Astrophysicists have always assumed that our Universe has a “mirror” symmetry. Researchers are now trying to test this assumption, and their first results show that our Universe might not be exactly as expected.

In a Universe with a mirror symmetry, the image of a counter-clockwise rotating galaxy is a galaxy with a clockwise rotation. The two kinds of galaxies should be present in equal amounts: if one of the two types is more abundant than the other, there is a breakdown of mirror symmetry, or parity violation.

Michael Longo of the University of Michigan’s Physics Department and his team used data from the Sloan Digital Sky Survey to study the rotation directions of spiral galaxies. After studying tens of thousands of galaxies, the team found an excess of left-handed spirals in the direction of the north pole. Although the excess is only of 7%, it is very unlikely to be a cosmic accident (one chance in a million). Our Milky Way Galaxy rotates the same way, and the effect extended out to distances over 600 million light-years.

The team’s results have recently been published in Physics Letters B.

What does this result mean, exactly? It is today accepted that according to what is called “the cosmological principle”, the Universe is homogeneous and isotropic on large scales: whoever and wherever you are, the Universe looks the same. If spiral galaxies tend to have their rotation axes aligned, that means there is a preferred direction in the Universe: it is not isotropic.

Most of the data analyzed came from the northern hemisphere of the sky, as the Sloan Telescope is located in the northern hemisphere. An important test of this new result will be to see if there is an excess of right-handed spiral galaxies in the southern hemisphere, once more data is available.

If these results are confirmed, this might have extremely important consequences on cosmology: the cosmological principle is a required assumption in many models and theories. If the Universe is shown not to be isotropic, it might completely change our view of the cosmos.



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