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How do you sense the presence of something that has never even been proven to exist?
No one has actually seen dark matter, even though is supposed to make up about 85% of the mass in the universe. The problem is that dark matter is incapable of absorbing or reflecting light, never mind producing it. There has been no definite proof of its existence found thus far. But while this mysterious stuff continues to be hypothesized about, astronomers from Swinburne University have now come the closest to detecting it yet, by using a method that uses gravity to show where matter exists — even if you could swear there is none.
Gravity is the one thing dark matter does have in common with visible matter. However small the amount of gravity any matter exerts, that gravity still has an effect on the objects around it. Gravitational lensing happens when the gravitational pull from massive objects in space bends or distorts light from distant galaxies as it passes through the void. When the effects of gravitational lensing are powerful enough, that distortion can actually lead to the discovery of new objects or galaxies that emit light. There is, however, an alternate version of this phenomenon that can give astronomers an assist in the search for dark matter.
"First, and short-term, is looking at how stars move inside the galaxy. So far we have looked at how gas moves inside of galaxy as a proxy for how the galaxy moves," astronomer and Swinburne doctoral candidate Pol Gurri, who led a study recently published in Monthly Notices of the Royal Astronomical Society, told SYFY WIRE. "Second is getting telescopes to obtain velocity maps for thousands of galaxies. We have observed around 100, when we get to thousands we will start to see what dark matter does to galaxies."
Galaxies are thought to be surrounded by invisible dark matter haloes. While these haloes elude telescopes, dark matter is still capable of slightly distorting the image of anything behind it. This is weak gravitational lensing (WL). You can’t see the dark matter, but what you can observe is its effects. With the usual WL approach, you already know what a galaxy is supposed to look like, so measuring the distortion can give you an idea of how much dark matter is hiding there. The problem is the risk of error in this WL approach. It means only statistical measurements of the signal can reveal the actual shape of the galaxy in the face of shape, noise, or uncertainty about its shape and position.
Observing galaxies through the Australian National University telescope, Gurri and his colleagues instead used a new WL method they call precision lensing. This is supposed to measure the lensing effects of a galaxy’s velocity field, a region with a certain distribution of velocity — in this case, the galaxy itself. Gurri's way of observing is much more precise.
“Regular weak lensing studies look at how the shape of galaxies is deformed, but there is so much uncertainty on the ’true’ or ‘undeformed’ shape of the galaxy that they need to combine thousands of galaxies to output a measurement," he said. "What they get then is an average over those thousands of galaxies. Our method is much more precise because we can put stronger constraints on what is the ’true’ or ‘undeformed’ velocity fields of the galaxies.”
They assumed that if an unlensed galaxy had a stable circular rotation, then its velocity would be the same across its axis. Say WL slightly warped the image of that galaxy; the galaxy would no longer appear symmetrical across its axis, leaving the astronomers to figure out how much dark matter it would take to warp it to that extent. Dark matter’s gravitational effects would finally be brought to light.
"There is lots of gas inside galaxies, and most information on how they move comes from looking at gas like hydrogen or similar. We know that if gas and galaxies move the same way, there are lots of chances that the galaxy is ’stable’ which allows us to put even tighter constraints on the ’undeformed’ velocity field of galaxies and get to even better precision," Gurri said.
Using both shape and velocity gives a much clearer image of how dark matter is in a galaxy and where it lurks. This could also help scientists understand the involvement of this invisible matter in how galaxies form. Does it have an active role in galactic formation, or is it a by-product? Nobody really knows. Upcoming telescopes like NASA’s Nancy Grace Roman Telescope and ESA’s Euclid Space Telescope have been designed with more advanced technology to make similar measurements to those made by Gurri’s team on terra firma.
"Both missions will capture information for many many galaxies, in part to allow finding groups of galaxies that are very similar and use regular weak lensing," Gurri said. "This will help a lot in the search of what dark matter does to galaxies. But it will be a combined effort, you will have these missions getting detailed shape information on many many galaxies, and other telescopes will be getting velocity information to use our technique."
Both of those telescopes launch in 2022, and they may finally see what continues to elude us.