As measured from Earth, the spectrum of the galaxy NGC 7512 reveals a receding velocity of about 7,000 km/s.If an observer in that galaxy takes a spectrum of the Milky Way they would
Measure ________.
A) the same 7,000 km/s receding speed
B) only 1,000 km/s, because for that observer the Hubble expansion is a lot slower
C) something that we cannot possibly know, because the known laws of physics apply only within the Milky Way
D) 23,000 km/s, because the speed of light has a different value for that observer
E) no relative motion, because we are in fact measuring Earth's motion about the Sun.

Question

Quizplus · Accepted Answer

According to the principle of relativity and the cosmological principle, the laws of physics are the same everywhere, and the universe is homogeneous and isotropic on large scales. Therefore, if the Milky Way is receding from NGC 7512 at 7,000 km/s, an observer in NGC 7512 would measure the Milky Way receding at the same speed of 7,000 km/s.

As Measured from Earth, the Spectrum of the Galaxy NGC