If the galaxies in the Universe have been observed to be expanding ,how do we know that the Universe expands?.

Question

and why should it expand to go where?

If something is moving in a curved trajectory just like the curvature of the Universe ,whould we still ofserve the same red shift doppler effect or wave dispersion effect?

Answer 1

this may be a simpler answer.As a train moves away from you the sound frequency changes.Similarly an observer on Earth(powerful telescopes)are receiving light from galaxies at changing frequencies.So we assume that they are moving away from us.Since every thing is moving away from us, the universe is expanding from the point of view of an observer on earth.

Answer 2

We know that the universe is expanding because we can measure the doppler shift in frequency of light from galaxies. The doppler shift is due to the movement of the galaxies, and it is always towards lower frequencies (red shift) which means the galaxies are receding from us. The only way this could happen is for the universe to be expanding.

Answer 3

Curvature of trajectory is not the same as curvature of the Universe.

One is curvature of path of motion, the other is curvature of spacetime.