Special Edition: Missing mass

Comic Transcript

Panel 1.
Zwicky: Missing mass, dark matter, was the only reasonable explanation for why galaxy clusters don’t fly apart.
Klassen: But doesn’t gravity keep all the galaxies together?

Panel 2.
Zwicky: Yes, of course! But all the galaxies are moving. Fast! The mass we can see in these galaxies is not enough to keep them together!

Panel 3.
Zwicky: They seem to be traveling so fast they should have escaped the cluster! But you do not see this happening. Look at any cluster of galaxies you want.

Panel 4.
Zwicky: I looked at the Coma Cluster… They’re not flying apart, they’re held together by gravity from what I called ‘missing mass.’
Epo: Actually, now it is called dark matter.

Panel 5.
Zwicky: And who’s this spherical…
Teacher: This is Epo.

What does it mean?

Galaxy Cluster – Thousands of nearby galaxies that are gravitationally bound together into a large group. Clusters contain at least as much mass in hot x-ray emitting gas as they do in galaxies. However, they are dominated by dark matter, which comprises 80% to 90% of their total mass.

Coma Cluster – A galaxy cluster located in the Coma Berenices constellation. The cluster contains over 1,000 galaxies and is 320 million light years from Earth.

In human speak please!

When Fritz Zwicky was studying the Coma Cluster in 1933 he noticed that the measured mass of the galaxies in the cluster, which was based on the visible matter, combined with the high orbital speeds of the galaxies inside the cluster did not add up. There did not seem to be enough total mass in the cluster’s galaxies to provide the gravity required to hold in galaxies that were moving so fast.

One way to understand this is to consider a yo-yo on a string. If you swing the yo-yo very fast, you can feel it pulling on the string. As you swing the yo-yo faster the tension in the string grows. Eventually, if you swing the yo-yo fast enough, the string will break. It will no longer be strong enough to hold the rapidly moving yo-yo. In the Coma Cluster, gravity plays the role of the string, and each galaxy is like the yo-yo.

Fritz Zwicky realized that the mass he could measure was not sufficient to provide the gravity needed to hold the cluster together; the Coma Cluster should have flown apart like so many spinning yo-yo’s with their strings cut. But it hadn’t flown apart. The galaxies were still orbiting each other. He calculated that in order to keep the Coma Cluster together there must be over 100 times more mass than is visible.

Zwicky called this extra source of gravity the “missing mass.” Because his result seemed so strange, no one at the time of Zwicky’s discovery believed him. While today the evidence for the missing mass, now commonly called “dark matter” is overwhelming, it isn’t difficult to understand why other scientists in the 1930s were skeptical of Zwicky’s claims. Unfortunately they were too skeptical and it would take decades before anyone took the idea of dark matter seriously.