"What is essential is invisible to the eye," said the Fox to the Little Prince. And although in Saint-Exupéry's fable the "invisible" referred to love, the Fox was also right on when it comes to about 95 percent of the universe.
The stuff we can't see controls the destiny of the cosmos and remains deeply mysterious to us. Strange to think that we are surrounded by intangible matter, stuff that connects with us in ways we are unaware of. Just think that, even at a less mysterious but no less fascinating way, trillions of neutrinos born at the heart of the sun are going through your body every second. Feel violated? Better to feel connected with the rest of the cosmos, through this ethereal thread linking us to our nearby star.
It all stated during the 1930s, when Swiss-educated, Caltech astronomer Fritz Zwicky noted that galaxies belonging to large clusters move much faster and more erratically then they should. Zwicky was eyeing the Coma cluster, which has more than a thousand identified galaxies, swarming around like busy bees. The glue keeping the cluster together is, of course, gravity, the force that alone reigns supreme in the cosmos. Or so we think.
Zwicky proposed that there was much more matter around the galaxies, which he dubbed "dunkle Materie," dark matter. He had started one of the biggest hunts in the history of astronomy, the hunt for dark matter.
In the past three decades, it has become clear that not only galaxy clusters but also galaxies have a lot of dark matter. This can be seen by tracking the speeds of the stars belonging to a galaxy, mapping their speeds from the center to the visible edge. What is found contradicts our naïve expectations of what gravity should be doing: instead of tapering down to very low speeds at the galaxy's edge, the stars seem to keep their rotational speeds approximately constant, even when far from the center.
Our interpretation is quite remarkable: like an invisible cocoon, there is a veil of dark matter surrounding the galaxy. This matter is supposedly made of particles; but not of ordinary particles, like electrons or protons. This is some other stuff, of a composition unknown to us. All we know is that the particles must have mass since their effect is gravitational. If they interact in any other way with ordinary matter (the stuff we are made off), it does so very feebly.
Dozens of experiments have been set up to find these elusive dark matter particles. If they do swarm the galaxy, we should be hitting one once in a while. So, placing slabs of sensitive material on the ground (or better, underground, to avoid interference from the other particles that constantly shower us) we should get a few hits now and then. When a dark matter particle hits an atom in the slab's crystal lattice it will set off vibrations that can be detected with ultra-sensitive sensors.
Last week, the LUX experiment — the largest-ever to hunt for dark matter particles — made public the first three months of their labors. LUX stands for Large Underground Xenon dark matter experiment, a large vat filled with about 816 pounds of liquid xenon one-mile underground in an old mine in the Black Hills of South Dakota.
The results were as clear as they were disappointing: no detection or promising signal was found; in fact, the results already cleared up some controversial data from other purported detections by other groups. So, at least for now, dark matter continues to elude us (as Adam explained yesterday).
The good news is that this is only the beginning, and there is much more data-gathering to follow. In a couple of years we will have a much cleaner data analysis and, who knows, perhaps a detection. On the other hand, the many negative results — and these include direct searches of the so-called supersymmetric particles at the Large Hadron Collider in Switzerland, where the Higgs boson was found last year — do make us ponder the whole scenario.
Could we be going the wrong way?
The situation is complicated, since there are other indications that dark matter is out there. The beautiful gravitational lensing effect, a distortion of the paths of light due to large masses, points to its existence. We also have a strong correlation from measurements of the cosmic microwave background radiation (CMB), the left-over photons from the epoch of hydrogen-atom formation in the early cosmic infancy, a mere 400,000 years after the Big Bang. The CMB pins down the cosmic recipe quite decisively, and the amount of dark matter comes out to about 25 percent of the total. Our stuff, the stuff of stars and people, comes to 5 percent. The last 70 percent is supposed to be the even more mysterious dark energy, something for another week.
So we have this weird situation where different astronomical observations indicate quite strongly that dark matter exists: motions of galaxies in clusters, motions of stars in galaxies, gravitational lensing and the cosmic microwave background. But we can't find it, or haven't found it yet. Other ideas, for example, that the dark matter is made of small black holes made a long time ago, have also been ruled out, or at least tightly constrained.
This is science in the making, when different groups of scientists converge to try to figure something out by taking as many different routes as possible. We are trying to tease out a secret that nature clearly enjoys keeping very well-guarded. The challenge remains, which is a good thing.
As any fisherman or hunter knows, an easy prey makes the whole exercise rather boring. But if you never catch anything, you are either a very bad fisherman or hunter, or there is no prey to catch. For the dreamers in all of us, things are much more interesting when the prey is something we haven't even dreamed about yet.
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