Question

(a) Explain how a galaxy with z > 1.25 could have been closer at the time it emitted the light that we now see compared with a galaxy with z = 1.25. (b) Explain how a galaxy can be further away at the present time than the distance light can travel in the age of the universe. (c) Explain how a galaxy can be moving away from us faster than the speed of light.

Answer 1

(a) In a universe which is expanding, at a red shift of z = 1.25 and q₀ = 0.5, the angular diameter distance becomes maximum and beyond this distance, the farther objects appear larger and dimmer. Hence, the various galaxies can be compared at various red shifts.

Therefore, for a galaxy at z >1.25, it appears larger and closer.

(b) The expanding of the universe is taking place at a faster speed than the speed of light. Hence, a galaxy might also be further away at the present time than the distance light which can travel in the age of the universe.

(c) The light emitted by the galaxies is moving towards us while the galaxies are moving away from us. Being moving in the opposite direction than that of the direction of the light emitted by the galaxies, these start moving with a speed faster than the speed of light.