One of the most tantalizing enigmas in science is dark matter, a bizarre substance that accounts for about 85 percent of the universe’s mass. Dark matter is tricky to observe because it does not emit light, but that doesn’t mean that it doesn’t interact with light at all.
In fact, the gravitational fields of dark matter clumps may create an abundance of “efficient lenses” that can magnify light from distant objects, according to a study published on Thursday in Science.
These dark matter lenses, which warp light like cosmic funhouse mirrors, could help astronomers observe remote objects located behind the lenses from our perspective on Earth and test out fundamental theories about the universe.
Scientists led by Massimo Meneghetti, a cosmologist at the Astrophysics and Space Science Observatory of Bologna, set out to estimate how many of these small dark matter lenses might be embedded in galaxy clusters, which are huge structures that can contain thousands of gravitationally-bound galaxies.
“Studying the matter distribution in cluster galaxies is important for several reasons,” said Meneghetti in an email. “First of all, we can test the predictions of the cold-dark-matter model. This is the commonly accepted model of dark matter, as it is able to reproduce very nicely several properties of the universe on large scales (much larger than the scales of galaxies and galaxy clusters).”
“The second important motivation for studying the matter distribution in cluster galaxies is to figure out how complex physical phenomena that take place in dense environments shape galaxy evolution,” he added.
If small dark matter lenses turn out to be abundant in galaxy clusters, it could help astronomers peer into otherwise unobservable corners of the universe. The lensing phenomena occurs when a gravitationally influential object, perhaps a dark matter clump, aligns with a source of background light, such as a distant galaxy. As the light travels through the gravitational field of the object, it can become substantially brighter, as if amplified by a cosmic telescope.
“Often the source is magnified, meaning that we have the chance to see tiny details in the lensed images that we would not be able to detect without gravitational lensing,” said Meneghetti.
However, he noted that the lenses tend to mangle the image of the background sources, which means that scientists have to figure out a way to reverse-engineer the original shapes of objects from the deformed lensed versions.
“If we can build a reliable model of how the matter is distributed in the lens, then we can use the model to correct the shape of the lenses’ sources,” Meneghetti explained. “This is very interesting! Using this effect, we can see what very distant and young galaxies look like.”
To better understand dark matter lenses, Meneghetti and his colleagues compared simulations of galaxy clusters to real observations of 11 clusters captured by the Hubble Space Telescope and the Very Large Telescope in Chile.
Surprisingly, there were about 10 times more lenses in the observations than predicted, suggesting that there are either “systematic issues with simulations or incorrect assumptions about the properties of dark matter,” according to the study.
“Our next step will be to investigate other dark matter candidates by running new simulations,” said Meneghetti. “In addition, we plan to extend our analysis to a larger number of galaxy clusters to enlarge our sample of strong lenses and have more data at our disposal to solve this puzzle.”
Meneghetti and his colleagues eagerly await new data from next-generation observatories, such as the Vera C. Rubin Observatory in Chile, which is on track to begin operations in 2021. The European Space Agency's Euclid space telescope and NASA's Nancy Grace Roman Space Telescope, which are both due to launch in the 2020s, will also be crucial instruments for this purpose.
The new telescopes will “produce images of nearly the whole sky and we will be able to discover thousands of new strong lensing clusters,” Meneghetti said. “These observations will be a gold mine for us!”