The farther you look into space, the farther back in time you’ll see. This is because light has a speed and it takes time for that light to travel across the universe. Because of this there is a limit to what we can see. It also means there are regions of space which we will never see because they are so far away, even though the light is travelling towards us, the space itself is expanding faster (nothing moving through space can travel faster than light, but the expansion of space can).
We calculate the age of the universe to be around 13.7 billion years old. Therefore we can only see light that has been travelling for this time. However, the rate of expansion of the universe has been changing. The most distant observed object is GRB 090423 which was a gamma ray burst detected in 2009, most likely caused by a star which collapsed when the universe was around 600 million years old. The light we’re seeing from this object has been travelling through the universe for about 13 billion years yet we are seeing the object as it was when it was closer to the Earth, about 36 million light years away.
However, in the 13 billion years that the photons of light given off by GRB 090423 have been travelling, that distance of 36 million light years has stretched to about 46 billion light years away. As a result the light itself
has become stretched, which causes it to be shifted more towards the red end of the spectrum – red shifted.