Every minute of every day, over a dozen dark matter detectors across the globe lie patiently in wait -- their bellies filled with ultra-pure liquid that is hungry for a dark matter particle.
Dark matter particles should, in theory, interact with the liquid inside of these detectors, leaving behind a unique signature. But what if dark matter is not comprised of particles?
A pair of theorists is considering the possibility. In particular, they are looking at a type of defect in space that might have been the result of cooling in the early universe and could be the dark matter for which everyone is searching.
|Before the arrival of a topological defect, the apparent time difference between the clocks is zero. As the topological defect passes the first clock, they two clocks de-synchronize. Then, as the topological defect passes the second clock, the de-sychronization disappears. Credit: Andrei Derevianko, arXiv:1311.1244|
Atomic clocks, the theorists argue, could be just the instrument to detect these defects. A network of atomic clocks, such as those aboard GPS satellites, could offer the next step in understanding this elusive material that makes up over a quarter, 26.8 percent, of the mass-energy of our universe.
When the universe was in its infancy, it was unimaginably hot, but as it expanded the universe underwent a cooling period. This cooling led to what some scientists think of as cracks, or topological defects, in space; one example being cosmic strings.
General consensus surrounding dark matter is that it is a form of particulate matter.
The structures of objects in our universe behave as if they contain more mass than what scientists can account for from observations. This invisible mass was given the name dark matter. But perhaps there is another explanation.
A second theory is that dark matter is not a particle at all but a series of topological defects – cracks throughout space that lead to these bizarre observations of missing mass that scientists do not fully understand.
|NIST-F1 Cesium fountain atomic clock, which serves as the US time and frequency standard. This clock will eventually lose one full second of precision, but it will take 100 million years to do so. Credit: NIST|
If this were the case, then dark matter detector experiments, like those at the Gran Sasso Laboratory in Italy and the Deep Underground Science and Engineering Laboratory in South Dakota, do not have the right technology to detect dark matter. However, atomic clocks could.
Atomic clocks are one of the most precise instruments in the world. Unlike your wall clock that lags multiple seconds in a year, it will take some atomic clocks 100 million years to lose one second. This precision is what makes atomic clocks a useful tool for identifying potential topological dark matter.
Global Positioning System (GPS) satellites in space are equipped with atomic clocks. As these clocks orbit Earth, they could come into contact with cracks in space. The crack would interact with atoms inside the clock – the atoms that the clock relies on to keep time.
The result would either speed up or slow down the clock’s time, depending on the nature of the interaction. This would de-synchronize the time that clock reads with a clock on the other side of Earth that had not passed through the topological defect.
Andrei Derevianko at the University of Nevada and his colleague Maxim Pospelov at the University of Victoria in Canada and the Perimeter Institute for Theoretical Physics argue that topological dark matter could be detected if scientists measured such a desnychronization between atomic clocks.
|A simulation of the original design of the GPS space segment, with 24 GPS satellites (4 satellites in each of 6 orbits), showing the evolution of the number of visible satellites from a fixed point (45°N) on Earth (considering "visibility" as having direct line of sight). Credit: El pak at en.wikipedia|
Derevianko and Pospelov calculated how long topological dark matter might de-synchronize a series of atomic clocks on GPS satellites -- essentially the window of time scientists would have to detect such a measurement.
They explain in a paper they published last November on the scientific paper repository arXiv that the clocks would be desynchronized for about 180 seconds. Since atomic clocks are precise to within one nanosecond, they will need to be desynchronized by at least that amount in order for scientists to detect topological dark matter.
Atomic clocks aboard GPS satellites are subject to other influences that could desynchronize them, such as solar flares. However, if scientists employed a network of clocks in the hunt, it would improve their chances of distinguishing between contaminated measurements said Dmitry Budker, a physicist at UC Berkeley.
“There are many reason why clocks can de-synchronize,” said Budker who was not involved with the research. “However, a network is very powerful in discriminating these parasitic events because the dephasing should occur in a coordinated manner among different members of the network.”
Budker is busy assembling the Global Network of Optical Magnetometer for Exotic (GNOME) physics experiment. Previous to his publication with Derevianko, Pospelov suggested that magnetometers could also serve as a tool to detect forms of dark matter other than dark matter particles.
“My personal hunch is that in the future we will have global networks looking for dark matter that contain both clocks and magnetometers, and maybe other types of sensors as well,” Budker said.
Derevianko and Pospelov will present their research at the upcoming 45th Annual Meeting of the American Physical Society Division of Atomic, Molecular and Optical Physics in Madison, Wisconsin.