Ask a Scientist: Time, space and the big bang

Otago Daily Times, 16 December 2016

Barin Das, of Wellington, asked:-

We read sometimes that a very distant galaxy has been observed, say 12 billion light years away whose light left not long after the big bang. How is that possible without the the two moving apart faster than the speed of light which is itself a limiting speed? If light from that object left 12 billion years ago was it closer to the center of the big bang? Consequently are we 10-15 billion light years away from that point?

David Wiltshire, a theoretical physicist and Alan Gilmore, an astronomer at the University of Canterbury responded:

Most current evidence indicates that the Big Bang happened 13.7 billion years ago. The universe expanded a lot before the first galaxies formed. It continues to expand.

When a galaxy is described as being 12 billion light years away we mean that its light has been travelling to us for 12 billion years. The galaxy wasn't 12 billion light years away when the light was emitted. It was much closer: only 5.5 billion light years away.

Due to the expansion of the universe distances between galaxies increase during the time the light is travelling. It took light from that galaxy 12 billion years to catch up with us. But today that same galaxy is 24.4 billion light years away.

It is wrong to think of the Big Bang as an explosion in a pre-existing space with a preferred centre. Rather every point is equally the "centre" of expansion. A good analogy is to imagine raisins in a uniform self-raising dough; any raisin will see the distances to other raisins increasing, regardless of which raisin is chosen. The expansion of space is the same. Space is nothing other than the distances between things. On the largest scales the distances between clusters of galaxies have been increasing for the entire age of the universe, and continue to do so. In a very literal sense space is being continuously created. Space is "nothing"; there's just more of it as time goes by.

The speed of light is a limit for objects which whizz by you at your location. This is because velocity, a rate of change of distance with respect to time, can only be defined with respect to fixed local rulers and clocks. There are no fixed rulers on the cosmological scales on which space is being created. So the rate of expansion of the universe which describes the rate of creation of space, rather than motion of objects through space, is not actually limited by the speed of light.

Professor David Wiltshire
Department of Physics and Astronomy
University of Canterbury