Constraints on conformal ultralight dark matter couplings from the European Pulsar Timing Array

TL;DR

This paper uses precise pulsar timing data to set new constraints on the coupling between ultralight scalar dark matter and gravity, improving existing bounds and informing scalar-tensor theories.

Contribution

It introduces a method to constrain conformal couplings of ultralight dark matter using pulsar timing, providing tighter bounds than previous studies.

Findings

Established new upper limits on dark matter coupling strength.

Demonstrated pulsar timing as a sensitive probe for ultralight dark matter.

Connected constraints to scalar-tensor gravity theories.

Abstract

Millisecond pulsars are extremely precise celestial clocks: as they rotate, the beamed radio waves emitted along the axis of their magnetic field can be detected with radio telescopes, which allows for tracking subtle changes in the pulsars' rotation periods. A possible effect on the period of a pulsar is given by a potential coupling to dark matter, in cases where it is modeled with an "ultralight" scalar field. In this paper, we consider a universal conformal coupling of the dark matter scalar to gravity, which in turn mediates an effective coupling between pulsars and dark matter. If the dark matter scalar field is changing in time, as expected in the Milky Way, this effective coupling produces a periodic modulation of the pulsar rotational frequency. By studying the time series of observed radio pulses collected by the European Pulsar Timing Array experiment, we present constraints on the coupling of dark matter, improving on existing bounds. These bounds can also be regarded as constraints on the parameters of scalar-tensor theories of the Fierz-Jordan-Brans-Dicke and Damour-Esposito-Far\`{e}se types in the presence of a (light) mass potential term.