UPDATED: WIMP search returns possible result!

Evidence of dark matter's presence

Indirect evidence is the life blood of astronomy as a science. When gauging the distances between our star—the sun—and objects we see in the sky, direct methods only work out to about 300 light years.

By direct measurement, I mean the use of a physical or geometric change (such as the Earth orbiting around the sun once a year) to measure a change in the position of stars as seen from the Earth. Take a picture of the stars in June, then another picture of the same stars in December, compare the results; knowing the distance the Earth has travelled, you can compute distances to stars, gas clouds or dust, directly. The primary limitations are how small are the changes in apparent position of objects in the sky that can be measured (in arc-seconds) and the distance the Earth has traveled (in millions of meters). If you’ve heard the term, parsec, it is named after this direct method of measurement, and is short for the parallax of one arcsecond.

300 light years is not very far on the scale of what we see in the universe. Our galaxy, The Milky Way, is estimated to be over 100,000 light years across. The nearest large galaxy outside of our own, 2,000,000 light years or over 600,000 parsecs away.

So how are these distances measured? Astronomers can indirectly measure the distances based off our understanding of how stars evolve over time (main sequence evolution), what we think the absolute brightness of certain types of variable stars are (and how dim they should be given a certain distance, Cephied type stars), how some stars appear to explode and brighten, then fade in a particular way (Type Ia supernovae) and how space itself appears to be evolving (redshift, …). These distance indicators are referred to as standard candles.

These distance methods allowed astronomers to determine how far away other galaxies are from our own, which directly lead to the hypothesises for the existance of Dark Matter. By having an idea of the distance to a galaxy, you can then move on to how quickly that galaxy is rotating and astronomers noticed that the outer edges of galaxies appear to rotate about the center of the galaxy much faster than would be expected[1].

Thus, there is something there that we have yet to observe. It could be a problem with our understanding of gravity, or, it could be some form of mass that we have yet to discover, or some form of matter that is just plain difficult to observe. Whatever it is, solving the mystery surrounding the idea of Dark Matter has been one of the hottest topics in astronomy for over 75 years.

So, what is a null result? Sometimes, an experiment that provides a negative result (not a morally negative, but answers a yes or no to a specific question), can be as important as an experiment that provides a positive result. A null result helps eliminate entire doctorates worth of hypothesis and helps guide further research and it is exactly that which is being reported today by the Cryogenic Dark Matter Search team.

The CDMS team is attempting to determine if dark matter observations are the result of a Weakly Interacting Massive Particle (WIMP). WIMPs could contribute to higher than expected rotational velocities of galaxies while weakly interacting with the electromagnetic environment around them (gamma-rays, x-rays, ultraviolet, visible light, infrared, radio waves, and so on), thus, making them dark and hard to observe.

The hard to decipher yet important part of their results being announced today:

This analysis sets an upper limit on the WIMP-nucleon spin-independent cross section of 6.6x10^-44 cm² (4.6x10^-44 cm² when combined with previous CDMS Soudan data) at the 90% confidence leve for a WIMP mass of 60 GeV/c², this work significantly restricts the parameter space for some of the favored supersymmetric models.

See: A Search for WIMPs with the First Five-Tower Data from CDMS and CDMS Web site and information about their experiement.

UPDATED: The results I mentioned were not from yesterday but from an earlier paper. It turns out that the current results have two candidate observations of WIMPs!

In this new data set there are indeed 2 events seen with characteristics consistent with those expected from WIMPs. However, there is also a chance that both events could be due to background particles. Scientists have a strict set of criteria for determining whether a new discovery has been made, in essence that the ratio of signal to background events must be large enough that there is no reasonable doubt. Typically there must be less than one chance in a thousand of the signal being due to background. In this case, a signal of about 5 events would have met those criteria. We estimate that there is about a one in four chance to have seen two backgrounds events, so we can make no claim to have discovered WIMPs. Instead we say that the rate of WIMP interactions with nuclei must be less than a particular value that depends on the mass of the WIMP. The numerical values obtained for these interaction rates from this data set are more stringent than those obtained from previous data for most WIMP masses predicted by theories. Such upper limits are still quite valuable in eliminating a number of theories that might explain dark matter.