This lesson explains what astronomers mean when they say the universe is expanding. Rather than imagining galaxies moving through fixed, empty space, we will examine how space itself stretches and how that stretching affects light.
A model of the universe must include its observed expansion. It also commonly relies on the cosmological principle: at sufficiently large scales, the universe is approximately homogeneous and isotropic. Homogeneous means broadly similar from place to place; isotropic means broadly similar in every direction.
All flashcards
What is the cosmological principle?
It is the assumption that, on sufficiently large scales, the universe is broadly homogeneous and isotropic—similar from place to place and in every direction.
What does the scale factor R describe?
It represents the relative scale of the universe. As R increases, distances between widely separated points that participate in cosmic expansion increase.
What is cosmological redshift?
It is the increase in the wavelength of light caused by the expansion of space while the light travels through the universe.
Does cosmic expansion have a central starting point in space?
No. Expansion is the stretching of space throughout the universe, not movement outward from a single location within it.
What does a measured cosmological redshift tell astronomers?
It indicates how much the universe expanded while the observed light was traveling from its source to us.
Quiz questions
What does it mean for the universe to be homogeneous on large scales?Answer: It is broadly similar from place to place across sufficiently large regions.
What does the scale factor R represent in cosmology?Answer: The relative scale of cosmic distances that expand with the universe.
Why is there no special location in space where cosmic expansion began?Answer: Space itself stretches throughout the universe rather than expanding outward from one point.
If the universe expands by a factor of 2 after light is emitted, what happens to the light’s wavelength?Answer: It doubles as the wave stretches with space.
Why can light from a distant galaxy show more cosmological redshift than light from a nearby galaxy?Answer: It usually travels longer through expanding space, giving its wavelength more time to stretch.
Based on Astronomy 2e by Andrew Fraknoi, David Morrison, Sidney Wolff (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.