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Dark Matter — "Alternative Gravity" Team Responds
An anonymous reader writes, "Following previous results, an international team of astronomers answers, defending the case for a modification of the theory of gravity. This article presents an alternative to dark matter and states constraints on the neutrino mass. In short, dark matter is still not a necessity, provided that neutrinos weigh 2eV. This is allowed by what we currently know and should be tested in the KATRIN experiment in 2009."
err, neutrinos do have mass, but not as much as stated in the paper. As far as current experiments go neutrinos come in three flavours and interchanging between them is only possible if they have mass. It has been shown in experiments that they change type and hence must have mass.
I'd like to clear this up because there are very common misconception that photons are massive or that something has to be massive to feel gravity, both of which are false.
THEORY: In our current understanding, photons are forbidden from having mass because of the way quantum electrodynamics (the most precisely tested theory in the history of science) works. It's an exercise in field theory to show it, but the gist is that electromagnetism (light, charge conservation, electric and magnetic forces...) are a consequence of a symmetry of nature, and that symmetry only works if the associated carrier particle (the photon) has exactly zero mass.
EXPERIMENT: If the photon had even a very tiny mass, it would also mean that the electromagnetic interactions would become short range (just like the weak interactions, which are mediated by a massive carrier). The usual inverse square law would become an exponential falloff. This has been tested for in laboratories (and in astronomy!) very precisely, so there are ridiculously strict upper limits on the photon mass.
This doesn't mean photons don't feel gravity!! Gravity interacts with all energy, not just mass, and so the energy of a photon is enough to cause it to bend around massive objects.
Their paths don't bend - it is the paths themselves that are distorted in space-time by the gravity well. This distortion appears to be bent in three dimensions - to the photon it is perfectly straight...
(ok, ok, simply *massive* oversimplification here - to the point of error, but I hope you understand my motives.)
People should NOT take the impression from this article that there is doubt that dark matter exists. The only doubt being raised is over what form the dark matter takes. Let me clarify:
(Note: Baryons are protons and neutrons. "Non-baryonic" means not made up of the building blocks of ordinary atoms.)
The beauty of the Clowes work (the "proof that dark matter exists" from a couple of weeks ago) is that the colliding clusters they worked on give simple, clean evidence that galaxy clusters are really dominated by invisible, non-baryonic dark matter. At it's core, it's a very simple argument. Two clusters collided, and the baryonic clouds (hot gas, seen with X-rays) experienced drag and got a bit hung-up passing through one another. Most of the mass, however (seen with gravitational lensing), passed straight through with no drag. We see the X-rays and lensing in two different places on the sky - they really are two different kinds of stuff. This is VERY direct proof that most of the mass in galaxy clusters is not the ordinary matter we see on earth - it's something non-baryonic that does not interact with light and does not interact much with ordinary matter. In other words, dark matter is real, physical stuff!
This article argues only about what that dark matter might actually be. It's generally believed that it can't be neutrinos, because neutrinos are so light that they would mess up galaxy formation, and so must be some new, exotic kind of particle. The logic here is that very light particles move so fast that they don't clump together well under their own gravity, which would disrupt the formation of galaxies and smaller clusters of galaxies. All this paper argues is that the dark matter might not be a truly new particle - the combination of modified gravity and neutrinos can be made to work. They still conclude that the invisible neutrinos must outmass the baryons in the clusters by a factor of at least 2.5.
Many people (particularly those who do not understand the evidence) dislike the idea of dark matter, thinking it sounds too much like epicycles. That's understandable, and it's good to be very skeptical of such a weird idea (I know I was). The truth is that there is now enough evidence to say that it really does exist, no matter how strange it may seem to us. The future lies is figuring out what the dark matter is actually made of, not bland assertions that "that just can't be right...".