Hubble Space Telescope has helped astronomers detect the tiniest clusters of dark matter ever found. Researchers have used a new method and an aspect of general relativity to identify the clumps, which are about 100,000 times less gigantic than our galaxy’s dark matter halo.
These relatively small clusters of dark matter are, in fact, agreeing with one of the most renowned dark matter theories, namely what is known as cold dark matter.
“We made a very compelling observational test for the cold dark matter model, and it passes with flying colors,” said astrophysicist Tommaso Treu of the University of California, Los Angeles.
Scientists do not know, as a matter of fact, what dark matter is. It cannot be directly detected, but what they know is that the Universe doesn’t act entirely as it should if the regular laws of physics were to be applied to what we can directly see. For instance, stars located on the outer margins of galaxies move faster than they should, as though influenced by some invisible mass.
This mass is called ‘dark matter,’ and there are numerous theories regarding the way it works. Some of them say of hot dark matter, with ‘hot’ meaning particles that move close to the speed of light, and clod dark matter, where ‘cold’ means particles are moving at speeds slower than relativistic velocities.
Two Kinds of Dark Matter
The majority of noticeable evidence and current models seem to suggest cold dark matter, but the case is not solved with this. A test can, however, provide scientists with clues on whether small dark matter clumps can be detected. Hot dark matter would be traveling at a too high velocity to allow for small clusters. If the dark matter is moving at a slower speed, as in the cold matter theory, those tiny clumps should exist.
Even so, detecting them is not an easy task. However, astronomers can base its presence on the gravitational impact it has on the noticeable matter surrounding it; for instance, stars that are traveling too fast around the outer margins of galaxies.
Another element that gravity impacts are light. If there is something gigantic, such as a galaxy cluster between Earth and a light source, the gravitational influence of that clump curves space-time, deflecting the patch of the light and generating numerous images of the light origins.
This process is known as gravitational lensing, an effect foreseen by Einstein’s general relativity hypothesis. In uncommon instances, the objects engaged are aligned in such a way that four twisted images are generated around the lensing object; this process is called an ‘Einstein cross.’
The gravitational impact of small dark matter clusters should, hypothetically, be noticeable in differences discovered in each of these images of the background light origin that are curved around the lens.
Detecting Clumps Using Light
The team of researchers utilized the Hubble Space Telescope to analyze eight different Einstein’s cross pulsars, which are incredibly bright galaxies energized by supermassive black holes, gravitationally lensed by gigantic obverse galaxies.
“Imagine that each one of these eight galaxies is a giant magnifying glass,” said UCLA astrophysicist Daniel Gilman.
“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.”
They calculated how the light of the pulsars is distorted by the lens. They observed the apparent brightness and location of each of the four images, comparing these with predictions of how the Einstein crosses should appear as without dark matter.
These comparisons enabled the team to measure then the mass of the dark matter clusters warping the images. These clumps appeared to be between 10,000 and 100,000 times smaller than the weight of the dark matter in and surrounding the Milky Way.
“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,” said astronomer and physicist Anna Nierenberg of NASA‘s Jet Propulsion Laboratory.
“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 study has been submitted at the 235th meeting of the American Astronomical Society (AAS) and published in the Monthly Notices of the Royal Astronomical Society.