Question

I am sorry to ask my question out of syllabus. My question is:

We know that the universe is expanding rapidly. Every star and planet is going away from one another faster than the speed of light! Then why it is said that our Galaxy (Milky way Galaxy) is coming nearer to our neighbour galaxy (Andromeda Galaxy) and after many years, these galaxies will collide with each other?

Answer 1

First thing that I want to say to you is that After the Big Bang the universe is assumed to expand at a speed greater than the speed of light. But there is a clause. The clause is that it happens only in the absence of any other effects.
That is galaxies have millions of stars and they are having a very large amount of mass. So they attract other galaxies which eventually makes them collide. Universe is assumed to expand only if these attractive forces are not present.
The outskirts or at the tip of the universe, there is very little amount of matter so there is no much of attracting forces present there and hence universe seem to expand at that point.
But inside the universe strong gravitational forces tend to pull the galaxies together and eventually these will collide to form clusters and superclusters.