Dark matter is one of the greatest mysteries of modern astrophysics and it's been in the news a lot lately. Everyday Einstein explains what it is and why it's been so elusive to find.
The same balance of forces governs the orbiting of the Earth around the Sun and the orbiting of the Sun and other stars around the center of our galaxy, the Milky Way. How fast a star has to move in order to stay in its orbit depends on the strength of the force of gravity pulling it in toward its galaxy’s center and thus on how massive that galaxy is.
Vera Rubin tracked the motion of stars on the edges of spiral galaxies and determined how much matter had to be in each galaxy in order to hold those stars in their orbits. Then she added up all the “normal” matter (gas, dust, and stars) known to be in each galaxy and found it wasn’t nearly enough.
There had to be a form of mystery matter that was not detected via any sort of emission, like starlight, but could be observed indirectly via its gravitational influence.
Why Is Dark Matter Making Headlines?
Since the 1970s, astronomers have found other forms of indirect evidence for the existence of dark matter, including velocities of stars in non-spiral galaxies, the motions of galaxies in clusters, and observations of the afterglow from the Big Bang, called the cosmic microwave background radiation.
But when it comes to the spiral galaxies that Vera Rubin studied (galaxies like our Milky Way), astronomers have debated whether or not the dark matter is sitting in the outskirts of the galaxy or in the core. Since we can’t get any sort of aerial view of our own galaxy, determining the matter content of the Milky Way is very difficult, like trying to judge the size of a large crowd from inside it.
A new study by astronomer Fabio Iocco and his collaborators that was recently published in the journal Nature tracked in detail the motions of stars in the inner portions of the Milky Way.
They found our inner galaxy is host to significant amounts of dark matter. Our Sun traces out a circular orbit around the center of our galaxy, called the solar circle, about halfway between the galaxy’s center and the galaxy’s edge. Iocco’s study found dark matter even within the solar circle!
Alternate Theories of Modified Gravity
An important part of being a scientist is to leave your own bias out of your research as best you can and to always look at a problem from as many angles as possible. In the case of dark matter, some scientists wondered if maybe having to invent an entirely new type of matter was too extreme and instead took a different approach.
Here's what they proposed: If the laws of gravity are telling us that they don’t work without dark matter, perhaps we just don’t understand the laws of gravity as well as we thought we did.
One such theory is that of modified Newtonian dynamics (or MOND) put forth my Milgrom in the 1980s. Supporters of MOND suggest that if we modify Newton’s laws at small accelerations, or where changes in motion are small like in the orbits of stars in the outskirts of galaxies, we can solve the mystery. Those slight tweaks give much lower prediction for the mass in galaxies, one that matches the amount of observed matter, and thus leave no need for any unknown form of matter. The predictions of the MOND theory actually reproduce the motions of stars in some galaxies really well.
The Bullet Cluster
Unfortunately for MOND, a pretty clear piece of evidence in favor of dark matter came with the discovery of the Bullet Cluster. This evidence is so clear, in fact, that some astronomers argue it is a direct observation of dark matter, rather than an indirect one.
The Bullet Cluster is the aftermath of two clusters of thousands of galaxies that have just had a head-on collision. There is a lot of space between the stars in all those galaxies, so throughout the collision they pass right through each other relatively unscathed. The hot X-ray gas that permeates many dense galaxy clusters, however, smashes together before getting dragged out and left behind.
The cluster further acts as what is called a gravitational lens for more distant galaxies behind it. The space around the Bullet Cluster gets warped due to its gravitational influence, causing the light from more distant galaxies in the background to curve around the Bullet Cluster and become detectable by our telescopes. How much and where that light gets curved relates to how much mass there is in the Bullet Cluster and where it is located.
Even though the X-ray gas in the Bullet Cluster is much more massive than the stars, lensing reveals that most of the Cluster’s matter is coincident with the starlight. Thus there must be some form of matter that astronomers are not observing other than gravitationally.
Although we’re not yet sure what constitutes dark matter, we are on the cusp of such a discovery with many experiments like SuperCDMS, Lux, and Axion. I predict we will be hearing from these groups soon and it will be a very exciting and rare day.
That ends our show for today. In the meantime, you can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at email@example.com..