Negative Refraction Might be Distorting our View of the Universe

In the late 1920’s, Igor Tamm, a Russian physicist, said that the movement of electromagnetic waves through space-time warped by stars and galaxies is equivalent to the movement of electromagnetic waves through non-warped space-time, but in an imaginary bianisotropic material which affects the components of an electromagnetic wave in different ways depending on the direction in which the wave moves through the material.

Light travels at different speeds in different substances; therefore, it bends, or refracts, when passing from air into another material, such as glass or water. Every known material slows light relative to its speed in a vacuum. Light normally always refracts in the same direction. In 1967, Victor Veselago, a Russian physicist, modified Maxwell’s equations describing the movement of electromagnetic waves through materials, and proved that it would be possible to create a material that would bend light the other way. In 2000, physicists announced that they had created a composite material, made up of an array of wires and copper crescents, that worked at microwave frequencies, that could cause light to refract opposite its normal direction.

Lakhtakia and Mackay began studying negative refraction three years ago. Lakhtakia and Mackay worked out the conditions in which Tamm’s bianisotropic material would create negative refraction. They then studied the cosmology literature for relativistic equations that would match their results. In August 2004, they discovered that the equations describing the region near a rotating black hole are very similar to those for materials producing negative refraction. While light cannot escape from a black hole’s gravitational pull, it can escape from the ergosphere, a zone just outside the black hole’s horizon. A spinning black hole appears to provide the right conditions for negative refraction. Other astronomical effects, including the space-time distortion caused by the inflation of the universe, might also cause light to bend opposite the way it is normally expected to bend.

It is possible that negative refraction may help solve the problem of dark matter. In studying the mass and spin rate of rotating galaxies, astronomers have found that the gravitational attraction of all the galaxies is not strong enough to hold them together, and they should be torn apart by centrifugal force. Some researchers have said that this is because the laws of gravity need to be modified, but the most popular solution has been to assume the existence of invisible dark matter that would add the necessary amount of gravity. Lakhtakia and Mackay, however, suggest that rotating black holes in our line of sight may be causing negative refraction and skewing our measurements of stellar positions. Therefore, the gravity in the Universe might be enough to hold the galaxies together, without the existence of dark matter. Another option they have considered is that dark matter could itself cause negative refraction.

Negative refraction might also provide explanations about dark energy and the hidden extra dimensions of space. Lakhtakia and Mackay have shown that negative refraction is linked to the cosmological constant, a constant that Einstein added to his space-time equations so that they could describe a static Universe that is neither expanding nor contracting. Edwin Hubble later discovered that the Universe is, in fact, expanding, and recent observations of distant supernovae show that the expansion is accelerating. It has been suggested that this expansion is caused by a dark energy permeating the universe. The dark energy corresponds to a small, positive value for the cosmological constant.

The rate at which the supernovae appear to be traveling indicates that dark energy accounts for 73 percent of all the energy in the Universe. But this is too much for some physicists to believe. They say that we can’t be sure if the Universe’s expansion really is accelerating. The cosmological constant could be zero. It could also be negative, if one subscribes to the brane-world model of the Universe, where the three spatial dimensions we experience are part of a multidimensional reality, in which the cosmological constant can be either positive or negative.

Using a description of space-time developed by Stephen Hawking and Gary Gibbons at the University of Cambridge in the 1970s, Lakhtakia and Mackay have shown that negative refraction is only consistent with a Universe that has a positive cosmological constant. So if astronomers find evidence of negative refraction away from the vicinity of a rotating black hole, it will confirm that the universe’s expansion is accelerating, and may rule out some brane-world models.

Christopher Kochanek, an astronomer based at Ohio State University in Columbus, argues that cosmologists already use the equations of general relativity to compute photon trajectories throughout the universe, including the areas around rotating black holes. Since there is no change in the way electromagnetism works, or how photon trajectories are calculated, Lakhtakia and Mackay’s calculations don’t make a difference. Lakhtakia’s response is that until five years ago, researchers who used Maxwell’s equations to calculate the path of light through materials discarded results that led to negative refraction. So astrophysicists are basing their knowledge on assumptions that do not include the discarded results.

Alexei Starobinsky, a cosmologist based at the Landau Institute for Theoretical Physics in Moscow, objects to Lakhtakia and Mackay’s findings by stating that black holes capable of producing negative refraction constitute a negligible part of the Universe. Mackay’s response is that while this is true, it does not mean that the effects are negligible. Astronomical distances amplify tiny effects. In addition, some cosmologists suggest that dark matter is composed of black holes. If so, there could be a substantial negative refraction effect contributed by these black holes.