Mar 27 2015

Dark Matter Collisions

The existence of dark matter is one of the coolest science stories of my lifetime. When I was growing up I was in love with pretty much every field of science, but particularly with astronomy, and at that time we had no idea that 85% of the matter in the universe even existed. We now know that the astronomy I was so fascinated with was actually the study of 15% of the matter in the universe. If you count dark energy, which makes up 68% of the universe, then dark matter makes up 27%, and everything that we thought was the entire universe is actually only 5% of the universe.

This story dramatically contradicts every crank and pseudoscientist who tries to tell you that scientists only support the status quo. In my lifetime we discovered 95% of the universe, without any prior theory telling us to expect this result. (Dark matter was hinted at in observations of galaxy rotation, but not generally accepted.) The discoveries were based on unexpected observations, that broke our models of how we thought the universe worked. Scientists met these new ideas with skepticism, but explored them further, and were slowly convinced by mounting evidence. Now dark matter and dark energy are generally accepted – because the evidence convinced the scientists. That is how science works, kids, so don’t believe the cranks.

Dark matter is a powerful idea, even though we currently don’t know what it is, because it not only explains observations that otherwise don’t make sense, it makes predictions. Predictions are the key to scientific progress. They provide the opportunity to test ideas against reality. If an idea does not make any testable predictions, right or wrong it’s just worthless. (Not even wrong.)

The first hint of dark matter’s existence was in the observation of galaxy rotations. In short, the stars in the outer parts of galaxies are moving faster than can be explained by Newtonian gravity and the mass of observable stars. There were some preliminary observations to this effect, but it did not come to serious scientific light until Vera Rubin published her influential paper in 1980, based on her work over the previous decade plus. She showed, using the most accurate observations to date, that stars in spiral galaxies all orbited at roughly the same speed. This means that the mass of spiral galaxies increased linearly as you go out from the core, but this could not be explained by observable matter. Galaxies must be composed of at least 50% invisible, or dark, matter.

An alternate theory, that of modified Newtonian dynamics (MOND), states that the gravitation constant is different at really large (galactic) scales and this explains the observations. This remains a minority opinion, and more recent observations seem to have killed MOND in favor of dark matter.

Those more recent observations indicate that dark matter is not just regular matter that is obscured by dust clouds or hidden from view for some other reason. Dark matter is actually a new and unknown type of matter, because it behaves unlike other known matter. The clincher comes from the observation of collisions between galaxy clusters. The first such observation was of the bullet cluster in 2006. 

Using the Chandra X-ray telescope, astronomers observed this collision of galaxy clusters. What they found was the the visible matter in the cluster slowed as they crashed into each other, as would be expected. This is because the matter in the two clusters interact with each other, causing the slow down. The astronomers also observed the gravitational lensing effect of the cluster. In essence, they could see the shape of the gravity of the colliding clusters. What they found was that the centers of gravity of the two clusters had continued to move past each other, hardly interacting at all, even while the visible matter had crashed into each other and slowed down.

What this means is that dark matter, which contains most of the gravity, interacts very weakly with regular matter and with itself. Its non-gravitational interaction is very minimal. In fact, astronomers could use the observation of the bullet cluster to set limits on this non-gravitational interaction of dark matter.

One data point is never satisfying to scientists, however. This brings us to a new study and the inspiration for today’s post. Astronomers have just published observations of 72 similar collisions of galaxy clusters. They write:

Using the Chandra and Hubble Space Telescopes, we have now observed 72 collisions, including both major and minor mergers. Combining these measurements statistically, we detect the existence of dark mass at 7.6σ significance.

The 7.6σ significance (or sigma) is a huge degree of confidence. Five sigma is generally considered to be the level of certainty in a data set where the results can be taken as true. This corresponds to a one chance in 3.5 million that the results were due to random chance. A result of 7.6 sigma is well beyond significant.

In other words – dark matter exists. It is a real thing. It is an unknown type of matter that has a strong gravitational field, but otherwise only very weakly interacts with other matter, including other dark matter.

We don’t know what dark matter is, but this new result gives us one more piece to the puzzle. The authors note that the results are, “disfavoring some proposed extensions to the standard model.”

I do hope I live to see this decades-long science news story progress to the point where we know what dark matter actually is. But even if that discovery is still decades away, it has been an amazing ride. When I was born we thought we knew what the universe was made of, but our knowledge only encompassed 5% of the actual universe. Now, at least, we have a better idea of the extent of our ignorance, which is the beginning of discovery.

We now have very good evidence that 27% of the universe is made of an unknown type of matter that has gravity but otherwise only weakly interacts. We are gathering clues as to what this dark matter is and what it isn’t. Eventually we’ll figure out what it is. That knowledge will likely just create further mysteries for scientists to explore.

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