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Scientists plan to use dead star “clocks” to shed light on the universe’s most mysterious subject: dark energy.
These timekeepers are actually pulsars, or rapidly spinning 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.
Called “millisecond pulsars,” they can spin hundreds of times per second and emit beams of electromagnetic radiation from their poles that streak across space like cosmic lighthouses. They get their name from the fact that when first seen, these neutron stars appear to pulsate, and their brightness increases when their beams are directed directly at Earth.
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The extremely precise timing of millisecond pulsars’ brightness changes means they could be used collectively as cosmic timekeepers in “pulsar timing arrays.” These arrays are so precise that they can measure gravitational distortions in the fabric of space and time, combined into a four-dimensional entity called “spacetime,” which could be the ideal way to hunt for dark matter.
“Science has developed very precise methods for measuring time,” pulsar timing array researcher John LoSecco of the University of Notre Dame said in a statement. “We have atomic clocks on Earth, and we have pulsars in space.”
Time is up for the dark matter mystery
Dark matter is so mysterious because it doesn’t interact with light or ordinary matter — or if it does, it does so so weakly that we can’t detect it. “Ordinary matter” is made up of atoms, which are made up of electrons, protons, and neutrons that interact with light and matter, so scientists know that dark matter must be made up of other particles.
Despite not interacting with light, dark matter has a gravitational pull, and its presence can be inferred when it affects light and even ordinary matter. It is this gravitational pull on light that LoSecco and his colleagues aim to exploit using pulsars.
According to Albert Einstein’s theory of general relativity, objects with mass warp the fabric of spacetime, and gravity is caused by this curvature. When light passes through this curvature, its path is also bent. This can change the time light travels, causing light from the same distant object to reach Earth at different times, in theory “slowing it down” (the speed of light doesn’t actually change; it’s the distance it travels that changes).
Dark matter has mass, and so the densities of this mysterious form of matter can also bend spacetime. Therefore, the path of light from distant objects is curved and its arrival time is delayed when it passes through densities of dark matter. This effect is called “gravitational lensing” and the intervening object changes the path of the light, giving it the name “gravitational lensing”.
LoSecco and his colleagues examined data collected from 65 pulsars in the Parkes Pulsar Timing Array. They observed about 12 events that showed changes and delays in the pulsars’ timings, typically with nanosecond accuracy.
This suggests that the radio beams from these dead-star cosmic lighthouses are traveling around a curvature in space caused by an unseen concentration of mass somewhere between the pulsar and the telescope. The team theorizes that these invisible masses are candidates for dark matter “clusters.”
“We take advantage of the fact that the Earth is moving, the Sun is moving, the pulsar is moving, and even dark matter is moving,” LoSecco said. “We observe deviations in arrival time caused by changes in the distance between the mass we observe and the line of sight to our ‘clock’ pulsar.”
The deviations the team observed are absolutely tiny. For example, an object the mass of the Sun would cause a delay in pulsar radio waves of about 10 microseconds. The proposed dark matter delay deviations the team observed are 10,000 times smaller.
“One finding is that there is a decay of about 20% of the mass of the sun,” said Professor LoSecco. “This object could be a dark matter candidate.”
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A side effect of the team’s research is the increased sensitivity of the Parkes Pulsar Timing Array data collected to search for evidence of low-frequency gravitational radiation.
Clumps of dark matter can add interference, or “noise,” to the data; identifying and removing this noise will help scientists better use this sample set to search for low-frequency ripples in space-time, or gravitational waves. This could lead to the detection of gravitational radiation from more distant and therefore earlier black hole mergers, and perhaps even primordial gravitational waves in the background 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 precise pulsar data.”
The team’s results were presented at the National Astronomy Meeting (NAM) 2024 meeting held at the University of Hull on Monday (July 15).