In a recent paper in the journal Monthly Notices of the Royal Astronomical Society, a duo of University of Waterloo scientists has presented the first images of dark matter binding two galaxies together.
Dark matter is a proposed substance that is plentiful in the universe--in fact, there’s about five times more dark matter than visible matter. It is called dark because it does not interact with light in any way. In fact, the only way we can measure dark matter is through its gravitational effects, which are huge.
Since the 1930s, astronomers realized that there should be more matter out in the universe that they couldn’t account for. One of the most indicting discrepancies was discovered in the 1970s by astronomer Vera Rubin, a pioneer of women in STEM fields. She noticed that galaxies were spinning too fast; stars at the edges of galactic disks should be flung out of their orbits. After exhausting all other explanations, Rubin settled on the theory that there is much more dark matter than visible matter in galaxies, and that the gravity of dark matter drives this rotation--a theory that has received much support over the years.
Now, scientists are getting better at visualizing where the dark matter is and how it weaves through our universe. This most recent research shows how dark matter keeps clumps of galaxies together. “These results show that the dark matter filament bridge is strongest between systems less than 40 million light years apart, and confirms predictions that galaxies across the Universe are tied together through a cosmic web of the elusive substance,” writes Victoria Woollaston in Wired.
So, how were they able to visualize this dark matter if we can’t see it? Through a seemingly magical technique called gravitational lensing. According to Einstein’s theory of relativity, matter bends the space around it. In our daily lives, we only experience this as the pull of gravity--the Earth curves space toward it, making objects fall toward its surface. On grander scales, whole galaxies can curve space so much that they affect the path of light in ways we can detect.
For example, Einstein predicted that, because of such curving, we should be able to see the light from a star hiding behind a galaxy as the light the star emitted bent around the galaxy due to its gravity. This was dubbed an “Einstein Cross”, and one was captured by the Hubble telescope in 1990--the image of the same star appears four times around the central galaxy:
This mindbending way light behaves due to gravity allows scientists to indirectly measure the distribution and effects of dark matter throughout the universe. Figuring out what dark matter is? Well, that’s a different and much more difficult task. Watch Miles investigate the search for a dark matter particle for the PBS NewsHour... from the bottom of a mine:
Banner image credit: Mon Not R Astron Soc.