Clumps of dark matter can be surprisingly small -- and cold.
Astronomers love to give weird names to things, but "dark matter" is pretty self-explanatory. It's matter, or we think it is, because it exerts a gravitational pull. It's also dark, cause we can't see it (although we observe its effect) -- and that's pretty much all we know about it.
Dark matter is estimated to account for approximately 85% of the matter in the universe, and yet we don't really know what it is. But a new study might help us in that regard.
As weird as it may sound, dark matter seems to "clump together". Turns out, these clumps can be much smaller than we thought. This confirms a fundamental prediction about dark matter, and can help researchers make an important breakthrough in understanding this enigmatic phenomenon.
A dark hunt
Dark matter is invisible to all our instruments. It doesn't emit light or any detectable radiation. We never imaged it in any direct way. So when studying dark matter, astrophysicists look for its effects.
The most prevalent of these effects is its gravitational effect. According to such observations, dark matters appears to be the gravitational "glue" holding galaxies together.
We don't know what kind of particles dark matter would be made of, but it almost certainly wouldn't be the electrons, protons, and neutrons we're familiar with. A popular theory holds that whatever particles it may be made of, these particles wouldn't move very fast. This would help explain why dark matter tends to clump together, and while the dark matter concentrations across the universe can vary so much.
If this were the case, this would make for "cold" dark matter. A competing theory supports the idea of "hot" dark matter, where particles are moving at relativistic speeds (close to the speed of light).
Clumps of dark matter can help solve this dilemma. "Hot" dark matter wouldn't allow the formation of small clumps, they simply move too fast to allow small chunks to form. So if we could detect small clumps, this would lend support to the "cold" dark matter hypothesis.
But remember how we said that dark matter can't be imaged? Yeah, that's still a problem.
So instead, researchers took to an old tool: gravitational lensing. But they gave it a new twist.
Gravitational lensing, as the name implies, is the technique of using gravitational attraction as a lens. Everything has a gravitational pull, but objects that are really massive can distort even light itself. While this is often a very subtle distortion, it's still detectable.
Think of it this way: if we're looking at a distant, bright galaxy through a telescope, and another massive object is interposed between our telescope and the galaxy, its gravitation can act as a lens, bending the light. This is what was done in this study.
As you might have guessed, this requires a very particular alignment -- which means that gravitational lenses must be found -- and they may not exist in the directions we want them.
But sometimes, ever so rarely, the objects involved are lined up in such a way that four distorted images are produced around the lensing object. This is called an Einstein cross. This is where things get really interesting.
You might be wondering what any of this has to do with dark matter. Well, the gravitational influence of dark matter clumps should be observable -- even that of smaller clumps.
The team used the Hubble Space Telescope to study eight Einstein cross quasars -- extremely luminous galactic cores powered by supermassive black holes. These quasars were gravitationally lensed by massive foreground galaxies.
"Imagine that each one of these eight galaxies is a giant magnifying glass," said UCLA astrophysicist Daniel Gilman, one of the study authors.
"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 eight quasars and galaxies were aligned so precisely that the warping effect produced four distorted images of each quasar, almost like looking at a carnival mirror. Such alignments are very rare and were fortunate for this study.
The presence of the dark matter clumps altered the apparent brightness and position of each distorted quasar image. The researchers measured how the light was warped by the lens, and then looked at the brightness and position of each of the images, comparing these against predictions of how the Einstein crosses would look without dark matter. These comparisons allowed them to calculate the mass of the dark matter clumps causing the distortion.
According to the results, small dark matter clumps could exist -- and these observations support the existence of colder dark matter.
"Dark matter is colder than we knew at smaller scales," said Anna Nierenberg of NASA's Jet Propulsion Laboratory in Pasadena, California, 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."
This does not rule out the possibility of hotter dark matter, but lends more weight to the colder theory. To make matters even more complex, there is also a mixed dark matter model that includes both types. However, this is almost certainly not the last study of this type.
Astronomers will be able to conduct follow-up studies of dark matter using future NASA space telescopes such as the James Webb Space Telescope and the Wide Field Infrared Survey Telescope, both infrared observatories.
It's remarkable that after decades of service, the Hubble telescope still provides extremely useful information, allowing us to understand aspects of the surrounding universe.
As for dark matter, we won't unravel its secrets today or tomorrow. We're still taking baby steps, but one at a time, we're getting closer to understanding what it really is -- and maybe then, it won't be dark matter anymore.
The team will present its results at the 235th meeting of the American Astronomical Society in Honolulu.