In a comment on an earlier post, a reader asks if the "Axis of Evil" in the Cosmic Microwave Background doesn't indicate that the Earth enjoys a privileged spot in the universe. Isn't this similar to how the axis of the Earth passes through Earth's center?
Not really, though it's understandable how it might seem that way. The problem is that the term "axis" is used in two different ways by physicists. Sometimes it is used as a directed line, as when calculating a torque. A directed line can either pass through a point or not. At other times, though, it is really meant to indicate a direction only. A good example of this would be the "axis of symmetry" of a liquid crystal (properly called the director). A direction need not be associated with any point -- "parallel to the x-axis" is a perfectly good direction that treats every point exactly the same.
That is what happens in liquid crystals, at least in the simplest case.
In the illustration above, the "isotropic liquid" has no preferred direction whatsoever; a molecule is equally likely to point in any direction, so no direction is special. This is how we normally think of the universe; the laws of physics should not depend on where you are or what direction you face. You might think that the difference between "down" and "sideways" when describing the motion of a baseball show that this is wrong; the ball falls down, it does not fall sideways. Actually, though, this happens not because "down" is a special direction, but because the Earth is a big object, and all objects exert a gravitational pull on each other. It is the circumstances that make "down" seem special, not the laws -- just like his behavior might send one man to prison while another man's behavior leaves him free.
At low temperatures, though, we see a crystalline solid. In this case there are three special directions: "up/down", which is the direction the long axes of the molecules point, "left/right" and "in/out", which specify the directions of the axes of the square lattice on which each plane of molecules is ordered.
At intermediate temperatures, though, there is sometimes (as in this case) a liquid crystal phase can emerge. In this illustration, the horizontal planes are disordered -- there is no hint left of the square lattice. That means two of the special directions have been eliminated, but one remains.
In fact, the idea that liquid crystals can be used to model the evolution of the universe goes back to the 1980's, when liquid crystals were used to understand theoretical objects called cosmic strings. However, the liquid crystal analogy was never taken to mean that there was an actual preferred direction in space. After all, very detailed experiments have failed to show such a preference.
Anyone who follows science news knows that claims are made of exciting new observations that will force the overturn of cherished ideas are made on a weekly, if not daily, basis. Because most of these claims end up being shown to be erroneous, most scientists adopt a skeptical, wait-and-see attitude, especially towards the more extravagant claims. Regarding the Axis of Evil, I'm still in the wait-and-see camp.
It's worth noting, though, that the possible violation of isotropy is NOT being buried by scientists. Scientists are the ones who found the Axis of Evil, and scientists are the only ones talking about it. Scientists are the ones testing the hypothesis and the ones coming up with possible explanations for it. The idea that the Axis of Evil "is being assiduously avoided for obvious reasons" is so untrue as to be borderline dishonest. If it were really being avoided, the public would never have heard of it.
UPDATE: I never clearly stated that the Axis of Evil, if it is real, would in fact be expected to be closely analogous to liquid crystals -- that is, due to spontaneous symmetry breaking in some particle field. This still would not make Earth a "special place" from the point of view of physics, but it would be very exciting in that it might help us get beyond the Standard Model, where particle physics has been stuck for 30 years or more.
Thank you for this excellent article. It is true that a liquid crystal analogy can model the preferred direction in our cosmos, but the problem is that such an explanation would also require the abandonment both of the Copernican Principle, and of all our present day physics.
ReplyDeleteThe other factor is that many other phenomena have been observed to align along this same preferred Axis, including:
1. Radio sky (dipole in same direction as CMB dipole, but *cannot be attributed to an assume motion of the local group*, as is commonly assumed in the case of the CMB dipole)
2. Type 1a Supernovae dipole
3. Quasar polarization preferred vector
4. "Dark flow"
5. Galaxy spin "handedness" preferred direction
It is one thing to suggest that space is organizing photons along a preferred axis. It is quite another to suggest that the photons are reporting to us an actual intrinsic anisotropy of the cosmos.
EITHER possibility demolishes the Copernican Principle.
You'll need to provide accessible citations, like http://arxiv.org/abs/1310.4184, http://arxiv.org/abs/1310.5211, http://arxiv.org/abs/1305.2701, or http://arxiv.org/abs/1201.5371. I am a physicist, but I don't have a cosmological axe to grind, so I do not carefully monitor everything from that field. At any rate, it is not exactly healthy for someone outside the field to take bleeding-edge sciene TOO seriously; a good rule of thumb is to wait 5 or 10 years and see how that development is viewed by experts then.
ReplyDeleteI have only had time to glance at the abstracts for the citations I just provided, and it does look like there is some interesting research being done, but none of the authors give it the same meaning you do.
You are wrong, in any event, about the Copernican Principle. The fact that we are looking at an apparent axis in the CMB means that it would look exactly the same to a hypothetical observer in the Andromeda Galaxy or the Whirlpool Galaxy or the Sombrero Galaxy. They would not be able to identify the Milky Way Galaxy, let alone Earth, as being in a special position. If these findings hold up, they may mean that the Cosmological Principle is wrong, but not the Copernican Principle.
http://beforeitsnews.com/science-and-technology/2014/06/top-scientists-freak-out-over-new-documentary-questioning-copernican-principle-which-they-appeared-in-2699776.html
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