Researchers are putting a global network of the most precise timekeepers ever made to the task of hunting for dark matter, the invisible and largely intangible substance that researchers think makes up about five-sixths of all matter in the universe.
The existence of dark matter is suggested via its gravitational effects on the movements of stars and galaxies. However, it remains a mystery as to what it might be composed of, and projects ranging from the most powerful atom smasher ever built to vats of chilly liquid xenon have failed to find a trace of it so far, lead study author Piotr Wcisło, a physicist at Nicolaus Copernicus University in Toruń, Poland, told Space.com.
Scientists have largely eliminated all known particles as possible explanations for dark matter. One remaining possibility is that dark matter is made of a new kind of particle; another is that dark matter is not made of particles at all, but rather a field that pervades space much like gravity does. [8 Baffling Astronomy Mysteries]
Previous research suggested that if dark matter is a field, structures could emerge within it — "topological defects" shaped like points, strings or sheets and potentially reaching at least the size of a planet, Wcisło said. These structures might have formed during the chaos after the Big Bang, and essentially froze into stable forms when the early universe cooled down.
Now scientists are testing the existence of dark-matter fields by looking for disturbances in some of the most accurate scientific instruments ever constructed — atomic clocks. These instruments keep time by monitoring the quivering of atoms, much as grandfather clocks rely on swinging pendulums. Nowadays, atomic clocks are so accurate that they would lose no more than 1 second every 15 billion years, longer than the 13.8-billion-year age of the universe.
Interacting with a topological defect could make an atomic clock's atoms temporarily shake faster or slower. By monitoring a network of synchronized atomic clocks that are spread far enough apart for a topological defect to have an effect on some clocks but not others, scientists could detect the existence of these ghostly structures and measure some of their properties, such as their size and speed.
The researchers employed optical atomic clocks, which use laser beams to measure the motions of atoms when they are slowed down by cooling them to temperatures near absolute zero. They calculated that passing through a topological defect could increase or decrease the fine-structure constant, which describes the overall strength of the electromagnetic force. Such changes would alter how atoms respond to lasers and the rate at which those clocks ticked.
Another possible explanation for dark matter is that its effects are caused by fields that vary in strength over time, which in turn lead to regular fluctuations in the strength of the electromagnetic field. Atomic clocks could, in theory, help detect such "coherently oscillating classical scalar fields," the scientists noted.
By analyzing four atomic clocks on three continents — in Colorado, France, Poland and Japan — the researchers could look for subtle variations in the fine-structure constant with about 100 times greater sensitivity than previous experiments. However, they did not detect any signal consistent with dark matter.
One of the major problems of optical atomic clocks is that they can currently only operate continuously for about a day, Wcisło said. One reason for this is that optical atomic clocks need to keep many lasers operating in sync in order to work, and over time at least one of these lasers fall out of sync. However, Wcisło noted a key advantage of their network is that it does not require all its clocks to operate at the same time.
The scientists aim to double the number of clocks in their network in the next year or two, Wcisło said, which could increase the sensitivity and observation time of their network by a factor of 10 or more.
The scientists detailed their findings online today (Dec. 7) in the journal Science Advances.