Why the sky changes in certain scenes

In some of the scenes we see comets and double suns. Could these indicate the scenes where he is dreaming? Like on the train where she gives him another chance we see cosmic lights through the window is he just dreaming this and they never get back together in real life but in his alternate universe? And in the end scene when we see 2 suns and he stops to think and we assume he figures out she is lying to him because he told her he would only get thrown off by a lie when he is most venerable.

Also what's up with the sky from her apartment rooftop changing from it being light out at some points and dark with stars in other points? Its the only scene where it is different.