Hubble's observations showed that the universe is expanding because recessional velocities are proportional to what?

Hubble's observations showed that the universe is expanding because recessional velocities are proportional to what?

• A. Mass
• B. Distances
• C. Temperatures
• D. Brightness

Answer: (B)

The key idea is Hubble’s law: galaxies appear to be receding from us, and the speed at which they recede increases in direct proportion to how far away they are. When we look at the light from distant galaxies, its redshift tells us their recessional velocity, and plotting that velocity against distance shows a straight-line relationship: v is proportional to d. This linear pattern means space itself is expanding, carrying galaxies along with it as it stretches. The constants of nature that set this behavior aren’t the galaxy’s mass, its temperature, or how bright it appears. Those properties don’t govern how fast a galaxy should be moving away just because it’s farther out. Instead, what matters is distance: the farther a galaxy is, the faster it seems to recede, precisely because the universe is expanding.

The key idea is Hubble’s law: galaxies appear to be receding from us, and the speed at which they recede increases in direct proportion to how far away they are. When we look at the light from distant galaxies, its redshift tells us their recessional velocity, and plotting that velocity against distance shows a straight-line relationship: v is proportional to d. This linear pattern means space itself is expanding, carrying galaxies along with it as it stretches.

The constants of nature that set this behavior aren’t the galaxy’s mass, its temperature, or how bright it appears. Those properties don’t govern how fast a galaxy should be moving away just because it’s farther out. Instead, what matters is distance: the farther a galaxy is, the faster it seems to recede, precisely because the universe is expanding.