Welcome to the Dark Side

No one took Fritz Zwicky seriously when he pointed out that the universe does not follow the rules we think it does. Working at Caltech in the 1930s, he studied galaxies, supernovae, and other recently discovered phenomena in the heavens. The idea that galaxies were clusters of stars, gas, and dust had only been confirmed in the 1920s, but by 1933, Zwicky was convinced that was not the whole story.

By all rights, galaxies should not exist at all. All the millions of stars, planets, and other bits of matter they contain still do not exert enough gravitational pull to hold them together—especially since they rotate outrageously quickly. Zwicky noticed that according to the law of gravity, rotating galaxies should be spinning out of control, shedding stars left and right. He hypothesized that galaxies were being held together by a mysterious substance we could not see, which he called dark matter.

At the time, the scientific community ignored his proposal. With the recently developed theories of relativity and quantum mechanics occupying their time, most physicists stayed focused on matter they could actually detect and study. Astronomers were busy coping with Edwin Hubble’s discovery that the universe was expanding, which provided the first observational support for the Big Bang. The idea of dark matter was only seriously revisited 40 years later, when everybody began to realize Zwicky had it right all along. It turns out that there is over five times more dark matter than luminous matter in the universe.

Ten years ago, it got even stranger. Two rival teams of scientists set out to measure the expansion of the universe, which by 1998 was confirmed to be the result of the universe’s explosive beginning, the force of which continued to propel every point in space-time away from every other point. The scientists believed that after over 13 billion years of expansion, the propulsion provided by the Big Bang would be overcome by gravity, which would slow the expansion and maybe even cause the universe to contract. Instead, they discovered that the expansion of the universe was accelerating. The mysterious force driving this acceleration was dubbed dark energy. Later measurements determined it makes up 74 percent of the universe. Dark matter makes up another 22 percent. Luminous matter (the stuff of life, planets, stars, molecules, atoms, and the rest of everything we can “see”) fills in the last 4 percent. For the entire history of all branches of science, we had been studying something that made up less than 5 percent of the universe.

The name “dark matter” is fairly accurate, if a bit reductive. Luminous matter absorbs and reflects light in the electromagnetic spectrum, while dark matter does not. But dark matter is not simply invisible or difficult to see. It is nonbaryonic, meaning it is not made of the same building blocks as conventional matter and does not interact with it.

There is nothing we know of that will bounce off of dark matter and be reflected back to our eyes or telescopes. Considering that is how we learn about most everything else in the sky, dark matter is extremely difficult to study.

Dark energy is another story. While scientists are convinced that it is there, they have no idea what it actually is. A force? A substance? Something in between? They are currently just attempting to study how it behaves—whether it remains constant through space or changes with distance, for example. One idea is that it might turn out to be an unexpected property of gravity acting over large distances, or maybe even through extra dimensions. But we are years away from the experiments that could test such ideas.

Dark matter and dark energy present the problem of how to devise an experiment to study something we can’t even define. Columbia professor Elena Aprile is one of the scientists doing just that. Working on the XENON experiment at the Gran Sasso National Laboratory in Italy, she and her team attempt to detect weakly interacting massive particles, better known as WIMPs. These as-of-yet-undiscovered particles are theorized to be unaffected by the electromagnetic force and to be very heavy, which would cause them to clump together around galaxies—exactly as dark matter does. WIMPs are leading candidates for dark matter, but Aprile and her team first have to prove they exist.

Like many experiments trying to detect elusive particles, XENON takes place underground as a way to reduce interference from all the other particles whizzing through the atmosphere. They hope to observe rare and subtle interactions between WIMPs and atoms of liquid xenon. They have yet to detect a WIMP, but continue to reduce background interference and work to make their equipment even more sensitive. Narrowing down the energy scales in which WIMPs can exist will help other physicists determine where to look for the particles in the future, especially if and when they attempt to create their own WIMPs in particle accelerators like the Large Hadron Collider.

Despite the best efforts of XENON and other experiments, we still know nothing about 95 percent of everything. For all the knowledge we have gained over thousands of years of doing science, we have barely scratched the surface. The study of dark matter and dark energy is opening a window onto another universe—one that we happen to live in.

Elizabeth Wade is a Barnard senior majoring in comparative literature. Fear of Physics runs alternate Mondays. Opinion@columbiaspectator.com