The Case for Dark Matter
Galactic Rotation Curves
One 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.
Scaling 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).
One 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
One of the most significant discoveries of the last century was that the universe began with the Big Bang. The early universe was a hot, dense, highly uniform particle “soup” that cooled and rarefied as space itself expanded. Initial tiny density fluctuations were the seeds of formation that grew into the web of galaxy clusters and superclusters that make our universe. Dark matter provides the gravitational framework that attracts normal matter together so it can collapse into galaxies, stars, and planets.
In recent years our understanding of the composition of the universe has changed dramatically. When we account for everything we know about (protons, neutrons, electrons … regular matter), it constitutes only ~5% of the total mass-energy of the universe. The other 95% is made up of ~68% Dark Energy and ~27% Dark Matter. Understanding what these are is currently one of the highest priorities in physics.
Two of the earliest candidates for Dark Matter were MaCHOs and WIMPs:
One 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.
Another 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.
In 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
Building a Dark Matter Detector
The Super Cryogenic Dark Matter Search (SuperCDMS) uses an array of specially designed detectors (upper right) created from high purity germanium crystals roughly the size of hockey pucks. They are instrumented to measure two signals: Ionization & Vibration. An incoming particle knocks electrons out of atoms in the crystal—temporarily ionizing them. The electrons can then be counted to measure the total ionization energy of the event. In addition to ionization, the particle causes the crystal itself to vibrate. The vibrations are measured to give the total energy and location of the event, much like finding the epicenter of an earthquake.
The detectors are operated inside of a dilution refrigerator (upper left). This device is able to cool them to close to absolute zero. Temperature is a measure of the motion of particles, so by cooling the detectors down, there is no natural vibration and the effect of the interaction stands out like dropping a pebble into smooth water.
Dark Matter interactions are predicted to be extremely rare, so the biggest challenge is shielding the detectors. Moving deep underground allows us to use the earth itself to block out many of these background events. SuperCDMS operates almost a half mile underground at the Soudan Underground Laboratory in northern Minnesota (figure 1 & 2). A decommissioned iron mine, this has become one of the premier underground science laboratories in the world.
Once underground, we still have to deal with the background radiation from the cavern itself. The experiment is surrounded by layers of shielding including an innermost layer of ancient lead removed from a shipwreck more than 200 years old (figure 3). This lead has lower radioactivity because it has been underwater for many years, shielded from radioactive-inducing cosmic rays.
Information provided by the Cryogenic Dark Matter Search (CDMS) Group at UC Berkeley. Original poster design: Sarah Wittmer