Dark
matter is believed to outnumber the regular stuff by a ratio of five to one,
yet it remains frustratingly indefinable. But there might be ways to possibly
spot it, if you know where to look, and now astrophysicists have scanned
neutron stars for significant signals of a proposed dark matter particle known
as an axion.
Years of
astronomical observations have led researches to conclude that the universe is
filled with massive, invisible particles. This “dark” matter doesn’t emit or
reflect light, but it makes its existence known through its enormously 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 achievement to find an “invisible” substance. It’s
made even tougher 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.
Physical Review Letters,
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