Researchers from the University of Waterloo claim to have captured the first composite image of something that astronomers have long believed to exist but has not been directly seen and detected: dark matter.
The new composite image, a combination of individual images, strengthens the idea that dark matter is really there. It also confirms predictions that galaxies are connected together in a cosmic web of dark matter that, until now, had been unobservable.
Elusive Dark Matter
Dark matter is believed to exist through the gravitational effects that it exerts on visible matter in the cosmos. Scientists think it explains why galaxies stick together.
Dark matter is also thought to be five times more abundant than normal matter in the universe. Despite these, scientists have had difficulty detecting dark matter directly.
The absence of tangible proof that dark matter exists already had some scientists question the idea that it is behind the gravitational effect on luminous matter that can be seen by telescopes.
University of Amsterdam physicist Erik Verlinde claims that dark matter is not necessary to explain the effects that have been attributed to it and offers a new theory of gravity that does not necessitate the involvement of dark matter in the motion of stars in galaxies.
A team of researchers led by astronomer Margot Brouwer of Leiden Observatory in the Netherlands tested Verlinde's theory by looking at the lensing effect of gravity around more than 33,000 galaxies. They found that the observations agree well with Verlinde's theory once free parameters are considered. Free parameters are essentially values that can be adjusted to make theory and observations match.
"The dark matter model actually fits slightly better with the data than Verlinde's prediction," Brouwer says. "But then if you mathematically factor in the fact that Verlinde's prediction doesn't have any free parameters, whereas the dark matter prediction does, then you find Verlinde's model is actually performing slightly better."
Astronomer Stacy McGaugh from Case Western Reserve University and part of the LZ experiment that aims to detect particles of dark matter, says that the project should be the last dice in the attempt to hunt for the elusive dark matter.
"This generation of detectors should be the last," McGaugh says. "If we don't find anything we should accept we are stuck and need to find a different explanation, perhaps by modifying our theories of gravity, to explain the phenomena we attribute to dark matter."
Dark Matter Exists
For scientists who remain optimistic that dark matter does exist and is responsible for how objects in the universe behave, the image produced by Mike Hudson from the University of Waterloo and colleagues offers evidence that dark matter does exist.
The image is made up of combined lensing images of more than 23,000 galaxy pairs and shows that the dark matter filament bridge, believed to form the connection between galaxies, is strongest between systems that are less than 40 million light-years apart. The image also confirms predictions that galaxies are tied together through a cosmic web of the invisible substance.
"For decades, researchers have been predicting the existence of dark-matter filaments between galaxies that act like a web-like superstructure connecting galaxies together," says Hudson. "This image moves us beyond predictions to something we can see and measure."
