The Case for Dark Matter
Galactic Rotation CurvesOne of the earliest clear evidences for dark matter came by measuring the speed of rotation in galaxies. Solar System: When you measure the speed of planets as they revolve around the sun, the inner planets move faster than the outer planets (see figure 1). This is entirely a consequence of the nature of gravity and the fact that the majority of the mass of the Solar System is in the sun.
GalaxiesScaling up to the size of the galaxy, we would expect a similar pattern in the galaxy (lower curve on plot, figure 2). However, when we measure the rotational speed of the galaxy (upper curve on plot), the speed is actually close to constant as you move out from the center. This is consistent with mass that is relatively evenly distributed out far beyond the stars we can see. This unseen mass is dark matter, distributed in a massive “halo” enveloping the relatively small visible galaxy (see figure 3).
Bullet ClusterOne of the clearest evidences of the existence of Dark Matter is the Bullet Cluster. The image above shows one small galaxy cluster pass-ing through a larger one. Astronomers studied the distribution of mass in each cluster with two different methods: X-ray emissions (colored pink) that map the distribution of normal mass (e.g. Interstellar gas and dust); and gravitational lensing which maps the total mass distribution (colored blue). When the clusters pass through each other, the dust and gas interacts in a shock front, while the majority of the mass continues without any interaction.
This is dark matter.
The Universe as we know it
MaCHOsOne type of possible candidate are massive astronomical objects that don’t emit any light. These Massive Compact Halo Objects, or MaCHOs, include black holes and neutrons stars. They can be identified by the characteristic bending of light as it passes by in a process called gravitational lensing. Studies have shown that MaCHOs can’t account for more than a small fraction of the total dark matter.
WIMPsAnother explanation is that the dark matter is made up of an entirely new type of particle. A leading candidate, the Weakly Interacting Massive Particle or WIMP, could also resolve other open questions in physics. The WIMP can interact both with gravity and the weak nuclear force. Most of current research is directed towards finding WIMPs.
Other CandidatesIn addition to WIMPs and MaCHOs, a slew of additional dark matter candidates have been hypothesized and are currently under investigation. These include axions, gravitinos, qballs, wimpzillas, and many more.
SuperCDMS at UC Berkeley
Using the earth itself as a shield, SuperCDMS uses sensitive detectors deep underground,
cooled close to absolute zero to search for these elusive particles.