Detection of Gravitational Anomaly at Low Acceleration from a Highest-quality Sample of 36 Wide Binaries with Accurate 3D Velocities

TL;DR

This work tests gravity in the low-acceleration regime using precise 3D velocities of 36 carefully vetted wide binaries within 150 pc. It employs a Bayesian 3D elliptical-orbit framework to infer per-binary PDFs of the gravity parameter $\Gamma=\log_{10}\sqrt{\gamma}$ with $\gamma=G/G_{\rm N}$, then consolidates them to obtain a population-level constraint. The main finding is $\Gamma=0.102_{-0.021}^{+0.023}$, i.e., $\gamma=1.600_{-0.141}^{+0.171}$, which falsifies Newtonian gravity at about $5\sigma$ in this regime; four systems approach mildly super-Newtonian speeds consistent with MOND-like expectations. Extensive tests—Speckle imaging, Gaia–Hipparcos proper-motion checks, metallicity consistency, and multi-epoch RVs—support the purity of the clean sample and the robustness of the result, with future larger samples anticipated to tighten the constraints and further probe external-field MOND predictions.

Abstract

We set out to accurately measure gravity in the low-acceleration range $(10^{-11},10^{-9})$ m\,s$^{-2}$ from 3D motions of isolated wide binary stars. Gaia DR3 provides precise measurements of the four sky-plane components of the 3D relative displacement and velocity ($\mathbf{r}, \mathbf{v}$) for a wide binary, but not comparably precise line-of-sight (radial) separation and relative velocity $v_{r}$. Based on our new observations and the public databases/publications, we assemble a sample of 36 nearby (distance $<150$pc) wide binaries in the low-acceleration regime with accurate values of $v_{r}$ (uncertainty $< 100$ m\,s$^{-1}$). Kinematic contaminants such as undetected stellar companions are well under control using various observational diagnostics such as Gaia's ruwe parameter, the color-magnitude diagram, multi-epoch observations of radial velocities, Speckle interferometric follow-up observations, and requiring Hipparcos-Gaia proper motion consistency. For the parameter $Γ\equiv \log_{10}\sqrtγ$ with $γ\equiv G/G_{\rm N}$ (where $G$ is a parameter generalizing Newton's constant $G_{\rm N}$ in elliptical orbits), we find $Γ=0.102_{-0.021}^{+0.023}$, inconsistent with standard gravity at $4.9σ$, giving a gravity boost factor of $γ=1.600_{-0.141}^{+0.171}$. Four wide binaries have 3D relative velocities exceeding their estimated Newtonian escape velocities with $1<v_{\rm obs}/v_{\rm escN}\le1.2$. These systems are unlikely to be chance associations and are expected in a nonstandard paradigm such as Milgromian dynamics (MOND). The hypothesis that Newtonian gravity can be extrapolated to the low-acceleration limit is falsified by this independent study with accurate 3D velocities. Future radial velocity monitoring and Speckle interferometric imaging for larger samples will be useful to refine the present result.

Figures (26)

• Figure 1: (Adapted from Chae:2025b) A general 3D geometry of an elliptical orbit. Here $z^\prime$ represents the line-of-sight (radial) direction in observer's frame with the $+$ sign indicating the direction pointing to the observer. The relative radial velocity refers to $v_{z^\prime}(\equiv v_r)$. See Chae:2025b for the definition of all the other parameters.
• Figure 2: The distribution of nominal uncertainties of $v_r(\equiv {\rm RV}_A - {\rm RV}_B)$ for the raw sample of 306 unique wide binaries assembled from various observations and database/publications.
• Figure 3: Summary of individual (given in parentheses at the top row and the left column) and pairwise time baselines for the various observations of radial velocities used. All given entries represent approximate median values. Hipparcos epoch is for observations of proper motions.
• Figure 4: This figure shows the color-magnitude diagram of stars in the raw sample of 306 wide binaries using Gaia's BP$-$RP color and absolute magnitude $M_G$ in Gaia's $G$ band. All stars in the clean sample are fainter than the color-dependent $M_G$ cut line.
• Figure 5: The distribution of $v_{\rm obs}/v_{\rm escN}$ is shown for the basic-cut sample of 75 wide binaries that may include kinematically contaminated cases. Here $v_{\rm obs}$ is the magnitude of the observed relative 3D velocity between the pair while $v_{\rm escN}$ is the Newtonian escape velocity predicted by 3D elliptical orbit modeling with a fixed value of $G=G_{\rm N}(\Gamma=0)$ or $1.4G_{\rm N}(\Gamma\approx0.07)$, which encompass the current range of the effective gravitational constant for Newtonian and MOND models. Both the distribution of the individual median values and that of random values from the Bayesian MCMC samples are shown. The numbers indicated are from the median distributions. Pseudo-Newtonian (MOND) approximate bound of $1.2$ is based on numerical MOND solutions ChaeMilgrom:2022 in the regime with acceleration $< a_0$ for a test particle assuming one-particle equivalent description.