Using Pulsars to Probe Waves of Dark Matter

Dark matter is a type of matter that is predicted to make up most of the matter in the universe, yet it is very difficult to detect using conventional experimental techniques, as it does not emit, absorb, or reflect light. While some past studies gathered indirect hints of its existence, dark matter has never been directly observed; thus, its composition remains a mystery.

One hypothesis is that dark matter is made up of axionlike particles with an extremely low mass, broadly referred to as ultralight axionlike dark matter (ALDM). As these particles are exceedingly light, predictions suggest that they would behave more like waves than individual particles on a galactic scale.

The PPTA collaboration, a large team of researchers based at different institutes worldwide, applied a new approach to search for ALDM by cross-correlating polarization data of pulsars, neutron stars that spin rapidly and emit highly regular beams of radio waves. This approach, termed the «Pulsar Polarization Array (PPA),» entails measuring the polarization position angles of a series of pulsars and how they changed over time and with respect to pulsar spatial position.

The approach aims to uncover correlation patterns of these changes that could be explained by the presence of ultralight dark matter. The first results obtained from this PPTA study were published in Physical Review Letters.

«Back to 2021, we proposed to construct a PPA ‘ using data collected from pulsar timing array (PTA) programs, positioning it as an innovative polarimetry tool for exploring astrophysics and fundamental physics,» Tao Liu, corresponding author for the paper, told Phys.org.

«We identified the detection of ALDM through the cosmic-birefringence effect induced by its Chern-Simons coupling as a compelling scientific application for the PPA.»

One of the most intriguing characteristics of ultralight dark matter particles is their predicted wave-like behavior when observed on astronomical scales, which stems from their extremely low mass. The researchers hypothesized that ALDM would prompt the linearly polarized light of a series of pulsars to ripple with a distinct oscillation in its polarization while it is traversing the galactic halo.

«This makes the PPA a remarkable opportunity to investigate this representative dark matter paradigm and advance our understanding of the nature of dark matter,» said Jing Ren, another contact author of the paper.

«Recognizing the potential scientific impact of this proposal, we eagerly began exploring the feasibility of conducting the first real-data PPA analysis for ALDM. Fortunately, the Parkes PTA (PPTA), one of the world’s leading PTA programs, with its extensive long-term pulsar polarization data, presented an ideal platform for this scientific endeavor.»

Searching for ultralight dark matter with pulsar polarization arrays

To first assess their approach to searching for ultralight ALDM, the researchers collected polarization data from rapidly rotating pulsars that were observed by the Parkes radio telescope. This is a radio astronomy observatory located in proximity to the town of Parkes, in Australia.

«We embarked on this ambitious search for ultralight ALDM, scrutinizing the anticipated ripples in the polarization of PPA pulsars that signify a galaxy-wide influence, with strong support from the PPTA team,» explained Liu. «To establish the first PPA, we utilized polarization data from 22 millisecond pulsars provided by the PPTA’s third data release, which spans an observation period of up to 18 years.»

Liu and his colleagues examined the polarization position angle of each of the 22 pulsars and looked at how it varied over time. They removed sources of noise and random fluctuations from the data, then tried to uncover correlation patterns of the polarization signals across the pulsars.

«The development of a suitable analysis framework, including noise modeling, was crucial in our research given the innovative nature of this method,» said Ren.

«Ultimately, by effectively cross-correlating pulsar data within the galaxy, this framework enabled us to identify the key signature of ultralight axion-like dark matter: polarization residuals exhibiting a specific wave pattern over time and vast astronomical distances.»

Informing efforts aimed at detecting ultralight axions

By studying the PPA they identified, the researchers were able to set new constraints on the strength with which ultralight ALDM would interact with light. Their recent efforts suggest that PPAs could open a highly valuable avenue for the detection of the wave-like signatures associated with ultralight ALDM.

«Our research establishes the strongest limits on the Chern-Simons coupling of ultralight ALDM within its central mass range, representing a significant step forward,» said Liu.

«This is the first-ever PPA analysis, and with the capabilities of new-generation radio telescopes like FAST and SKA to monitor more pulsars with greater polarization precision, we are excited about the potential for the PPA to further explore this important dark matter scenario in the coming years.»

In the future, Liu and his colleagues could conduct further analyses and establish more PPAs using newly compiled pulsar polarization datasets. Meanwhile, other teams of physicists could draw inspiration from their work and introduce similar methods to probe ultralight ALDM leveraging astronomical observations.

«The current and upcoming pulsar polarization datasets from various astronomical programs are vast, with the PPTA data being just one example,» added Ren.

«This scientific endeavor can be elevated in the coming decade, further expanding our explorations. Additionally, we are exploring the synergy between pulsar polarization and timing signals. To facilitate a more systematic investigation of the dark matter puzzle and potentially other scientific inquiries, it is essential to combine the PPA with complementary astronomical tools like the PTA.»

More information in arXiv