The ability to observe, measure and model cosmological phenomena on large scales has improved significantly over the last few decades, bringing with it a wealth of new data. Analysis points to important discrepancies in the present theories of fundamental particles and the force of gravity. Various proposals have attempted to account for the missing matter and missing energy, also called the mass–energy discrepancy. EU-funded researchers working on the project 'Cosmological tests of gravity' (COSMOGRAV) have delivered an analytical framework to test hypotheses surrounding this most important open question in cosmology. Cold dark matter is postulated to be a missing component of matter invisible to photons. This simple amendment to Einstein's General Theory of Relativity explains much but predicts more structure than exists. It also shows that the expansion of the Universe is slowing down whereas, in fact, it is known to be speeding up. To address this problem, scientists have proposed the existence of dark energy as well, but its nature remains a mystery. Neither cold dark matter nor dark energy have ever been directly detected, but both are founded in Einstein's theory. COSMOGRAV set out to investigate whether a new theory of gravity is needed or a new theory of matter. A new theory of gravity could solve both problems with the same solution. To study alternative theories of gravity, scientists developed parametric deviations from general relativity on a cosmic scale. The framework has been used successfully to describe a very wide range of gravity theories. Researchers also developed techniques to measure deviations from general relativity and to evaluate dark energy using cosmological observations. Analyses led to important constraints on theories of gravity based on scalar rather than vector fields. Finally, scientists studied the possibility that dark matter and dark energy are coupled, providing insight into new ways this might happen. The COSMOGRAV framework paves the way to novel investigations of the interactions among dark matter, dark energy and gravity, many of which have already begun. Results will have far-reaching implications regarding the formation, nature and eventual fate of the Universe.
Gravity, universe, relativity, dark matter, dark energy, cosmological phenomena, mass–energy discrepancy, Theory of Relativity, theory of gravity, theory of matter