Scientists plan to use the “clocks” of dead stars to shed light on the most mysterious matter in the universe: dark energy.
These timepieces are actually pulsars, rapidly rotating neutron stars that are born when stars at least eight times more massive than the Sun die. The extreme conditions of neutron stars make them ideal laboratories for studying physics in environments found nowhere else in the universe.
“Millisecond pulsars” can spin hundreds of times per second and emit beams of electromagnetic radiation from their poles, like cosmic lighthouses, that scan space. They get their name because when they were first spotted, these neutron stars appeared to pulse, their brightness increasing when their beams were aimed directly at Earth.
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The precision of measuring millisecond variations in the brightness of pulsars allows them to be used collectively as cosmic clocks in “pulsar measurement networks.” These networks are so precise that they can measure gravitational perturbations in the fabric of space and time, combined into a four-dimensional entity called “spacetime,” which could be the ideal way to track dark matter.
“Science has developed very precise methods for measuring time,” John LoSecco, a pulsar timing researcher at the University of Notre Dame, said in a statement. “On Earth, we have atomic clocks, and in space, we have pulsars.”
Putting an end to the mystery of dark matter
Dark matter is so mysterious because it doesn’t interact with light or ordinary matter—or if it does, it does so very weakly and we can’t detect it. “Ordinary matter” is made up of atoms composed of electrons, protons, and neutrons that interact with light and matter, so scientists know that dark matter must be made up of other particles.
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Although it does not interact with light, dark matter exerts a gravitational influence, and its presence can be inferred when this influence affects light and even ordinary matter. It is the effect of this gravitational influence on light that LoSecco and his colleagues sought to exploit using pulsars.
a hazy blue scene showing warped spacetime and two supermassive black holes close together to the left.
Artist’s rendition of a set of pulsars affected by gravitational ripples produced by a supermassive black hole binary in a distant galaxy. (Image credit: Aurore Simonnet/NANOGrav)
According to Albert Einstein’s theory of general relativity, objects with mass bend the very fabric of spacetime, and gravity arises from this curvature. When light exceeds this curvature, its trajectory is also deflected. This can change the travel time of light, causing light from the same distant body to arrive at Earth at different times, which theoretically “slows down” it (the speed of light does not actually change; it is the distance traveled that changes).
Dark matter has mass, and so concentrations of this mysterious form of matter can also warp spacetime. Thus, the path of light from distant objects is bent, and its arrival time is delayed when it passes concentrations of dark matter. This effect is called “gravitational lensing,” with the intervening body altering the path of light, called “gravitational lensing.”
A diagram showing gravitational lensing when light from a distant object reaches Earth.
A diagram showing gravitational lensing caused by dark matter (Image credit: Robert Lea (created with Canva))
LoSecco and his colleagues examined data collected from 65 pulsars in the Parkes Pulsar Timing Array. They observed about 12 incidents that indicate variations and delays in the pulsars’ timing times, which are typically accurate to nanoseconds.
This indicates that the radio wave beams from these cosmic dead star lighthouses are traveling around a warp in space caused by an invisible concentration of mass somewhere between the pulsar and the telescope. The team hypothesizes that these invisible masses are candidates for dark matter “clumps.”
“We take advantage of the fact that the Earth, the Sun, the pulsar, and even the dark matter are moving,” LoSecco explained. “We observe the discrepancies in arrival times caused by the change in distance between the mass we are observing and the line of sight of our pulsar ‘clock.'”
The deviations observed by the team are absolutely tiny. For example, a body the mass of the Sun would cause a delay in the radio waves of pulsars by about 10 microseconds. The dark matter delay deviations observed by the team are 10,000 times smaller than that.
“One of the findings suggests a distortion of about 20 percent of the Sun’s mass,” LoSecco said. “This object could be a candidate for dark matter.”
One side effect of the team’s research is improved accuracy of data from the Parkes Pulsar Timing Array, which is collected to search for evidence of low-frequency gravitational radiation.
Dark matter clumps can add interference, or “noise,” to these data. Identifying and removing this noise will help scientists better use this set of samples in their search for low-frequency ripples in spacetime, called gravitational waves. This could help detect gravitational radiation from more distant and therefore older black hole mergers—and perhaps even primordial background 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 could also improve the accuracy of precision pulsar data.”
The team’s results were presented at the National Astronomy Meeting (NAM) 2024 at the University of Hull on Monday (July 15).
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