Three years ago, researchers found evidence that dark matter might not be as dark as first thought, with the potential for some subtle interaction with forces beyond gravity. Now follow-up work from the same team collected more observations and showed that the subtle interactions might not be there after all. The new study is being presented at the European Week of Astronomy and Space Science (EWASS).
The team originally used the Hubble Space Telescope to observe galaxy cluster Abell 3827. The system acts as a gravitational lens. Thanks to that, they were able to estimate the mass distribution of the system, and it looks like it had a slight offset compared to where the stars in the galaxies were. Now, by using the Atacama Large Millimeter/submillimeter Array (ALMA), they were able to get a better look and saw that dark matter stays in its galaxies.
“The search for dark matter is frustrating, but that’s science. When data improves, the conclusions can change,” lead author Dr Richard Massey, from Durham University, said in a statement. “Meanwhile, the hunt goes on for dark matter to reveal its nature. So long as dark matter doesn’t interact with the Universe around it, we are having a hard time working out what it is.”
The key to the correction was ALMA's exceptional infrared capability. Hubble was not able to see the full distortion created by the gravitational lens, because some bits of the background galaxy were only visible in infrared. By using ALMA, they charted the dark matter position, which is consistent with the star distribution.
“We got a higher resolution view of the distant galaxy using ALMA than from even the Hubble Space Telescope,” co-author Professor Liliya Williams, from the University of Minnesota, added. “The true position of the dark matter became clearer than in our previous observations.”
The previous research was not in vain. Thanks to the 2015 data, several theories were proposed to explain the offset phenomenon, and many researchers suggested conditions that a cluster would need to show subtle signatures of interacting dark matter. These can now be used to rule out dark matter interactions by observing a large number of clusters.
"These theoretical calculations agreed that, if dark matter feels forces other than gravity, dark matter offsets would indeed be visible in some (but not all) galaxy clusters. It all depends on our viewing angle, the speed of collisions, and the time since impact. Galaxy cluster Abell 3827 may just not be seen from the right angle," Dr Massey told IFLScience.
"With a slight change of strategy to reflect this new theory, we’re now planning to observe lots of clusters. Mapping dark matter (by measuring gravitational lensing) needs the clear view of a telescope in space, but Hubble can’t point at lots of clusters. So we’ve built a telescope that can!"
This telescope is called SuperBIT, and it will be attached to a giant helium balloon. It will be above 99 percent of our planet's atmosphere, thus not getting too much interference, but at a fraction of the cost of sending it to space. It will float around Earth several times and will observe about 200 galaxy clusters.
While the nature of dark matter remains mysterious, it is a fundamental component of the universe. It is estimated that dark matter outweighs regular matter six to one and is responsible for the shape of galaxies and their distribution across the universe. Recently, astronomers discovered a galaxy that seems to lack dark matter, strengthening the idea that it is a substance.