Why is the observable universe 93 billion light years in diameter if the Big Bang was 14 billion years ago?
Indeed, when we look at astronomical objects, we are looking at their past. So looking at the Sun we see it as it was about 8 minutes ago. That’s how long it takes photons released by the Sun to reach our eyes. Looking at the star closest to us, Proxima Centauri, we see its past 4 or so years ago. Sirius is 8.6 years ago, Betelgeuse is about 650 years ago, and so on.
Continuing with this sequence, we would have to see no further than 13.8 billion years, which is when the Big Bang occurred. However, it is common knowledge that the size of the observable universe is 93 billion light years, i.e. about 46.3 billion light years each way. Let’s examine why this is so.
As we know, the universe is expanding. This was established by observing the red and blue displacement of light from distant galaxies. Because of the Doppler effect, the light emitted by objects that are moving away from us is shifted to the red side, and objects that are approaching us are shifted to the blue side. Most galaxies far away from us have a red shift. Only some closest to us are blue. And the farther a galaxy is from us, the greater its red shift. That is, the farther a galaxy is from us, the faster it is moving away from us.
