Dark matter is thought to outnumber the regular stuff by a ratio of five to one, yet it remains frustratingly elusive. But there might be ways to potentially spot it, if you know where to look, and now astronomers have scanned neutron stars for telltale signals of a proposed dark matter particle called an axion.
Decades of astronomical observations have led scientists to conclude that the universe is filled with massive, invisible particles. This “dark” matter doesn’t emit or reflect light, but it makes its presence known through its extremely strong gravitational effects on stars and galaxies.
Directly detecting dark matter particles would be considered a Holy Grail of physics, but of course it’s no easy feat to find an “invisible” substance. It’s made even harder by the fact that we don’t know much else about it – dark matter could be any number of hypothetical particles, such as superheavy gravitinos, sterile neutrinos, dark photons, or weakly interacting massive particles (WIMPs), each with their own properties.
One particularly promising candidate is the axion. If they exist, these particles are expected to be very light, have a neutral electric charge, and float around the universe in waves. But what makes them most exciting is that unlike other candidates, axions should occasionally interact with regular matter through forces other than gravity – namely, electromagnetism.
Experiments have been run in the past to try to detect axions as they produce electric or magnetic fields in certain conditions, or by affecting the spin of electrified neutrons. But for the new study, the researchers turned their gaze away from the lab and up to the stars.
Another predicted property of axions is that when they encounter a strong electromagnetic field, they should sometimes spontaneously convert into photons – particles of light that are easily detectable.
Neutron stars have some of the strongest magnetic fields in the universe, and their huge masses should attract large numbers of axions. So the researchers reasoned that these objects would be the perfect places to scan for axions converting into photons.
That conversion would be expected to produce an ultra-narrow peak of radio waves at a particular frequency, depending on the axion’s mass. The team analyzed data from two radio telescopes – the Robert C. Byrd Green Bank Telescope in the US and the Effelsberg 100-m Telescope in Germany – as they observed two nearby neutron stars, as well as a wider scan of the Milky Way center, where there should be an estimated 500 million others.
The researchers sampled radio frequencies of around 1 GHz at these locations, which is the range expected to be produced by axions of masses between 5 and 11 micro electron-volts. And they detected no such signals.
A null result isn’t a washout though – it allows the researchers to rule out axions existing in that mass range. With more and more experiments of different types hunting for candidates with different masses and properties, each one potentially brings us closer to an answer.
The research was published in the journal Physical Review Letters.
Source: Kavli Institute for the Physics and Mathematics of the Universe