We present the first measurement of the Weyl potential at four redshifts bins using data from the first three years of observations of the Dark Energy Survey (DES). The Weyl potential, which is the sum of the spatial and temporal distortions of the Universe's geometry, provides a direct way of testing the theory of gravity and the validity of the $Λ$CDM model. We find that the measured Weyl potential is 2.3$σ$, respectively 3.1$σ$, below the $Λ$CDM predictions in the two lowest redshift bins. We show that these low values of the Weyl potential are at the origin of the $σ_8$ tension between Cosmic Microwave Background (CMB) measurements and weak lensing measurements. Interestingly, we find that the tension remains if no information from the CMB is used. DES data on their own prefer a high value of the primordial fluctuations, followed by a slow evolution of the Weyl potential. A remarkable feature of our method is that the measurements of the Weyl potential are model-independent and can therefore be confronted with any theory of gravity, allowing efficient tests of models beyond General Relativity.

Séparer la distorsion du temps de celle de l'espace dans l'espace-temps: tel est l'objet de recherche d'une équipe de l'Université de Genève. Les scientifiques mettent à l'épreuve les équations d'Einstein et d'Euler, aux confins de l'Univers, pour résoudre les mystères de la matière et de l'énergie noires.

Tous les vendredis, "CQFD" reçoit un homme ou une femme de science pour parler de son travail et de ses recherches. Aujourd'hui, Stéphane Délétroz a invité la cosmologiste Camille Bonvin, professeure à la Section de physique de l'Université de Genève (Unige). Une heure pour faire connaissance avec cette spécialiste de l'Univers et parler de son expansion, d'énergie sombre, de matière sombre et d'ondes gravitationnelles.

Modifications of General Relativity generically contain additional degrees of freedom that can mediate forces between matter particles. One of the common manifestations of a fifth force in alternative gravity theories is a difference between the gravitational potentials felt by relativistic and non-relativistic particles, also known as "the gravitational slip". In contrast, a fifth force between dark matter particles, due to dark sector interaction, does not cause a gravitational slip, making the latter a possible smoking gun of modified gravity. In this article, we point out that a force acting on dark matter particles, as in models of coupled quintessence, would also manifest itself as a measurement of an effective gravitational slip by cosmological surveys of large-scale structure. This is linked to the fact that redshift-space distortions due to peculiar motion of galaxies do not provide a measurement of the true gravitational potential if dark matter is affected by a fifth force. Hence, it is extremely challenging to distinguish a dark sector interaction from a modification of gravity with cosmological data alone. Future observations of gravitational redshift from galaxy surveys can help to break the degeneracy between these possibilities, by providing a direct measurement of the distortion of time. We discuss this and other possible ways to resolve this important question.