Question

All galaxies farther away than about 50×10^6 ly exhibit a red shift in their emitted light that is proportional to distance, with those farther and farther away having progressively greater red shifts. What does this imply, assuming that the only source of red shift is relative motion? (Hint: At these large distances, it is space itself that is expanding, but the effect on light is the same.)

a) The galaxies are moving at constant velocity.

b) The galaxies are accelerating away from us.

c) The universe is contracting.

d) The universe is expanding.

Answer 1

The progressive red shift in light from galaxies at distances greater than 50 million ly suggests that the universe is expanding, as the stretching of space itself during the transit of light increases the wavelength, resulting in red shift.

Step-by-step explanation:

Observations show that galaxies farther away than about 50×106 light-years (ly) exhibit a red shift in their emitted light that is proportional to their distance, with the red shift increasing for galaxies that are farther away. This phenomenon implies that the universe is expanding.

The red shift is analogous to the Doppler Effect, but instead of being due to the actual motion of galaxies through space, it's caused by the expansion of space itself. This expansion stretches the light waves as they travel through the universe, resulting in a longer (red-shifted) wavelength by the time they reach us. Applying Edwin Hubble's groundbreaking discoveries, it's clear that the space within the universe is expanding, with galaxies receding from each other. The correct answer to the question is d) the universe is expanding.