According to the leading theory of the universe, the cosmos is filled with an unseen substance known as dark matter. This outweighs regular matter five to one and evidence of its existence comes from how galaxies' gravity shapes their internal and external environments.
New observations from the Hubble Space Telescope have now shown what could be the smallest agglomeration of dark matter. The approach the researchers employed for this discovery required the use of eight gravitationally lensed quasars.
Quasars are galaxies whose supermassive black holes are extremely active, outshining everything else in the galaxy. If there’s another galaxy in the line of sight between us and these quasars the gravity of the foreground object warps space-time and it's possible that the light of the distant quasar is distorted and magnified.
The gravitational lenses in the survey produce four distorted images of the quasars. By comparing the four images for each quasar with what they should look like, the team was able to produce better estimations of the size of the clumps of dark matter along the line of sight to the quasars.
"Imagine that each one of these eight galaxies is a giant magnifying glass," team member Daniel Gilman of UCLA explained in a statement. "Small dark matter clumps act as small cracks on the magnifying glass, altering the brightness and position of the four quasar images compared to what you would expect to see if the glass were smooth."
The clumps are between 1/10,000th and 1/100,000th times smaller than the mass of the Milky Way’s dark matter halo. As they don’t have any galaxies in them, they couldn’t have been detected using other methods.
The finding has important consequences for what we think we know about dark matter. We usually refer to is as cold dark matter, meaning that the velocity of these particles is much lower than the speed of light. The size of the dark matter clumps gives a better idea of how cold it is.
"Dark matter is colder than we knew at smaller scales," said Anna Nierenberg of NASA's Jet Propulsion Laboratory in Pasadena, California, who is leader of the Hubble survey. "Astronomers have carried out other observational tests of dark matter theories before, but ours provides the strongest evidence yet for the presence of small clumps of cold dark matter. By combining the latest theoretical predictions, statistical tools, and new Hubble observations, we now have a much more robust result than was previously possible."
The existence of dark matter has not been confirmed yet but work such as this provides strong evidence that there’s something out there that only interacts with gravity and is invisible to us. The research was presented at the 235th meeting of the American Astronomical Society in Honolulu, last week.