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The figure illustrates the notion that the universe might be behaving
like a birefringent crystal,
in which light moving in one direction behaves differently from light
travelling in another direction.
A 250-dpi GIF version of this image is available.
Summary by Borge Nodland (bnod@lle.rochester.edu), University of Rochester
In the cosmos, there are many galaxies that emit so-called "electromagnetic
synchrotron radiation.'' This radiation is highly plane-polarized, which
means that its electric field oscillates predominantly within a plane,
called the polarization plane of the radiation. In their journey
through the cosmic expanse, these plane-polarized waves pass through
intergalactic magnetic fields and charged particles, which rotate the
polarization plane of the waves via a well-understood physical process
called the "Faraday rotation effect.''
During the past few years, John Ralston of the University of Kansas and I have studied data published by several independent research groups on the polarization of radio waves emitted by synchrotron galaxies. Surprisingly, we found that a wave's polarization plane undergoes an additional rotation that is very different from Faraday rotation. The amazing thing is that the new rotation depends on the direction the wave moves through space. This is reminiscent of how a birefringent - or electromagnetically anisotropic - crystal changes the polarization of light passing through it in a way that depends on the direction the light travels through the crystal.
To be more specific, we found that the rate of rotation depends on the angle between the direction of travel of the wave and a fixed direction in space, pointing approximately toward the constellation Sextans from Earth. The more parallel the direction of travel of the wave is with this fixed direction, the greater the rotation. The amount of rotation is also proportional to the distance of travel of the wave. These are the only two dependencies of the rotation.
The curious effect is illustrated in the diagram above. In this diagram, Earth is at the center, and the direction toward Sextans is represented by a red "anisotropy axis.'' The axis extends from Earth toward Sextans in one direction, and toward the constellation Aquila in the opposite direction. A plane-polarized radio wave emitted by Galaxy A (green) travels toward Earth in a direction almost parallel to the anisotropy axis (red). On the other hand, a plane-polarized radio wave emitted by Galaxy B (blue) approaches Earth in a direction almost perpendicular to the anisotropy axis.
As the two waves propagate through space, their planes of polarization rotate, as shown by the green and blue helices. The distances of travel are the same for both waves, but the nearly parallel wave (green) has its polarization plane rotated more than the nearly perpendicular wave (blue). In general, we find that the rotation increases systematically as a wave's direction of travel approaches that of the fixed anisotropy direction. For illustrative purposes, the rotation effect in this diagram is exaggerated. The actual effect is extremely tiny, we find that one full revolution of the polarization plane is completed after the wave has voyaged for about a billion years.
It is important to note that the anisotropy axis running through Aquila, Earth and Sextans, as shown in the figure, only represents a direction, or, in the vernacular of mathematics, a vector, in space. Any other axis - possibly vastly remote from Earth, Sextans and Aquila - parallel to the anisotropy axis shown here, will suffice in defining the anisotropy vector. No particular location in space, like the location of Earth for example, is relevant - only directions are relevant.
As in any analysis of experimental data, analysis of the synchrotron radiation data pinpointed only approximately the orientation of the anisotropy axis. We found that the data strongly indicated that the anisotropy axis lies within an "anisotropy cone'' that has its vertex at Earth, its central axis pointing from Earth to Sextans, and its surface making a 20 degree angle with the central axis. The data provided no support for an anisotropy vector pointing anywhere outside this cone. In the opposite direction, from Earth to Aquila, the same axis is confined within a similar cone, so that the anisotropy cone is really a "double cone.''
In this figure, the double anisotropy cone is shown in red,
positioned with its vertex at Earth, at the center of the
figure, and opening up toward the constellation Sextans in one
direction, and toward the constellation Aquila in the opposite
direction. Our data consisted of 160 radio galaxies, shown as yellow
dots. The most distant galaxies in the data are about 7 billion light
years away.
A 250-dpi GIF version of this image is available.
In a curious way, the anisotropy direction reveals itself as that
orientation of the needle of a cosmic compass along which
electromagnetic radiation "twists'' the most as it journeys through
the fabric of space. What a coincidence, then, that the constellation
Sextans stands for the sextant, the ancient navigational
instrument by which seafarers would orient themselves. And in mythology, Aquila is the messenger from heaven - the mythological Eagle leading
souls to immortality.
Since the new rotation we find has such a systematic directional dependence, it is implausible that it is generated by cosmic ions and fields via some mechanism similar to the Faraday effect. One may therefore surmise that it is the vacuum itself that flaunts a form of electromagnetic birefringence, or anisotropy - similar to the birefringence exhibited by many crystals.
Thanks to Borge Nodland, University of Rochester, for the figures and text.
This research is described in the 21 April 1997 issue of Physical Review Letters.
