Low-Acceleration Gravitational Anomaly from Bayesian 3D Modeling of Wide Binary Orbits: Methodology and Results with Gaia Data Release 3

Abstract

Isolated wide binary stars provide natural laboratories to directly probe gravity for Newtonian acceleration $g_{\rm{N}}\lesssim 10^{-9}$ m s$^{-2}$. Recent statistical analyses of wide binaries have been performed only with sky-projected relative velocities $v_p$ in the pairs. A new method of Bayesian orbit modeling exploiting three relative velocity components including the radial component $v_r$ is developed to measure the gravitational anomaly parameter $\Gamma\equiv\log_{10}\sqrt{\gamma_g}\equiv\log_{10}\sqrt{G_{\rm{eff}}/G_{\rm{N}}}$ where $G_{\rm{eff}}$ is the effective gravitational constant for pseudo-Newtonian elliptical orbits, while $G_{\rm{N}}$ is Newton's constant. The method infers individual probability distributions of $\Gamma$ and then combines the independent distributions to obtain a consolidated distribution in a specific range of $g_{\rm{N}}$. Here the method is described and applied to a sample of 312 wide binaries in a broad dynamic range $10^{-11.0}\lesssim g_{\rm{N}}\lesssim 10^{-6.7}$ m s$^{-2}$ with $v_r$ uncertainties in the range $168<\sigma_{v_r}<380$ m s$^{-1}$ selected from the Gaia DR3 database. The following results are obtained: $\Gamma = 0.000\pm 0.011$ ($N_{\rm{binary}}=125$) for the high-acceleration regime ($10^{-7.9} \lesssim g_{\rm{N}} \lesssim 10^{-6.7}$ m s$^{-2}$) agreeing well with Newton, but $\Gamma = 0.085\pm 0.040$ or $\gamma_g=1.48_{-0.23}^{+0.33}$ (35) for a MOND regime ($10^{-11.0}\lesssim g_{\rm{N}}\lesssim 10^{-9.5}$ m s$^{-2}$) and $\Gamma = 0.063\pm 0.015$ or $\gamma_g=1.34_{-0.08}^{+0.10}$ (111) for a MOND+transition regime ($10^{-11.0}\lesssim g_{\rm{N}}\lesssim 10^{-8.5}$ m s$^{-2}$). These results show that a gravitational anomaly is evident for $g_{\rm{N}}\lesssim 10^{-9}$ m s$^{-2}$ and $\Gamma$ in the MOND regime ($\lesssim 10^{-9.5}$ m s$^{-2}$) agrees with the prediction ($\approx 0.07$) of MOND gravity theories.