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Stepping Stones in the Search for Dark Matter

Euclidean

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One subject that has received a lot of attention, speculation, and research in the last twenty or more years is the elusive Dark Matter. It gets its peculiar name from the fact that dark matter does not look or behave like normal matter. In fact, we don't even know if it really exists! However, recent experiments, both here on Earth and up in outer space, seem to give clues as to the nature of dark matter.

So where did the idea of dark matter come from? It came from a discrepancy in the mass of galaxies, similar to the discrepancy we observe in atoms. The nucleus of an atom consists of protons and neutrons, and since we know the mass of both particles, one would think that we could find the mass of a particular atomic nucleus simply by adding the masses of its constituent protons and neutrons. However, if you take the mass of the nucleus directly, and compare that mass to the sum of the masses of the protons and neutrons in that nucleus, you will find that some of the mass is missing from the nucleus. Einstein explained that when protons and neutrons (which are essentially a proton and electron smashed together) come so close together, as in a nucleus, a Strong Force is needed to hold all those charges together. The discrepancy in mass happens because some of the mass from the protons and neutrons is converted to nuclear energy that fuels the strong force holding the nucleus together. Thus, the mass of the nucleus itself is less than the sum of the masses of its parts.

Interestingly, scientists noticed a similar discrepancy in the mass of galaxies. Apparently, a galaxy's stars, planets, dust, and other matter do not fully account for that galaxy's mass. Much of the mass is missing. So where did it go? Why can't we see or detect it? Theoretical physicists proposed the idea that this missing mass is hidden in dark matter, which is supposed to outweigh normal matter in mass by more than 5 to 1.

Recently, two experiments have yielded clues about the nature of dark matter. One experiment, known as the Cryogenic Dark Matter Search, used silicon and germanium crystals cooled to near absolute zero. Why? Scientists think that dark matter may consist of Weakly Interacting Massive Particles (WIMPs). In theory, if WIMPs exist, they should very occasionally strike the nucleus of a silicon or germanium atom, which would cause a release of energy and therefore a small vibration in the extremely cooled crystals. The crystals were placed about 700 underground to keep out background noise like stray protons and electrons that could trigger false positives in the crystals. Over the course of about a year, the crystals picked up three signals that could be WIMPs, but scientists have guarded optimism about the results, largely because so few signals have been detected. As a result, they plan to move the crystals further underground (about 2 kilometers) and they plan to use just germanium crystals, which are supposed to be more sensitive than the silicon ones, primarily because the three detected signals occurred at the very lower limit of the crystal's sensitivity, an observation which clashes with the current theories about WIMPs and dark matter.

Another, more hopeful experiment, took place on the International Space Station. There, a device known as the Alpha Magnetic Spectrometer (AMS) detected and observed billions of electrons and positrons buzzing in space around the Earth. The purpose of the experiment was to uncover the link between dark matter and cosmic rays, which are essentially charged particles zipping through space. Scientists believe that dark matter particles can collide and annihilate each other, causing bursts of matter and antimatter pairs, such as electrons and positrons or protons and antiprotons. The AMS collected data which showed that the number of positrons observed in cosmic rays increases as their energies rise, but after a certain point, the increase trails off. Scientists want to collect more data from the AMS because, if the number of positrons at high energies suddenly plummets, it could be a sign of dark matter.

Of course, some scientists view the AMS data skeptically, remarking that data about positrons and cosmic rays will never reveal anything about dark matter. I, however, am hopeful that this new data, once its collection has finished, will open more avenues of research into dark matter.



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