Periodic Reporting for period 4 - GRAMS (GRavity from Astrophysical to Microscopic Scales)
Okres sprawozdawczy: 2023-10-01 do 2025-03-31
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.
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.
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 gravitational wave 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 was exactly to find such a "well posed" formulation, which allowed us to perform the very first numerical simulations of binary neutron stars, black hole - neutron star systems, and gravitational collapse in an effective field theory of interest for cosmology. We were also able to reproduce aspects of these results analytically, further validating our numerical solutions.
Thanks to our well posed formulation and analytic results, we have been able to compute the gravitational wave signal from a system of two neutron stars, from mixed binaries consisting of a neutron star and a black hole, 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 subtle deviations from the general relativistic behavior arise in these systems, as a result of a partial breakdown of the screening mechanism. Indeed, we have found that both binary neutron stars (or pulsars) and mixed binaries generate gravitational waves that differ quite significantly from those predicted by general relativity in the quadrupole channel. The effect is even more pronounced for stellar collapse, which yields a large scalar gravitational signal (absent in general relativity) at low frequencies. When low frequency gravitational wave data (by next generation ground detectors or LISA) become available, these results will allow for testing effective field theories of Dark Energy and thus understanding the Dark Sector.