by Katelyn Rousch

Dr. Clowe explains what scientists really mean when they talk about “dark matter,” provides possible theories as to what it might be, and describes his past and present research in the field to discover more about dark matter and the universe.

Meet Dr. Douglas Clowe, the current head of the Ohio University Astrophysics Institute. His specialties include gravitational lensing, clusters of galaxies, galaxy evolution, and cosmology. His work with the Bullet Cluster, a collision of galaxy clusters, helped to verify the existence of dark matter.

The Debate on Dark Matter

Since the 1930s, scientists have collectively noticed something weird going on with clusters of galaxies. Dr. Douglas Clowe, the current head of the Ohio University Astrophysics Institute, describes the problem with movement of galaxies in a cluster as being, “Too fast with respect to each other to be held together with just the gravity of the galaxies.” 

He explains that researchers have developed accurate methods of determining things like the velocity and position of stars, and have also known the approximate mass of stars and “space dust” between them. It is possible to calculate these things by working with the differences, or Doppler shifts, in the emission and absorption spectrum of galaxies, which may change based on the elements in their makeup. 

However, these numbers had been telling scientists for nearly a hundred years that there was mass missing from their current equation. So the question was: is there missing, undetectable mass, known as “dark matter,” or is the current understanding of how gravity works wrong?

Super-Heated Soup

According to Dr.Clowe, the debate was given a jump-start when x-ray was introduced as a new galactic imaging method. With this technology, scientists found that ionized hydrogen and helium atoms superheated at 200 million degrees Kelvin are a part of galaxy clusters. This sounds like something that would melt even the Millenium Falcon, but because these atoms are so far apart and so small, the rate of heat transfer is far too small to have any effect. 

Though this super-heated soup of atoms may not hinder the future of space travel, it is made up of matter, which has mass. While the mass of a single atom is nearly negligible, an entire galaxy-cluster’s worth of them is not. At first, scientists thought they had answered their question, but really, this cloud only accounted for 10 percent of the missing matter in these systems. So the mystery remained – at least until a few years ago.

Light is a Highway

A key breakthrough happened a few years ago with the observation of two galaxy clusters colliding. Dr. Douglas Clowe at Ohio University was among the scientists researching this collision known as the Bullet Cluster. In the Bullet Cluster, two galaxy clusters were observed smashing into one another. Dr.Clowe explains that the makeup density of the galaxies within the clusters is different than cloud of charged particles that surround them, a collision separates the galaxies from the “soup” they are in. The galaxies will pass right through, because of the sheer enormity of the space between them. The cloud, however, will “lag behind.” This separation gives scientists an opportunity to determine where the majority of mass is concentrated. 

As explained before, there are two main theories at this point: (1) dark matter and (2) the current understanding of gravity is wrong. Either way, the solution is essentially intended to fix some bad math. Explanation (1) accounts for an unknown mass. Explanation (2) modifies gravity in a way that is outside of general relativity.

While bad math might seem like something that stays in college physics classrooms, it is a very real problem that researchers have to account for. Problems are redone hundreds, if not thousands of times, by intellectuals around the world before their results are accepted. 

Scientists already have to correct Newton’s Laws using Einstein’s General Relativity outside of the galaxy. They have to do this because gravity does some weird things to light as it travels through space, making it bend and twist on its way to a destination. The real path of light is much more like an interstate than a plane flight, and some heavy-duty mathematics are needed to straighten it out.

Fortunately, this “highway” of light can be used to researchers’ advantage. 

Using something called Gravitational Lensing to study how light bends in relation to the gravity of other galaxies, scientists like Dr.Clowe can determine where the major source of gravity is coming from. In the case of the bullet cluster, researchers can observe the effect the collision has on the way surrounding galaxies’ light is bent, allowing them to determine where the majority of the mass is located. 

In theory, dark matter should not interact with the particles that make up the cloud, or “soup.” Since it only interacts with gravity, it should have similar properties to the galaxies within the cluster itself, meaning that if there is dark matter, it will show as a greater mass in the galaxy cluster rather than its cloud. The numbers were run, and the mass was indeed sitting alongside the galaxy cluster rather than the cloud.

With this data, Dr.Clowe and other researchers were able to confirm that in galaxy clusters, most of the mass must be some form of dark matter.

A Mystery Remains

So what is it? Dr.Clowe defines dark matter as a mysterious substance that reacts and affects gravity, but not light. It explains why clusters of galaxies move faster than they should be with respect to one another by filling in a missing mass value in some pretty complicated equations.

Dr.Clowe gave a few examples of the different theories that are out there:

Some scientists theorize that dark matter may be some undiscovered subatomic particle. There are many ongoing experiments where researchers stare at an isolated object, one they know should not be affected by nearly anything but potentially dark matter, and wait for something to happen. While this may sound uneventful, lack of results from these tests are currently ruling out the potential for dark matter to have an interaction with a part of atoms known as the weak nuclear force, and would a reaction ever happen, it could be another revolutionary step in the field.

Another theory is that dark matter really just interacts with itself, and has the capability to annihilate itself, releasing something that could link back to its ability to affect gravity. However, Dr. Clowe says that if this occurs, “it would probably release energy and other types of particles that do interact with light, so could be detectable as emission lines in X-ray or Gamma-ray parts of the spectrum.”

One of the stranger possibilities is related to string theory, which proposes an additional six dimensions in space. Currently, scientists theorize five, which are the three spatial dimensions, time, and one that has to do with the strong and weak forces of atoms. This theory suggests that there are actually two universes that are connected by the sixth dimension, and otherwise don’t interact. The best way to understand this is to think of the universe as a trampoline. When an object is added, the material will stretch to form a pocket for that mass, similar to the way gravity does. Objects of smaller masses will be affected by those indentations. If someone were to sit in the center, and a friend placed a ball on the edge, the way the fabric is stretched would cause the ball to be pulled toward the center. With this theory, the sixth dimension is the fabric of the trampoline, and dark matter would be anything pushing or pulling from the other side, affecting the indentations in the fabric, or gravity in the case of the actual universe.

When asked what dark matter is, Dr. Clowe replied, “I tell people dark matter can basically be whatever they want it to be, except atoms.”

At this point, there hasn’t been a theory proven to be any more valid through experimentation, so all, even the more outlandish theories, are possibilities.

The Quest for Knowledge Continues

Currently, Dr. Clowe is working on improving measurements to reduce errors for an upcoming project known as LSST, or the Large Synoptic Survey Telescope. Today, the best telescopes only survey a tiny portion of the night sky each night. This program aims to change that by using a massive, eight-meter telescope that is designed to capture a sliver of night sky every thirty seconds, giving it the ability to map the entire southern hemisphere in three days. 

With 10 years of data, scientists will be able to use that data to, as Dr.Clowe puts it, “Find things we don’t know exist yet.”