Oct 05 2009
Dark matter is cool – not necessarily its actual temperature, but as a scientific concept. It is one of the scientific mysteries of our generation, and it’s a fascinating story that is unfolding before our eyes with each new discovery. It’s just a good science story. A recent observation has just added to the drama, perhaps calling into question dark matter’s existence.
Dark matter is a helpful example of how the scientific process sometimes works. Scientists first proposed the notion of dark matter to explain anomalies in the rotation of galaxies – they rotate faster than they should, indicating that they have more mass than is visible. It’s as if they have lots of matter that is not visible – dark matter.
Dark matter became a working hypothesis to explain our observations about the universe, especially how galaxies move. But all scientific theories, in order to be of any value, must not only have explanatory power, they also need predictive power – they must make predictions that allow the theory to be tested.
One piece of evidence that is thought to be confirmation of dark matter is the bullet cluster. This is an area of space where two galaxy clusters are colliding. Astronomers can see that the gas clouds slowed down as they crashed into each other. But the stars mostly passed right by each other. It also appears that the dark matter in the clusters also just went past each other, as the gravitational lensing of the areas where there are lots of stars is much greater than the stars alone can account for.
However, there is an alternate theory to dark matter – called MOND for modified Newtonian dynamics. Since dark matter only appears to interact with other matter through gravity (like in the bullet cluster, it was not physically slowed down by the collision – only its gravity is evidence), then perhaps, says MOND, all we need to do is tweak gravitational theory to account for the apparent effects of dark matter.
Specifically, if gravity does not fall off as quickly as we currently think then we could explain the rotation of galaxies without having to invent a new kind of matter. Perhaps studying gravity on the scale of our solar system only gives us an approximation of how gravity works. Perhaps there is some mathematical factor in the gravitational equations that only becomes measurable at galactic scales.
Right now dark matter is the dominant theory and MOND is a minority alternative. A new piece of data, however, may shift the balance a bit.
A thorough study of 28 galaxies has uncovered a curious feature – all of the galaxies (despite their age or shape) have “five times more dark matter than normal matter where the dark matter density has dropped to one-quarter of its central value” (according to New Scientist). Dark matter theory cannot account for this remarkable mathematical symmetry. Such symmetry implies that we are dealing with a force that has precise mathematical features. There is no property of matter that can explain it.
In other words, this feature might suggest that MOND is correct – we do need to tweak the gravitational equations, and if we do it in the right way we might not only explain the “extra” gravity we are seeing but this curious symmetry also, and dark matter will go the way of the ether – an invention to help explain cosmology that turned out to not exist. It will be fun trivia for science buffs in about a century, though.
Of course, this observation, while incredibly interesting, is not enough to kill dark matter. There are still other lines of evidence, like the bullet cluster, that tip the scales toward something like dark matter as the explanation. But perhaps this new observation will give MOND more life as an alternate theory. Or perhaps there is some third alternative no one has thought of yet. Or perhaps there is some truth to both theories – there is some dark matter but our gravitational models also need tweaking.
I’m not currently taking a position on this controversy. I’m just standing on the sideline watching astronomers fight it out over their competing theories. It’s like a science soap opera, although the story unfolds much more slowly.
So far, each new piece of evidence has seemed to deepen the mystery, raising more questions than they answer. That is also often typical of how science progresses. In the early phase of any new theory each new bit of data increases the complexity and confusion of the question, until we get enough pieces to start to put the big picture together. We are right about at the point of maximal confusi0n with dark matter, or at least it seems that way to this neurologist. I’m happy to be corrected by any astronomers out there.
We still have no idea what dark matter is. Until we solve that puzzle, I suspect that MOND theory or something like it will persist. It may even turn out to be true.
I can’t wait to see how it all turns out.
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