How is the universe expanding faster than the speed of light?

1 Answer
Mar 30, 2017

The resolution of this apparent problem comes down to understanding the difference between the idea of expansion, and that of motion. Read on...


The problem is resolved if we realize that what we mean when we say "the universe is expanding" is that spacetime is expanding outward. Spacetime is the rather abstract construct of three spatial dimensions and time that serves as the "container" for all material in the universe, and is not a material body. This expansion is different from the motion of the material objects that are inside the universe.

My understanding is that the galaxies we see travelling at speeds greater than that of light are being swept along with the continual growth of spacetime. They are not moving through spacetime at a speed greater than #c#, but are being carried away from each other by the expansion of spacetime.

The common analogy is to imagine two dots on the surface of a balloon. As we inflate the balloon, the dots move apart, not because they are travelling over the surface, but because the surface is expanding.

In the case of galaxies, these are moving apart, and are also being carried to greater distances by the expansion. Thus, they can appear to move away from each other at greater than #c#.

During the inflationary phase of the universe (when it was still very, very young, and smaller than your house), the universe is thought to have expanded at a rate that was many times the speed of light!