I understand that the Big Bang theory estimates the age of the universe to be about 14 billion years. I also understand that the farthest galaxy seen by the Hubble telescope is about 12 billion light years away. How can this be? If the light took 12 billion years to get here that means it left only 2 billion (roughly) years after the Big Bang. How did the galaxy get that far away in that short time?
It takes light 12 billion years to get to us from such a galaxy, in which time the universe itself has expanded to its current size. When the light was emitted from the galaxy, it was much closer to us than it is now, and space has been expanding ever since.
After that distant galaxy emitted the light that we are seeing today, that light had to travel across a rapidly expanding universe before it could get to us. Like an ant trying to get to the other side of a stretching rubber band, the steady progression of the light in our direction is retarded by the stretching of space between us. So even though the galaxy was much closer than 12 billion light years at the time of emission, we're not seeing it until 12 billion years later. So we say that its distance today is 12 billion light years.
This page was last updaed on December 3, 2016.