Scientists plan to use “clocks” on dead stars to illuminate the most mysterious thing in the universe: dark energy.
These timekeepers are actually pulsars, or rapidly spinning neutron stars, 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.
So-called “millisecond pulsars” can spin hundreds of times per second and shoot beams of electromagnetic radiation from their poles like cosmic beacons that sweep through space. They get their name because when they were first spotted, these neutron stars appeared to pulsate, increasing in brightness when their beams were aimed directly at Earth.
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The ultra-precise timing of the brightness change of millisecond pulsars means that they can be used collectively as cosmic clocks in “pulsar timing systems”. These arrays are so precise that they can measure gravitational perturbations in the fabric of space and time, united as a four-dimensional unit called “space-time,” which could be the perfect way to hunt dark matter.
“Science has developed very precise methods for measuring time,” John Loseco, a pulsar timing matrix researcher at the University of Notre Dame, said in a statement. “On Earth we have atomic clocks, and in space we have pulsars.”
Calling Time for 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 consisting of electrons, protons and neutrons that interact with light and matter, so scientists know that dark matter must be made up of other particles.
Although it does not interact with light, dark matter has 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 his colleagues seek to exploit using pulsars.
According to Albert Einstein’s general theory of relativity, objects with mass warp the very fabric of spacetime, and gravity arises from this curvature. When light passes this curvature, its path also deviates. 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 doesn’t actually change; the distance it travels does).
Dark matter has mass, and thus concentrations of this mysterious form of matter can also warp spacetime. Thus, the path of light from distant objects is curved and its arrival time is slowed as it passes through concentrations of dark matter. This effect is called “gravitational lensing”, with the intervening body changing the path of light, called “gravitational lensing”.
LoSecco and colleagues examined data collected from 65 pulsars in the Parkes Pulsar Timing Array. They observed about 12 incidents that show variations and delays in the timing of pulsars, which typically have nanosecond precision.
This shows that the radio wave beams from these dead star cosmic beacons travel around a warp in space caused by an invisible concentration of mass somewhere between the pulsar and the telescope. The team theorizes that these invisible masses are candidates for “clumps” of dark matter.
“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,” LoSeco said. “We observe variations in arrival times caused by the change in distance between the mass we observe and the line of sight to our ‘clock’ pulsar.”
The deviations the team observes are very small. To illustrate this, a body with the mass of the sun would cause a pulsar radio wave delay of about 10 microseconds. The proposed dark matter delay deviations observed by the team are 10,000 times smaller than this.
“One of the findings suggests a distortion of about 20% of the sun’s mass,” Professor Loseko said. “This object could be a candidate for dark matter.”
One side effect of the team’s research is improving the precision of data from the Parkes Pulsar Timing Array, which is being collected to look 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 make better use of this set of samples in the search for low-frequency ripples in space-time called gravitational waves. This could enable the detection of gravitational radiation from more distant and therefore 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,” Loseco said. “This study sheds new light on the nature of dark matter and its distribution in the Milky Way, and may also improve the accuracy of precision pulsar data.”
The team’s findings were presented at the National Astronomical Meeting (NAM) 2024 meeting at the University of Hull on Monday (July 15).
