Periodic Reporting for period 4 - LSSgrav (Testing the law of gravity with novel large-scale structure observables)
Período documentado: 2025-05-01 hasta 2025-10-31
Hubble and Friedman observed in the 30' that our universe is in expansion. According to Einstein's theory of gravity, General Relativity, it was expected that this expansion should slow down as time evolves. Surprisingly, in 1998 two groups of astrophysicists observed the opposite: that the expansion of our universe is accelerating. Since then cosmologists have tried to understand what is causing this acceleration. Two types of mechanisms have been proposed: the first one consists in adding a new form of energy in our universe, called dark energy, that would be responsible for the accelerated expansion. The second possibility is to modify the laws of gravity at large distances, such that gravity itself would make the universe expand faster and faster. In this context, the aim of my project is to test the laws of gravity at very large distances, to understand if General Relativity is still valid at the scale of the universe or if it has to be replaced by another theory of gravity. Since gravity is one of the fundamental laws of physics, it is of great importance to determine what its true nature is.
To achieve this goal, with my team we have designed new methods to test some fundamental properties of gravity at large distances. First we have identified a key quantity, gravitational redshift, which impacts our observations of galaxies. We have constructed a method to measure this quantity and we have demonstrated that it will be detectable with the coming observations from the DESI and Euclid surveys. We are now applying the method to the first data delivered by DESI. Second, we have demonstrated that gravitational redshift, which is sensitive to the distortion of time, is a key quantity to robustly test gravity. Without this observable, deviations from General Relativity would be degenerated with additional forces acting on dark matter, preventing us to test gravity robustly. The distortion of time provides the missing ingredient to test gravity and dark matter without ambiguity and to clearly discriminate between their signatures. Finally, in the course of the project we have developed a method to extract the Weyl potential, which is the sum of the spatial and temporal distortions of the geometry, from gravitational lensing. We have measured this observable in the Dark Energy Survey and used it to test gravity and dark matter. Together, the distortion of time, the velocity of galaxies and the Weyl potential allow us to look for deviations from our standard model of cosmology in an optimal and robust way.
To achieve this goal, with my team we have designed new methods to test some fundamental properties of gravity at large distances. First we have identified a key quantity, gravitational redshift, which impacts our observations of galaxies. We have constructed a method to measure this quantity and we have demonstrated that it will be detectable with the coming observations from the DESI and Euclid surveys. We are now applying the method to the first data delivered by DESI. Second, we have demonstrated that gravitational redshift, which is sensitive to the distortion of time, is a key quantity to robustly test gravity. Without this observable, deviations from General Relativity would be degenerated with additional forces acting on dark matter, preventing us to test gravity robustly. The distortion of time provides the missing ingredient to test gravity and dark matter without ambiguity and to clearly discriminate between their signatures. Finally, in the course of the project we have developed a method to extract the Weyl potential, which is the sum of the spatial and temporal distortions of the geometry, from gravitational lensing. We have measured this observable in the Dark Energy Survey and used it to test gravity and dark matter. Together, the distortion of time, the velocity of galaxies and the Weyl potential allow us to look for deviations from our standard model of cosmology in an optimal and robust way.
With my team, we designed a novel method to measure the distortion of time at cosmological distances. The distortion of time is an effect predicted by General Relativity, which tells us that massive objects in the universe (like stars, galaxies and clusters), distort not only the spatial geometry of the universe, but also the passing of time. This effect has been measured with great precision on Earth and in stars, confirming the validity of General Relativity. With my team we showed that with our new method, it will be possible to measure the distortion of time at cosmological distances for the very first time, with the coming generation of galaxy surveys. To test the validity of our method, we built synthetic catalogues of galaxies, reproducing what we expect to see with the survey DESI, and we successfully applied our method to these catalogues. We are now working on the first measurement of this effect with data from DESI.
We then showed that this new measurement will allow us to perform two important tests. First we will be able to compare the distortion of time with the distortion of space. General Relativity predicts that these two distortions are the same, while alternative theories of gravity generically predict a difference between these two quantities. By measuring them and comparing them we can therefore test the predictions from Einstein.
