According to the Big Bang Theory, the known universe started 13.7 billion years ago in a hot dense state and expanded explosively. As it expanded, it cooled down to the point where atoms could form. By the force of gravity, these atoms coalesced into gas clouds that eventually became stars. These stars eventually formed young galaxies. Which of the following is true about the current position of the galaxies in the universe?
A.
The galaxies are now drifting further apart.
B.
The position of the galaxies cannot be predicted.
C.
The galaxies are now moving closer together.
D.
The position of the galaxies is not changing.

Hubble’s law states that galaxies that are farther away have a greater red shift, so the speed at which a galaxy is moving away is proportional to its distance from the Earth. Red shift is a phenomenon due to Doppler shifting, so the shift of light from a galaxy to the red end of the spectrum indicates that the galaxy and the observer are moving farther away from one another. Doppler shifting also is found in the Energy and Waves section of this course.