Graduate School News and Events
Shedding Light on Dark Matter
It is generally accepted in the scientific community today that 96% of the universe is made up of something that has never been seen and is not really understood.
About a quarter of that unknown stuff—representing 85% of the matter of the universe—is “dark matter,” studied at Yale by physicists and astronomers.
Dark matter has mass—that is, it interacts with gravity—but not with light, and it is therefore impossible to see. This makes it “unlike the ordinary matter that makes up the Earth, people, the Sun, and everything else that we can see in the sky at night (the remaining 4%),” says fifth-year student Hugh Lippincott (Physics), who has posted a series of blogs titled “Physics for Mom.” Hugh works with Professor Daniel McKinsey and research scientist James Nikkel, as well as a team from Boston University. (Yale Professor of Astronomy Marla Geha studies dwarf galaxies surrounding the Milky Way, which are almost entirely made of dark matter, and Priya Natarajan, professor of astronomy and physics, observes the shape of dark matter halos by studying how they bend light and distort images in the background.)
The front page of Hugh’s website announces, “The goal of this blog is to explain what I do in grad school so that my mom can understand.” And because his explanation is so clear, the Graduate School News will now share Hugh’s research with its readers, whether they specialize in Spanish or Sociology, Pharmacology or Film Studies.
Here’s how he sums up the entire enterprise: “What do I do? I am trying to directly detect dark matter,” Hugh says.
Fifth-year student Hugh Lippincott looking for evidence of dark matter in a cave.
How do scientists know dark matter exists, if they can’t see it, and how do they hope to detect it?
Apparently, there is a lot of circumstantial evidence for dark matter’s existence, “but one of the simplest (and oldest) arguments comes from the rotations of galaxies,” Hugh says.
Because of gravity, galaxies rotate about their center. “The speed of the rotation can be determined using the Doppler effect, which says that the frequency of an observed wave will be shifted depending on the relative speed of the source and observer. A familiar example of the Doppler effect is that the frequency and pitch of a police siren will change to a listener on the sidewalk as the police car passes by. Since light is also a wave, the frequency and wavelength of light coming from a distant galaxy will also be shifted if the galaxy is moving. Specifically, the wave will be compressed if moving toward you, and stretched if moving away from you.
“In a rotating galaxy, one side is spinning away from us while another side is spinning towards us, and the difference between the wavelengths of light coming from the two sides can tell us how fast the galaxy is spinning. Simple Newtonian mechanics can predict the speed of rotation at a point in the galaxy as a function of mass and radius, and for a constant mass, the speed of rotation should decrease with increasing radius. This makes sense intuitively—the force of gravity decreases with increasing distance, so if the mass is held fixed but the distance increases, there just isn’t as much force to pull the galaxy around.
“If all the matter in the galaxy were in the central, bright part of the galaxy (the part that interacts with light), we would expect the speed of rotation to decrease once we left the bright part of the galaxy. In fact, the speed of rotation stays constant ...much farther than the extent of the bright part of the galaxy. Therefore, there must be matter in the galaxy that we cannot see.”
The more mass there is, the more gravity. The more gravity, the faster the rotation should be. In fact, these galaxies spin “much faster than we would expect, given how much ‘normal’ matter we see in them (stars, dust, gas, etc.), so there must be more mass there than we can detect. We call this invisible mass ‘dark matter.’
“There are a number of theories for what dark matter might be, but one of the most popular is that it is a Weakly Interacting Massive Particle, or WIMP (a cute name that is quite a literal description of a particle that has mass and interacts weakly).”
Four forces work in nature: gravity; electromagnetism; the “strong” force, which holds protons and neutrons together; and the “weak” force, which is involved in nuclear reactions (and possibly dark matter).
“The range of the weak force is very small. You have to be really, really close to something to interact weakly. For example, neutrinos interact weakly. The Sun is basically a giant nuclear reactor and it emits neutrinos all the time. Sixty billion solar neutrinos go through your fingernail every second, but they just don’t hit anything; basically, you are transparent to a neutrino and they pass right through.”
Dark matter could be a “particle that interacts weakly, a WIMP, which is why we’ve never detected it before. The goal of my research is to build a very sensitive radiation detector and directly detect a WIMP by observing the energy released on that rare occasion when a wimp does interact with something in the detector.”
The detector might run continuously for an entire year, and “we might see a single event that we could point to and say that it was a WIMP.”
Crucial to the success of the experiment is understanding and eliminating anything that the detector detects that is not dark matter. Unfortunately, that isn’t easy to do. For example, cosmic rays and other ambient sources of radiation cause a standard radiation detector (a Geiger counter) to go off about 100 times per second—10 million times a day. Hugh’s detector needs to be sensitive enough to pick up one event per year and filter out all the background noise.
How will the team accomplish this?
“First, we will put our detector underground in an active nickel mine in Sudbury, Ontario. This has been done with great success by neutrino experiments in the past. If the detector is underground, the earth helps shield it from cosmic rays, knocking the background down a few orders of magnitude.
“Second, we will use liquid neon or liquid argon in our detector,” eliminating many sources of unwanted background. These inert “noble” gases are very easily purified, and also “have the great property that when exposed to radiation, they ‘scintillate’ or produce light. That will be our signal.”
The team will look for flashes of light produced by a wimp interacting in the liquid. Because an argon or neon detector can be quite large, that very rare WIMP will have a big target to hit.
Finally, a lot of background noise will be eliminated by taking into account the characteristic timing of the light produced by an interaction. Most interactions in the detector are caused by “electronic recoils”: radiation scattering off of electrons. A dark matter event, in contrast, would occur from a wimp scattering off a nucleus, producing a “nuclear recoil.” These two kinds of scintillation light are visible for different lengths of time, and “we can use the timing to tell them apart. Our recent work has focused on describing that time distribution and understanding how well we can use it to separate the two types of events.... In addition, we’ve measured how much light is produced in the various interactions, which helps us understand the signal we might expect to see in a large underground detector.”
So far, the team has performed these measurements for liquid argon, and they are currently repeating them for liquid neon in the lab at Yale. They have also installed a prototype argon detector in the nickel mine in Canada and are finishing the design of a larger detector to be built in the next year. This device “we hope will be the most sensitive dark matter detector in the world” and enable Hugh and his colleagues to “see” dark matter.
And that’s what Hugh does.