In addition, this new measurement will also allow us to test the validity of Euler's equation for dark matter. Euler's equation describes how matter moves in a universe with distorted geometry. It has been tested with great precision for ordinary matter, but it has never been tested for the unknown dark matter. By comparing the distortion of time with the velocity of galaxies (that are made by 80 percent of dark matter) we will be able to test Euler's equation for dark matter. This will allow us to determine if additional forces or interactions act on dark matter. With my team, we have shown that these tests will be feasible with surveys like DESI, Euclid and the SKA.
In parallel to this project, we have developed a method to measure the Weyl potential, which is the sum of the temporal and spatial distortion of the geometry, from gravitational lensing. Gravitational lensing describes the deviation of light due to intervening matter on the trajectory. This matter distorts space and time, which in turns deviates light's trajectory. This effect has been used by the community to measure the distribution of matter in the universe. These measurements rely however on the validity of General Relativity. Since our aim is to test the validity of this theory, we exploited the data in a different way, using them to measure directly the Weyl potential at different moments of the history of the universe. We then used this quantity to test the theory of gravity and the properties of dark matter, by building novel model-independent tests.
The results have been published in peer-reviewed journal, including renowned ones like Nature Astronomy, Nature Communications and Physical Review Letters. They have also been presented at various conferences in cosmology, gravity and high energy physics.
Videos explaining these projects in a way accessible to non-scientists can be found on the youtube channel Cosmic Blueshift: https://www.youtube.com/channel/UCXdNkmSmao5QCKp06RzR5Tg(se abrirá en una nueva ventana)
We then showed that this new measurement will allow us to perform two important tests. First we will be able to compare the distortion of time with the distortion of space. General Relativity predicts that these two distortions are the same, while alternative theories of gravity generically predict a difference between these two quantities. By measuring them and comparing them we can therefore test the predictions from Einstein.
In addition, this new measurement will also allow us to test the validity of Euler's equation for dark matter. Euler's equation describes how matter moves in a universe with distorted geometry. It has been tested with great precision for ordinary matter, but it has never been tested for the unknown dark matter. By comparing the distortion of time with the velocity of galaxies (that are made by 80 percent of dark matter) we will be able to test Euler's equation for dark matter. This will allow us to determine if additional forces or interactions act on dark matter. With my team, we have shown that these tests will be feasible with surveys like DESI, Euclid and the SKA.
In parallel to this project, we have developed a method to measure the Weyl potential, which is the sum of the temporal and spatial distortion of the geometry, from gravitational lensing. Gravitational lensing describes the deviation of light due to intervening matter on the trajectory. This matter distorts space and time, which in turns deviates light's trajectory. This effect has been used by the community to measure the distribution of matter in the universe. These measurements rely however on the validity of General Relativity. Since our aim is to test the validity of this theory, we exploited the data in a different way, using them to measure directly the Weyl potential at different moments of the history of the universe. We then used this quantity to test the theory of gravity and the properties of dark matter, by building novel model-independent tests.
The results have been published in peer-reviewed journal, including renowned ones like Nature Astronomy, Nature Communications and Physical Review Letters. They have also been presented at various conferences in cosmology, gravity and high energy physics.
Videos explaining these projects in a way accessible to non-scientists can be found on the youtube channel Cosmic Blueshift: https://www.youtube.com/channel/UCXdNkmSmao5QCKp06RzR5Tg(se abrirá en una nueva ventana)
The coming years will see the avenue of fantastic data from various galaxy, like DESI, Euclid and the SKA. The studies we performed with my team show that with these data it will be possible to test new properties of gravity and of dark matter, that have never been tested so far at cosmological distances. During the grant, with part of my team, we have joined the DESI collaboration, as external collaborators, to apply our novel method to DESI. Our goal for the coming years is, on one hand, to pursue our theoretical work, by designing new methods to test General Relativity and unveil the nature of gravity and dark matter. And on the other hand, we will apply our tests to data, first DESI and later on Euclid and the SKA. This will allow us either to confirm or disprove the validity of General Relativity.