Does our universe have mirror symmetry? That is the question Prof. Michael Longo of the University of Michigan's Physics Department asked. The answer could perhaps be found by studying the rotation directions of spiral galaxies.
Physicists and astronomers have always assumed that the Universe has this symmetry. To test this, Longo and his team of five undergraduates used data from the Sloan Digital Sky Survey to study the rotation directions of spiral galaxies. The mirror image of a counter-clockwise rotating galaxy, like the example, would have clockwise rotation. An excess of one type over the other would be evidence for a breakdown of mirror symmetry, or, in physics speak, a "parity violation" on cosmic scales.
Longo and his team, after studying tens of thousands of spiral galaxies, found an excess of left-handed spirals in the part of the sky toward the north pole of our galaxy, the Milky Way. The excess is small, about 7%. However, Longo estimates the chance that the excess could be a cosmic accident is something like one in a million. The effect extended out to distances over 600 million light years. Our galaxy also rotates in the same sense.
"If verified, this data would be extremely important because it is almost universally accepted that on sufficiently large scales the universe is isotropic (no special direction)," he said.
If spiral galaxies tend to have their rotation axes aligned in one direction, it means that there is also a preferred direction in the universe. This violates another tenet of astrophysics that assumes the universe has no special direction or is "isotropic."
Because the Sloan telescope is in the northern hemisphere, the data that was analyzed came mostly from the northern hemisphere of the sky. An important test of this result will be to see if there is an excess of right-handed spiral galaxies in the southern hemisphere. Longo looked at the limited sample that is available now, and found that there does seem to be more right-handed ones there. More data from the southern hemisphere will provide an important test of this result.
Longo's paper has recently been published in Physics Letters B. An anonymous referee who reviewed the paper for the journal said: "In the paper the author claims that there is a preferred handedness of spiral galaxies indicating a preferred direction in the universe. Such [a] claim, if proven true, would have a profound impact on cosmology and would very likely result in a Nobel prize."
- Michael J. Longo. Detection of a dipole in the handedness of spiral galaxies with redshifts z∼0.04. Physics Letters B, 2011; 699 (4): 224 DOI: 10.1016/j.physletb.2011.04.008
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