunity to test the extent to which general relativity describes gravity at galactic scales. In the last decades, numerous codes had been developed to solve the Boltzmann equation in the presence of gravity. The output of all these codes is either used to constrain cosmological parameters or generate initial conditions for simulations of small-scale physics. The DRKFRCS team expanded the Cosmic linear anisotropy solving system (CLASS) to obtain constraints on modified gravity models with recent observational data. Additionally, Horndeski in the cosmic linear anisotropy solving system (hi-class) can be used to investigate relativistic effects on ultra-large scales. In the code publicly released in June 2016, changes had been made to allow for the modelling of the dynamics of dark energy models that belong to the class of Horndeski theories. To date, hi-class is still under development with new features added, to widen the choice of models. Before the end of DRKFRCS, scientists discovered an unknown connection between models of dark energy that modify general relativity as well as gravitational waves. They showed direct correspondence between the appearance of anisotropic stress in cosmological solutions and a modification in the propagation of gravitational waves. Anisotropic stress is frequently observed in Horndeski theories as well as other general classes of models extending gravity. Specifically, it splits the values of the two scalar gravitational potentials. The ratio of the two potentials is a model-independent observable that Euclid should be able to measure with high precision. Moreover, project results open the way for a completely different probe, the detection of gravitational waves to search for dark energy. Keywords Dark energy, dark matter, cosmic acceleration, DRKFRCS, gravitational waves
