The Expansion of Space
The universe is expanding
What expansion means
In cosmology, expansion means that the average distance between faraway galaxies increases with time. It does not mean galaxies are exploding outward from a central point in space; rather, space itself is stretching on large scales.
Important intuition
Expansion is a property of the space between galaxies, not usually the motion of galaxies through space like bullets.
Redshift and stretching
As space expands, the wavelengths of traveling light can stretch too. If a wave is stretched, its wavelength becomes longer and its color shifts toward the red end of the spectrum. This is one of the main observational signs of expansion.
Expansion vocabulary
In cosmology, expansion mainly means that:
Cosmic expansion describes the stretching of space on large scales.
Correct answer: The average distance between faraway galaxies increases
What happens to the wavelength of light as the universe expands?
Stretching space stretches the light, increasing wavelength.
Correct answer: It increases
A simple analogy
A useful analogy is dots on the surface of a balloon. As the balloon inflates, every dot sees other dots moving away. The surface represents a 2D version of expansion; our universe has three spatial dimensions, so the real picture is more complex.
Which statement is most accurate?
Cosmic expansion concerns the large-scale stretching of space.
Correct answer: Expansion is the stretching of large-scale space
Why is the balloon analogy only partly useful?
Analogies help intuition, but they do not perfectly match reality.
Correct answer: Because it is a lower-dimensional picture and the universe is not literally the surface of a balloon.