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.

As a result, light that travels at a constant speed has the very space it occupies expand 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.