Wide binaries (WBs) offer a unique opportunity to test gravity in the low-acceleration regime. Because they experience gravitational accelerations comparable to those in the outer regions of galaxies, yet are unaffected by dark matter, and their dynamics are well understood. Furthermore, they can be treated as isolated systems because the differential acceleration exerted by the Galaxy on the two stars is negligible.
Thanks to exceptional astrometry from Gaia, it has been possible in recent years to construct large samples of WBs with precise projected separations and transverse velocities (2D analyses). However, these studies have reached conflicting conclusions on whether Newtonian gravity best reproduces observations for systems in the low-acceleration regime (e.g., <~10-9 msec-2 ).
We focused instead on the study of 3D solutions by adding precise and accurate radial velocities to the Gaia data. High-quality radial velocities are essential for identifying and removing multiple systems; when combined with Gaia DR3 parallaxes and proper motions, they allow the orbits of binaries to be tightly constrained.
We first analyzed ESO 3.6m/HARPS spectra for 32 WBs, finding that 31 of them could be represented with a viable, bound Newtonian solution. From a second sample of 12 wide binaries with very large separations (greater than 13,000 AU) observed with ESPRESSO@VLT, we found that for three of the systems, no bound Newtonian orbits were possible. However, these binaries have very large separations and are expected to be disrupted by interactions with the Galaxy or encounters with other stars. Finally, we added a sample of 26 low-mass WBs, because these binaries experience low accelerations at much smaller separations, thereby minimising the probability of disruption by Galactic tides or third bodies.
Performing a Monte Carlo analysis on the entire sample, we find bound Newtonian solutions for 65 out of the 70 WBs; we map the orbital parameter solutions and find that their distribution behaves as expected. This agreement holds even when dividing the data into low- and high-acceleration samples, with the possible exception of the orbital phase. Considering that the unbound systems are likely disrupted binaries, we conclude that Einstein-Newtonian gravity provides a coherent view of wide binaries, and no evidence for new physics is required.
Looking ahead, Gaia DR4 and a new, large sample of WBs currently being observed with ESPRESSO@VLT will provide new insights into this fascinating test.
11/06/2026
Speaker: Luca Pasquini (INAF - Osservatorio Astrofisico di Arcetri)
Tuesday, June 16, at 12 CEST in Aula Gratton
Link to the streaming: https://meet.google.com/vjn-anyj-nww