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Comoving Distance [Q&A]


Atul of Canara College asks, how is it possible that we can see quasars that are 28 billion light years away, when the Universe itself is only 13.7 billions years old? How can light have traveled farther than the Universe has existed?

This can happen because the Universe is expanding.

Distances to quasars and similar distant objects are often given in terms of the comoving or proper distance. This point was also made during the discussion on the size of the observable Universe. Essentially, this is their distance from us right now, not the actual distance at the time that the light was emitted. While the comoving distance remains fixed (because it is defined for the current epoch, and thus doesn’t depend on the expansion of the Universe), the proper distance, which is a measure of the physical distance at a given epoch, does change, and is smaller at earlier times.

When the Universe was young, everything was closer together (smaller proper distances). Suppose a galaxy emits light in our direction then. As the light propagates towards us, the proper distance between us and that galaxy is increasing because of the expansion of the Universe. By the time the light actually arrives at Earth, the galaxy will have moved farther away, and the farther it gets, the faster it moves away. That is how we can get such large distances.

Conversely, light that leaves the Earth now, can only get to a point that is about 10 billion light years away now — anything beyond that distance would be moving away too fast for the light to catch up by the time it gets there.