Because of the time it takes for light from these galaxies to reach Earth, many in the picture look exactly as they did over 10 billion (10,000,000,000) years ago. Because of this it allows scientists to look at what the Universe was like at that time, and how it changed to become what it is now. Add to this the cosmological principle, which suggests that on large scales the Universe is roughly the same no matter what direction you look, and the HDF becomes a very important tool for astronomers studying how the Universe has changed.