|"In a curious way, . . . reveals itself
as the orientation of the needle of a cosmic compass . . . "
In 1997, Borge Nodland and John Ralston found in their analysis of astronomical polarization data that the universe had an optical axis: it was circularly birefringent! The universe appeared to behave in a similar way as a crystal with optical activity: it rotated the polarization direction of linearly polarized light! This cosmic "quartz crystal" had an optical axis parallel to the direction Aquila-Earth-Sextans . Is the universe birefringent? Is the universe rotating?
" . . . It is interesting to note that the constellation Sextans stands for the sextant, the ancient navigational instrument by which seafarers would orient themselves. Aquila, by the way, is the messenger from Heaven - the mythological Eagle leading souls to immortality" Borge Nodland
In the conventional view, the universe (matter, energy and space) is expanding symmetrically from its Big Bang origins and space has no preferential direction (at least within our inflationary bubble). Light should propagate in the same way coming to us from any direction. However, polarization measurements from distant radio galaxies that emit strongly polarized waves showed a rotation of the polarization plane. This rotation was proportional to the propagation distance as projected along a fixed direction in space: the Nodland-Ralston effect.
The rotation claimed was minuscule: one full revolution of the polarization plane is completed after the wave has traveled for about ten billion (1010) years. It was detected in the microwaves emitted by distant radio galaxies. Those galaxies emit synchrotron radiation by accelerating charged particles in helical paths. The radiation becomes linearly polarized parallel to the particle "orbit" (perpendicular to the bending axis). The emitted polarization is known from the orientation of the galaxy plane with respect to the Earth. The signal was separated from common Faraday rotation produced by magnetic fields in the intervening space using the later dependence with frequency.
The discoverers suggested that electromagnetic interactions may be intrinsically anisotropic and proposed a generalization of electrodynamics theory consistent with their data. Other researchers have attributed the results to the movement of the Sun over the background radiation and to several other origins.
Moreover, others have suggested that the data analysis is faulty and some even claimed an upper boundary for any such effect 30 times lower than the data on the original paper. The controversy is still going on but new measurements should soon decide if space is itself birefringent.
 Nodland, B., and J. P. Ralston, "Indication of anisotropy in electromagnetic propagation over cosmological distances" Physical Review Letters, 78:3043-3046, 1997. Also see this link