General relativity was proposed by Einstein more than a century ago to describe the workings of gravitation on scales ranging from those of our everyday experience to the largest scales of the cosmos. This theory has been tested to exquisite precision on Earth and in the solar system, but on large cosmological scales it can successfully explain what we observe only if we assume that about 70% of the universe is made of Dark Energy and about 25% of Dark Matter. Since no direct evidence exists of these dark components, the possibility remains that in order to describe and understand the universe on large scales, one should instead modify the theory of general relativity. Such modifications of general relativity, however, need to possess an intrinsic "screening" mechanism allowing them to reduce essentially to general relativity on the scales of the Earth/solar system (where experimental tests are very precise), screening unwanted deviations away from it.
A novel probe of these "screening mechanisms", and therefore of these effective theories of the Dark Sector, could be provided by the growing set of gravitational wave observations by LIGO and Virgo, as well as the future observations by LISA. These detectors are sensitive to gravitational wave signals from systems of two black holes, two neutron stars, or even a neutron star and a black hole. Gravitational wave observatories record even the merger of these systems, probing situations in which speeds are comparable to the speed of light and gravitational fields extreme. The consistency of general relativity with these observations has been ascertained by performing computer simulations of binary black holes/neutron stars, and comparing to the data. The same comparison has not been performed in effective theories of the Dark Sector, for which it is not even mathematically clear how to study the evolution of a two body system in fully dynamical regimes. The goal of GRAMS is exactly that of filling this gap, i.e. to perform computer simulations of the merger of two compact objects (black holes or neutron stars) in effective field theories of Dark Energy and Dark Matter, in order to compare the predictions of these theories to gravitational wave observations. This will give hints on the very nature of Dark Energy and Dark Matter, and therefore on what constitutes 95% of our Universe.
GRAMS reached this goal by simulating gravitational waves from several astrophysical scenarios: two neutron stars orbiting each other, a neutron star merging with a black hole, and the collapse of a neutron star—all within K-essence, the only remaining viable scalar-tensor theory of the Dark Sector. These simulations revealed small but clear deviations from the predictions of general relativity, due to a partial failure of the theory’s built-in screening mechanism. In particular, both neutron star binaries and neutron star–black hole systems were found to emit gravitational waves that differ notably from general relativity in the quadrupole mode. The effect is even stronger in the case of stellar collapse, which produces a significant low-frequency scalar monopole gravitational wave signal—something that general relativity does not predict. As low-frequency data from future detectors like LISA or next-generation ground-based observatories become available, these findings will offer a new way to test alternative theories of Dark Energy and probe the nature of the Dark Sector.
