Cosmic inflation
Cosmic inflation is the theory that the universe is expanding.
By measuring the spectral shift of galaxies Hubble was able to determine that the vast majority of galaxies are receding from us. He also discovered that the further a galaxy is from ours, the faster it is receding. One explanation for this observation is that the space of the Universe is expanding. That is, over time two points grow further apart; the very fabric of the Universe is growing. Template:TOC-right
The spectral shift of light traveling through space is often compared to the audible pitch shift that occurs when a train is approaching or receding from an observer. The additional relative speed of the train causes the compression waves of sound to bunch up (go higher) or spread out (go lower). However, the speed of light is constant. No matter what the relative speed of an object is, the light emitted from it travels at a constant rate. A different mechanism is responsible for the spectral shift of light - cosmic expansion.
While light travels at a constant speed, the very space it occupies expands causing the wave nature of light to lengthen, thus lowering its perceived frequency. The further the light travels, the more of this space lengthening it will experience, and the more frequency shift it will exhibit.
It is interesting to note that gravity holds space together. We do not see this expansion in areas of the universe that contain much matter. For example, the space between stars in our galaxy is not expanding, nor is the space between atoms in our bodies. However, the density of matter in the space between galaxies is so low that cosmic inflation can occur.