New constraints on the presence of ultralight dark matter in the Milky Way

Dark matter, composed of particles that do not reflect, emit or absorb light, is expected to make up most of the matter in the universe. Its lack of interactions with light, however, prevents its direct detection by conventional experimental methods.

Physicists have been trying for decades to devise alternative methods to detect and study dark matter, but many questions about its nature and presence in our galaxy remain unanswered. The Pulsar Timing Array (PTA) experiments attempted to probe the presence of so-called ultralight dark matter particles by examining the timing of a set of millisecond-long galactic radio pulsars (i.e., celestial objects that emit regular radio pulses of one millisecond).

The European Pulsar Timing Array, a multinational team of researchers based at different institutes that uses 6 radio telescopes across Europe to observe specific pulsars, recently analyzed the second wave of collected data. Their article, published in Physical Examination Lettersimposes tighter constraints on the presence of ultralight dark matter in the Milky Way.

“This paper was essentially the result of my first doctoral project,” Clemente Smarra, co-author of the paper, told Phys.org. “The idea came about when I asked my supervisor if I could conduct research focused on the science of gravitational waves, but from a particle physics perspective. The main goal of the project was to limit the presence so-called ultralight dark matter in our galaxy.

Ultralight dark matter is a hypothetical candidate for dark matter, composed of very light particles that could potentially solve long-standing mysteries in the field of astrophysics. The recent study by Smarra and his colleagues aimed to probe the possible presence of this type of dark matter in our galaxy, via data collected by the European Pulsar Timing Array.

“We were inspired by previous efforts in this area, including the work of Porayko and his collaborators,” Smarra said. “Thanks to the longer duration and improved precision of our dataset, we were able to place tighter constraints on the presence of ultralight dark matter in the Milky Way.”

The recent European Pulsar Timing Array paper makes different hypotheses than those made by other studies carried out in the past. Instead of probing the interactions between dark matter and ordinary matter, it assumes that these interactions occur only via gravitational effects.

“We assumed that dark matter interacts with ordinary matter only through gravitational interaction,” Smarra explained. “That’s a pretty strong statement: in fact, the only sure thing we know about dark matter is that it interacts gravitationally. Simply put, dark matter produces potential wells in which dark matter travels. radio beams from pulsars. But the depth of these wells is periodic in time, so the travel time of radio beams from pulsars to Earth also changes with a distinct periodicity.

By looking for this particular effect in the second wave of data released by the European Pulsar Timing Array, Smarra and his colleagues were able to establish new constraints on the presence of ultralight dark matter around pulsars. The European Pulsar Timing Array has been collecting this data for almost 25 years, using 6 sophisticated radio telescopes located in different locations across Europe.

“Based on our analyses, we can rule out that ultralight particles in a specific range of masses could constitute all of dark matter,” Smarra said. “Therefore, if they were there, we would still need something else to explain what we see. And this result is rather robust, because we focused on the gravitational interaction of dark matter, which is the only thing we know for sure.”

Recent work from the European Pulsar Timing Array shows that ultralight particles with masses of 10−24.0 eV≲m≲10−23.3 eV cannot constitute 100% of the measured local dark matter density and may have at plus a local density of ρ≲0.3. GeV/cm³. These new constraints could guide further research in this area, potentially informing future research into this elusive dark matter candidate.

“I am currently planning to explore whether pulsars have signatures that could tell us more about dark matter,” Smarra added. “Additionally, I am generally interested in PTA science; therefore I would also like to work on astrophysical modeling of supermassive black hole binary systems, which are considered a compelling explanation of the stochastic gravitational wave background that we recently observed.”

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