Skip to main content
European Commission logo print header

GRavity from Astrophysical to Microscopic Scales

Periodic Reporting for period 3 - GRAMS (GRavity from Astrophysical to Microscopic Scales)

Reporting period: 2022-04-01 to 2023-09-30

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. These detectors have collected tens of 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.
In the class of effective field theories with screening mechanisms (i.e. those reproducing general relativity on local scales and deviating from it on large cosmological scales), we have selected those in agreement with experimental constraints, and notably with the recent observation that the speed of light and the speed of gravitational waves must be equal to within a mind-boggling precision of a part in 1.e15 (i.e. a part in a million of billions!). This has restricted our "theory space" essentially to a single gravitational theory, K-essence. The latter features a screening mechanism protecting local scales from unwanted deviations from general relativity, while allowing those deviations to run unbridled on cosmological scales. Even though the theory passes the existing constraints on the speed of gravitational waves, studies of the generation of these waves were not available at the start of the project, preventing a detailed comparison of the predictions of these theories with LIGO/Virgo data. The main problem was exquisitely mathematical, as no one knew how to set up a computer simulations of neutron star/black hole binaries in a "stable" and "well posed" way (i.e. in a way in which numerical inaccuracies would not blow up as the simulation proceeds). The main technical achievement of GRAMS' first half was exactly to find such a "well posed" formulation, which allowed us to perform the very first numerical simulations of binary neutron stars (and also of supernova collapse) in an effective field theory of interest for cosmology.
Thanks to our well posed formulation, we have been able to compute the gravitational wave signal from a system of two neutron stars and from the collapse of a neutron star in K-essence, the only effective scalar-tensor theory of the Dark Sector that remains viable to date. We have observed that the screening mechanism is inefficient in these systems, unlike in the case of the solar system, potentially allowing one to use existing and future gravitational wave data to test these theories and thus to understand the Dark Sector. Indeed, we have found that both binary and collapsing neutron stars generate gravitational waves that differ quite significantly from those predicted by general relativity, at least at low frequencies. In the second part of the project, we will therefore attempt a detailed comparison of these predictions to existing low frequency data, e.g. from binary pulsars and with future (simulated) data from space borne gravitational wave interferometers such as LISA.
Snapshots (at different times) of a simulation of two neutron stars in K-essence