More from the latest SciAm ... one of the big "feature articles" in this issue is all about dark matter and dark energy, and it reminded me of something that's always bothered me about the whole business of dark matter.
The dark-matter theorists say that "dark matter" does not interact with radiation, and interacts only gravititationally with other dark matter and with normal matter. Because of this, they way light pressure in the early universe did not keep dark matter spread out, and so it was able to clump together into diffuse spheroidal blobs or halos. When the universe cooled sufficiently, the theory goes, normal matter accreted into these halos via gravitational attraction and formed stars and galaxies. This supposedly explains the invisible and unobservable dark-matter halos that the dark-matter theorists say surround every galaxy.
Now, consider this a moment. Normal matter didn't just form vast galactic-scale halos, but went on collapsing under the influence of gravity to form galactic disks, and on the small scale to collapse even further into tight knots and clumps that we call stars and planets. Beyond even those, normal matter collapsed still further and formed black holes, some of them gigantic.
Yet dark matter magically stayed suspended in these vast spherical galactic halos. It clumped together on galactic scales, and then just ... stopped. But dark matter, we are told, doesn't interact with radiation. Radiation pressure cannot stop it from collapsoing further. In fact,we're told, the ONLY force that acts on it is gravity.
So what I want to know is, why are these dark-matter halos still halos? Why haven't they collapsed further under the effect of gravity, which we're told is the only force that acts on them? Why didn't all dark matter in the universe collapse into dark-matter black holes? If nothing but gravity acts on dark matter, then what's holding it up?
Some of these issues, I believe, are where the necessity came from for the dark-matter theorists to invent "dark energy". And lately, the astronomical press has been filled with arguments over the Bullet Cluster, two colliding star clusters off in space somewhere which the dark-matter theorists are touting as being conclusive proof of the existence of dark matter.
Except that it doesn't quite work. Even when they take into account both dark matter and dark energy, they STILL can't make the math work out right on the gas clouds surrounding those colliding star clusters. In fact, as reported in New Scientist, in order to make the math work, they're now mumbling vaguely about invoking a second Mysterious Unknown Force that they haven't even coined a name for yet, this one somehow vaguely gravitational in nature.
So let's see, now we're up to three players in the dark-matter bestiary -- one undetectable form of matter, visible only through gravitational effects, and two kinds of mysterious energy, neither of which they can tell us anything about except "Well, we don't know what it is or what it's made of or where it came from, but our calculations say it must behave something like this." I find myself inescapably reminded of the Far Side cartoon showing a group of scientists standing around a blackboard covered with equations, except for a gap near the bottom containing the words "And then magic happens". The caption has one of the scientists saying, "I think you need to be a bit more specific here."
I think dark-matter theory "needs to be a bit more specific". I've been skeptical about dark matter for years, and some time back (some time last year, I think), I posted in my journal about some of my doubts, commenting that it seemed reminiscent of the epicycles once used to try to explain planetary motion before astronomers gave up and accepted that the Earth wasn't the center of the universe. I speculated then that, just like those astronomers kept finding themselves having to add more and yet more epicycles, so dark-matter theorists would find themselves having to add still more layers of mysterious spooky-action-at-a-distance forces to try and make the math work out. I think we may now be seeing the next "epicycle" of dark-matter theory.
Occam's Razor tells us that when two alternative explanations of a phenomenon are proposed, the most likely one to be true is the one that requires the least number of new assumptions. Right now, the choice is between dark-matter theory, with its growing bestiary of exotic matter and mysterious energies that we're basically asked to take on faith, or the TeVeS (tensor-vector-scalar gravity) theory, derived from the earlier and less sophisticated MOND theory (Modified Newtonian Dynamics). TeVeS essentially grew out of a theoretical effort to reconcile MOND with general relativity. TeVeS/MOND simply proposes that our understanding of gravity is incomplete and that it behaves slightly differently at very large scales and very low accelerations than we have hitherto believed. The principal argument which dark-matter theorists have been able to level at TeVeS/MOND is that there is as yet no theoretical basis as to why gravity should behave in this way; however, this is the pot calling the kettle black, as dark energy and the new mystery pseudo-gravitational force they're invoking to make the math work in the Bullet Cluster are equally devoid of theoretical underpinnings. Arguing against TeVeS on this basis is one naked man in a crowd pointing at another and shouting "He ain't got no clothes on!" in hopes of distracting attention from his own state of undress.
When all is said and done, my gut still tells me that the day will come when dark matter and dark energy are consigned to the trashcan of scientific history, along with phlogiston and the luminiferous aether, mentioned only in tones of slight embarrassment in discussions of places where scientific theory went so far astray as to be, in the words of Wolfgang Pauli, "not even wrong".