Scientists plan to use dead star “clocks” to illuminate the universe’s most mysterious stuff: dark energy.
These timekeepers are actually pulsars, or rapidly spinning neutron stars born when stars at least eight times as massive as the sun die. The extreme conditions of neutron stars make them ideal laboratories for studying physics in environments found nowhere else in the universe.
So-called “millisecond pulsars” can spin hundreds of times a second and blast beams of electromagnetic radiation from their poles like cosmic lighthouses, which sweep across space. They get their name because when they were initially spotted, these neutron stars appeared to be pulsing, increasing in brightness when their beams were pointed directly at Earth.
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The ultraprecise timing of millisecond pulsars’ brightness variation means they can be used collectively as cosmic timepieces in “pulsar-timing arrays.” These arrays are so precise they can measure gravitational disturbances in the fabric of space and time, united as a four-dimensional entity called “spacetime,” which could be the ideal way of hunting dark matter.
“Science has developed very precise methods to measure time,” pulsar timing array researcher John LoSecco, of the University of Notre Dame, said in a statement. “On Earth, we have atomic clocks, and in space, we have pulsars.”
Calling time on the dark matter mystery
Dark matter is so mysterious because it doesn’t interact with light or with ordinary matter — or, if it does, it does so very weakly and we can’t detect it. “Ordinary matter” is made up of atoms comprised of electrons, protons and neutrons that interact with light and matter, so scientists know dark matter must be made of other particles.
Despite not interacting with light, dark matter does have a gravitational influence, and its presence can be inferred when this influence affects light and indeed ordinary matter. It is the effect of this gravitational influence on light that LoSecco and colleagues aimed to exploit using pulsars.
According to Albert Einstein’s theory of general relativity, objects with mass curve the very fabric of spacetime, and gravity arises from this curvature. When light passes this curvature, its path is also diverted. This can change the travel time of light, causing light from the same distant body to arrive at Earth at different times, in theory, “slowing it down” (the speed of light isn’t actually changed; it’s the distance it travels that changes).
Dark matter has mass, and thus, concentrations of this mysterious form of matter can warp spacetime, too. Thus, the path of light from distant objects is curved, and its arrival time is delayed when it passes concentrations of dark matter. This effect is called “gravitational lensing,” with the intervening body changing the path of light called a “gravitational lens.”
LoSecco and colleagues examined data collected from 65 pulsars in the Parkes Pulsar Timing Array. They observed around 12 incidents that indicated variations and delays in the timings of the pulsars, which usually have nanosecond accuracy.
This indicates the radiowave beams from these dead star cosmic lighthouses are traveling around a warp in space caused by an unseen concentration of mass somewhere between the pulsar and the telescope. The team theorizes these invisible masses are candidates for dark matter “clumps.”
“We take advantage of the fact that the Earth is moving, the sun is moving, the pulsar is moving, and even the dark matter is moving,” LoSecco said. “We observe the deviations in the arrival time caused by the change in distance between the mass we are observing and the line of sight to our ‘clock’ pulsar.”
The deviations observed by the team are absolutely tiny. To illustrate this, a body with the mass of the sun would cause a delay in pulsar radiowaves of about 10 microseconds. The proposed dark matter delay deviations seen by the team are 10,000 times smaller than that.
“One of the findings suggests a distortion of about 20% of the mass of the sun,” Professor LoSecco said. “This object could be a candidate for dark matter.”
One side effect of the team’s research is the improvement of the precision of the Parkes Pulsar Timing Array data, which is collected to look for evidence of low-frequency gravitational radiation.
Dark matter conglomerations can add interference, or “noise,” to this data; identifying and removing that noise will help scientists to better use this sample set in searches for low-frequency ripples in spacetime called gravitational waves. This could enable the detection of gravitational radiation from more distant and thus earlier black hole mergers — and, perhaps even background primordial gravitational waves left over from the Big Bang.
“The true nature of dark matter is a mystery,” LoSecco said. “This research sheds new light on the nature of dark matter and its distribution in the Milky Way and may also improve the accuracy of the precision pulsar data.”
The team’s results were presented at the National Astronomy Meeting (NAM) 2024 meeting at the University of Hull on Monday (July 15).
