Searching for signs of dark matter outside of its gravitational effect has failed – but this time it's controversial.
Astronomers have not found that the X-ray glow is the product of one particular candidate for dark matter: sterile neutrinos.
The researchers argue that this is a result that casts a bit of a shadow on this candidate as the leading contender for dark matter, but does not reject it entirely.
Dark Matter is a huge old question mark. Although we cannot detect it directly, we know that it exists because it has a really strong gravitational effect on the matter we see.
For example, matter at the outer edges of galaxies moves faster than it would if it were under the gravitational influence of visible matter alone. And gravitational lensing – the way gravity bends the path of light – is also stronger than we expect. Based on these effects, astronomers have calculated that 85 percent of the matter in the universe is dark matter.
Since we cannot find it, we do not know what it is. There are a number of hypothetical candidates, and astronomers are trying to come up with ways to detect them.
A sterile neutrino is a hypothetical particle. Normal neutrinos, the most abundant particles in the Universe, are similar to electrons, but have no charge and have very little mass, so they barely interact with normal matter. Physicists have suggested that a sterile neutrino will not interact with normal matter at all, except gravitationally.
But these hypothetical neutrinos are also unstable. They should decay into normal neutrinos and electromagnetic radiation. And if they decay like this, then this radiation should be detected. Very weak, but detectable.
As stated in a 2014 study, faint X-rays from sterile decay of neutrinos from distant galaxies were detected, radiation called the 3.5 keV line. But then follow-up studies – one in 2016 on a dwarf galaxy 260,000 light-years away and another in 2017 on a cluster of galaxies 240 million light-years away – failed to confirm the finding.
We know that the Milky Way has a significant dark matter halo, so if sterile neutrinos decay in it, they should be detected around the galaxy.
The team conducted a meta-analysis of 20 years of raw archived X-ray data on the empty space around the Milky Way, where other luminous objects would not interfere with the search for signs of 3.5 KeV emissions. They didn't find one.
'This 2014 paper and subsequent work confirmed that the signal has generated significant interest in the astrophysics and particle physics communities because of the opportunity to learn for the first time exactly what dark matter is at the microscopic level,' said physicist Ben Safdie of the University of Michigan.
“Our results do not mean that dark matter is not a sterile neutrino, but it does mean that – contrary to what was stated in 2014 – there is no experimental evidence to date to indicate its existence.”
The research was published in the journal Science.
Sources: Photo: Dark matter halo of the Milky Way galaxy. Zosia Rostomian and Nicholas Rodd / Berkeley Lab; and Christopher Dessert and Benjamin Safdi / University of Michigan
