Friday, March 06, 2009

Looking for dark matter

Dark matter is one of those topics in astronomy that really catches the public's interest. By mass, there is at least five times more dark matter in the Universe than "normal" matter (baryons). The evidence for dark matter is strong, but we don't really know what it is. How can we not know what 80% of the matter in the Universe is?

This week, I visited with some astrophysicists who are trying to detect dark matter in a laboratory on Earth. If they succeed (and are the first team to do so), it may well mean a trip to Stockholm. If they and all other groups searching for dark matter fail, it could pose a serious challenge to our current views of the Universe. It's a high stakes game, with a lot of pressure to be first and to be right.

Dark matter was first suggested by Fritz Zwicky, who was an astronomer with a, um, unique personality. Zwicky had many outlandish ideas well outside the mainstream. Many of these ideas were wrong, but many were right. One of Zwicky's ideas grew out of his studies of clusters of galaxies. The galaxies were moving faster than the gravity of individual galaxies would have predicted, so Zwicky posed that unseen matter was responsible for these fast motions. It took decades to show that this matter could not be normal matter (atoms and protons and electrons), but many lines of evidence have now shown that dark matter exists and cannot be normal matter.

But there remains a possibility that dark matter may not exist, but rather something else is going on. Perhaps our understanding of gravity is wrong, and gravity works differently on very large scales than on scales we can test in our laboratories. Perhaps the Universe folds back on itself in multiple dimensions and gravity from normal galaxies "leaks" through these folds while normal light cannot. (Don't worry if you don't understand it, I only have a vague idea myself.)

Assuming that dark matter does exist as a previously-unknown type of matter, it should be possible to detect. There are many proposed types of dark matter particles, with names such as axions, neutralinos, WIMPS, Kaluza-Klein particles, and many others. These particles react with normal matter mainly only by gravity, but very, very rarely they can actually bounce off of normal matter.

It is these rare bounces that Earth-based laboratories hope to detect. The physicists running these experiments cool a detector down to near absolute zero so that the atoms in the detector are barely moving. If a dark matter particle hits one of these atoms, the atom will move with a lot more vigor, and this extra movement can be detected with very precise measurements.

The problem is that there are lots of other, much more common events that cause the detector atoms to get excited and move around. Cosmic rays from outer space slam into these detectors with huge energies, heating many atoms up. Placing the detectors in mines deep underground protect them from cosmic rays. But normal particles called neutrinos can also penetrate the Earth and, on rare occasions, interact with the detector (in fact, many of these detectors double as neutrino detectors). Telling neutrino hits apart from dark matter hits is hard but not impossible. Another problem is radioactive decay. Even in a very pure detector, some atoms of radioactive material will be present, and radioactive decay also causes signals in the detector.

So, the entire game of dark matter detection is to look at hundreds of thousands of events and try to explain each one as a known process. The few unexplained events that remain are then considered to be dark matter candidates. Then even more tests are needed to see if these events are true dark matter. For example, as the Earth moves around the sun, the direction of dark matter particles should appear to change. So, if the average direction of the candidate dark matter events changes over the course of a year, then these events may be even more likely to be due to dark matter.

If one of the teams looking for dark matter actually finds it, and the experiment can be confirmed by other means, then we can finally be positive that dark matter exists. If the experiments are perfected yet fail to find dark matter, then many physicists will start to question if dark matter actually exists, or if some of the other, (currently) less-popular explanations like modified gravity may actually be correct.


  1. Anonymous10:36 AM

    Fritz Zwicky is the Father of Dark Matter, Gravitational Lensing, The Sky Survey Technique, and JATO, holding many patents in jet propulsion as well. He coined the term Supernova, and was the first to conduct a systematic search for them, discovering more Supernovae than any one individual. He mapped the heavens and catalogued over 30,000 Zwicky Galaxies and 10,000 Zwicky Clusters of Galaxies in his Catalogue of Selected Compact and Post-Eruptive Galaxies. Fritz Zwicky's work continues to withstand any challenge and holds true, and stands as restorative truth against all literary assault and the seemingly never-ending malignant hearsay by the scientific community. The incessant complainers in the scientific community have had voice for so many years, and set themselves up as experts on his work, while seeking credit for his work. Most have miserably failed in their own accomplishments, while arrogantly posturing with their critiques of Fritz Zwicky. All of Fritz Zwicky's theories have been correct, and have all been proven true in time. The incompetence of the scientific community is evident in their thus far failed, and desperate attempts to explain Dark Matter. It took the scientific community over 80 years to even grasp his theory of Dark Matter. Certainly, my family and I would appreciate a cessation of commentary by just another know-it-not, and should not have to redirect compass to a respectful dialogue in my father's memory.

    Dr. Barbarina Zwicky, h.c.

  2. Anonymous5:57 AM

    This article has been added to the Astronomy Link List.

  3. I apologize if I came off as disparaging Zwicky; virtually every astronomer I know (including myself) holds Zwicky in very high regard. About a year ago some of us stumbled across some notes made by a local astronomer from talks and lectures given by Zwicky, and it is amazing how much foresight he had in a lot of areas. The achievements you mention are indeed crucial many areas of current astrophysics.

    It is incorrect to say that all of Zwicky's work was correct (see, e.g., his claims on "Pygmy Stars"), where Zwicky's interpretations have not withstood the tests of additional scrutiny. All astronomers, even the best and most prominent, have wrong ideas, and even Zwicky's incorrect claims pushed the science forward through vigorous scientific debate, refined observing techniques, and stronger theoretical underpinnings.

    But, as you point out, Zwicky was absolutely correct that Dark Matter (or some other exotic physics mimicking dark matter) exists; those who disparaged his findings were wrong. Zwicky certainly deserves (and receives) credit for that discovery. Zwicky also deserves (and, again, receives) full credit for some of the fundamental work on clusters of galaxies and the evolution of galaxies.

    Lastly, though, Zwicky did have a forceful personality and argued passionately for what he worked on and believed in against many other strong and unique personalities. This has led to stories (many undoubtedly apocryphal) that still roam the halls of astronomy departments. Similar stories circulate about other astronomers of that era (Hubble, Baade, Eggen, and de Vaucouleurs, to name a few). These stories are not meant in disrespect, but are part of a larger-than-life mythos that pervades early 20th century astronomers. There's probably a sociological study to be done there.

    Thanks for your comment and spirited defense of your father.