Why should galaxy collisions have been more common in the past than they are today?
A)Galaxies were closer together in the past because the universe was smaller.
B)Galaxies were more active in the past and therefore would have collided with each other more frequently.
C)Galaxies were much bigger in the past since they had not contracted completely.
D)Galaxies attracted each other more strongly in the past because they were more massive;they had not yet turned most of their mass into stars.

Question

Quizplus · Accepted Answer

Galaxies were closer together in the past because the universe was smaller. As the universe expands, the distances between galaxies also increase, reducing the likelihood of collisions.

Answer: B
 Galaxies were more active in the past and therefore would have collided with each other more frequently. Galaxies had more gas and dust in the past, leading to increased star formation and galactic activity, which increases the likelihood of collisions.

Answer: D
 Galaxies attracted each other more strongly in the past because they were more massive; they had not yet turned most of their mass into stars. The gravitational force between galaxies is proportional to their masses, so more massive galaxies would have experienced stronger gravitational attraction and were more likely to collide.

Why Should Galaxy Collisions Have Been More Common in the Past