4.2 Shedding New Light on Dark Matter
In the 1930’s, a physicist named Fritz Zwicky measured the speeds at which galaxies move around in the Coma cluster.
- He found that they were moving far too quickly for the cluster to be held together by the gravity of the galaxies.
- Zwicky knew that if the Coma cluster had as much mass as his observations implied, it would cause greater light deflection due to general relativity.
- Unfortunately, the light deflection was still too small to measure with contemporary instruments.
In the 1970’s, Vera Rubin and Kent Ford found that the rotational speed of galaxies should have flung their stars out of them.
- The fact that galaxies stay together implied that they contained ten times as much mass as had been observed.
- Similarly, if you visualize the dark matter a galaxy contains, you will see that the galaxy is roughly ten times larger than it first appears.
The theory of dark matter requires that it is inert.
- It must interact with other matter via gravity to explain astronomical observations.
- However, it must have no electrical charge, no strong nuclear charge, and no weak nuclear charge.
- According to the Standard Model of particle physics, the right density of dark matter could have been created in the early universe in the form of neutralinos, or WIMPs.
Gravitational lensing is a phenomenon that occurs when massive objects bend the light waves that pass around them. By calculating the effect of gravitational lensing on Hubble Space Telescope observations of galaxy clusters, Professor Natarajan was able to map the density of dark matter within those clusters.
- Galaxies tend to have clumps of dark matter near them within a galaxy cluster. This introduces a second, smaller lensing effect on top of that of the cluster itself.
- The galaxy-galaxy strong lensing probability is a metric defined to measure the capacity of a cluster to perform this smaller scale lensing.
- Observations show that clusters are an order of magnitude more concentrated than simulations predict using this metric.
- This points to either the existence of some process or something about the nature of dark matter itself that we don’t yet understand.
- The full study, “An excess of small-scale gravitational lenses observed in galaxy clusters,” is available here.