Astronomers think they may have found giant “invisible” structures lurking in the Milky Way. These things seem to be pretty big – roughly the size of Earth’s orbit around the Sun – and they could help to explain where a bunch of missing matter in the universe is, known as the missing baryon problem.
The detection of these structures was made by astronomers working at the CSIRO’s Compact Array telescope in eastern Australia. Although previous research had hinted at their presence before, this new paper – published in the journal Science – helps to constrain their size and shape a bit better.
Don’t think these are big, solid objects though. Essentially, the structures appear to be large clumps of some sort of material, possibly clouds of cool gas, in the existing thin gas that lies between stars. And they appear to be in odd shapes. The astronomers described them as looking like hollow “noodles” or hazelnuts – with material on the outside and a hollow center. Some may even be sheet-like, and we could be looking at them edge-on.
Where they came from, how old they are, or how many there could be in our galaxy remains a mystery, though. “It’s all guesswork at this stage,” lead author Keith Bannister of CSIRO told IFLScience. “There could be many thousands of these in the galaxy.”
The CSIRO's Compact Array in Australia, shown, was used to make the discovery. Alex Cherney
The structures were found by observing the “lensing” effect they produced on distant galaxies. In this case, one in particular was seen refracting, or bending, the light from a distant bright object called a quasar inside a galaxy, named PKS 1939–315, in June 2014. As the light passes through the structures on its way to us, it gets distorted, specifically in the radio wavelength band.
It’s not quite the same effect as gravitational lensing, where a massive object bends the light of a more distant one, producing effects like an Einstein Ring. But the interesting thing is that the structures don’t have an effect on the optical brightness, suggesting they are devoid of dust or other more solid material.
“The lensing here is plasma lensing, and it’s exactly the same as refraction, such as how eye glasses work,” said Bannister. He also likened it to light passing through a wine glass.
We also know their speed. They are moving at about 50 kilometers (30 miles) per second through the interstellar medium, and the ones that have been detected so far reside about 3,000 light-years away. But the really interesting part of all this is that these previously unseen structures could be accounting for a large fraction of hidden mass in the Milky Way.
Bannister was quick to point out that they have nothing to do with dark matter. The existence of dark matter is often inferred from how galaxies are able to hold themselves together, despite their rotation. These structures are not thought to play a part in that.
Instead, they may play a part in the lesser-known missing baryon problem. “That arises from the very successful theory of the Big Bang,” explains Bannister. “The quirky bit is that it tells us that roughly 4 percent of the universe should be made of atoms, baryons, things that make up you and me.”
The problem is, counting up all the matter we can see doesn’t account for 4 percent of the universe. So to resolve the problem, something else must be out there – and perhaps these invisible structures, at least in optical wavelengths, hold the answer.
“We’re hoping that with our work, it could spur a bit of theoretical postulation as to how these things might have formed,” said Bannister.