Question

Question
Two galaxies on opposite ends of the universe are moving away from the Earth. Each has a velocity of 200,000 km/s relative to the Earth. How fast would an observer in one of those galaxies see the other galaxy moving away? (1 point)
Responses

between 300,000 and 400,000 km/s
between 300,000 and 400,000 kilo meter per second

between 200,000 and 300,000 km/s
between 200,000 and 300,000 kilo meter per second

200,000 km/s
200,000 kilo meter per second

400,000 km/s
400,000 kilo meter per second

Answer 1

Use relativistic velocity addition: u = (v+v)/(1+v^2/c^2) with v = 200,000 km/s, c = 300,000 km/s.

u = 400,000 / (1 + (200,000^2)/(300,000^2)) ≈ 400,000 / 1.44444 ≈ 277,143 km/s.

So between 200,000 and 300,000 km/s.