Faster Than Light Expansion of the Universe

What Does it Mean for a Galaxy to Recede at Superluminal Speed?

Sep 15, 2009

Although Einstein's special relativity says that nothing can move through space faster than light, space itself can expand and carry away galaxies faster than light.

Many sources discussing deep space astrophysics or cosmology claim that certain galaxies move away from the Milky Way Galaxy at faster than light speeds. To the reader who is familiar with even the most basic principles of Einstein's theory of Special Relativity, this statement may be very puzzling. How can anything go faster than the speed of light when Special Relativity strictly forbids it?

The resolution to this apparent contradiction turns out to be partly from a loop hole in Special Relativity and partly from choosing less ambiguous wording.

An Expanding Universe Bends the Rules of Special Relativity

The most well-known rule of Special Relativity states that nothing can travel through space faster than the speed of light. This rule has been verified by physicists time and time again and is now taken to be a scientific fact. There is, however, a loophole in that rule which emerges from the words "through space". Space itself can stretch, warp and do many strange things without violating this rule. If the space between two galaxies stretches then to each galaxy the other will appear to be moving away at some speed. However, this is not motion in the usual sense of the word.

Thanks to Edwin Hubble it is now common knowledge that the universe is expanding. The space between galaxies is being stretched at a slowly accelerating rate. This causes galaxies to appear to move away from us in all directions. Furthermore, if we compare the perceived speeds of two galaxies, a galaxy twice as far will appear to move away from earth approximately twice as fast. That is, as the distance between the Milky Way and a galaxy increases, the faster it appears to move away from the Milky Way. At certain large distances, galaxies will appear to be moving away with speeds faster than that of light, but again this is simply due to the expansion of space and not the motion of galaxies through it.

Looking Back in Time at the Speeds of Younger Galaxies

There is one more subtlety that keen readers may have noticed. If light travels through space and distant galaxies are being carried away by the expanding space faster than the speed of light, how does the light they emit ever reach earth? The answer is: it doesn't. Light emitted from a galaxy takes a long time to reach earth. When gazing upon images of galaxies taken by the Hubble telescope, one must realize that it is an image of the galaxies as they looked billions of years ago.

When astronomers claim to see a galaxy which is travelling faster than light, what they mean is that they observe light which left the galaxy at an early time when it was closer to the Milky Way's current location. At that early time it wasn't being carried away by the expansion so quickly, so the light had a chance to reach earth. The light, however, had taken so long to reach earth that now the galaxy is so far away and being carried away so fast that the light it emits right now will never reach earth in the future.

Galaxies which are now too far away – or equivalently, being carried away by the expansion too fast – for their light to reach us, are said to be outside of the horizon. Once a galaxy is outside of the horizon, the light it emits will never reach earth.

A Very Lonely Future for the Milky Way Galaxy

Astrophysicists now believe that the universe will continue to expand faster and faster. This effect will carry most of the galaxies astronomers see today, outside of the horizon. Cosmologist Lawrence M. Krauss, in a lecture entitiled "Our Miserable Future" given at McGill University in 2007, has pointed out that in the very distant future, humans will not be able to see most of the galaxies they see today because light from those galaxies will never reach eath once they pass outside of the horizon. This paints a picture of a very lonely future universe indeed.

References

Freedman, Wendy L., The Expansion Rate and Size of the Universe, Scientific American Inc., 1998.

The copyright of the article Faster Than Light Expansion of the Universe in Deep Space Astronomy is owned by Jasper Palfree. Permission to republish Faster Than Light Expansion of the Universe in print or online must be granted by the author in writing.