The night sky is like a cosmic time machine. Whenever we observe a distant planet, star or galaxy, we are seeing it as it was hours, centuries or even millennia ago. This is because light travels at a finite speed (the speed of light) and given the large distances in the Universe, we do not see objects as they are now, but as they were when the light was emitted. The time elapsed between when we detect the light here on Earth and when it was originally emitted by the source, is known as the 'lookback time'.|
For very distant objects, the lookback time is increased by the Hubble expansion of the Universe, which is causing the space between galaxies to increase with time.