Final answer:
The cosmic scale factor that represents the average distance between galaxies in the past, when it was half of the current distance, is 0.5 or 1/2.
Step-by-step explanation:
The student is asking about the cosmic scale factor that describes the uniform change in scale of the universe due to its expansion. In the context of the question, the present scale factor a(t) is 1.0, which represents the current average distance between galaxies. If this distance is twice what it was at some time in the past, then the past cosmic scale factor would be 0.5 (or 1/2), because the scale factor is directly proportional to the distances between galaxies. Thus, if the current factor is 1 and the distances were half of what they are now, the scale factor at that time must have been half of the current factor, which is 0.5.