Testing the universality of free fall towards dark matter with radio pulsars

TL;DR

This paper proposes using radio pulsar timing of neutron star-white dwarf systems to test the universality of free fall towards dark matter, aiming to improve constraints on dark matter couplings by leveraging celestial dynamics.

Contribution

It introduces a novel method using binary pulsar orbital dynamics to constrain dark matter's differential coupling to matter, surpassing previous terrestrial experiments.

Findings

Set a new limit on dark matter coupling parameter using pulsar data.

Showed potential for significant improvement near the Galactic center.

Demonstrated the method's sensitivity to dark matter density spikes.

Abstract

The violation of the weak equivalence principle (EP) in the gravitational field of the Earth, described by the E\"otv\"os parameter $\eta_\oplus$, was recently constrained to the level $\left|\eta_\oplus\right| \lesssim 10^{-14}$ by the MICROSCOPE space mission. The E\"otv\"os parameter $\eta_{\rm DM}$, pertaining to the differential couplings of dark matter (DM) and ordinary matter, was only tested to the level $\left| \eta_{\rm DM} \right| \lesssim 10^{-5}$ by the E\"ot-Wash group and lunar laser ranging. This test is limited by the EP-violating driving force in the Solar neighborhood that is determined by the Galactic distribution of DM. Here we propose a novel celestial experiment using the orbital dynamics from radio timing of binary pulsars, and obtain a competing limit on $\eta_{\rm DM}$ from a neutron star (NS) -- white dwarf (WD) system, PSR J1713+0747. The result benefits from the large material difference between the NS and the WD, and the large gravitational binding energy of the NS. If we can discover a binary pulsar within $\sim 10$ parsecs of the Galactic center, where the driving force is much larger in the expected DM spike, precision timing will improve the test of the universality of free fall towards DM and constrain various proposed couplings of DM to the Standard Model by several orders of magnitude. Such a test probes the hypothesis that gravity is the only long-range interaction between DM and ordinary matter.