Final answer:
Redshift, a stretching of light's wavelength due to the expansion of the universe, provides information about the galaxy's distance and the universe's size when the light was emitted. This information is central to understanding cosmic expansion and Hubble's law.
Step-by-step explanation:
We relate redshift to the size of the universe at the time of photon emission through the understanding of cosmic expansion history. When light from a distant galaxy reaches us, its wavelength is stretched, or redshifted, due to the expansion of the universe. This redshift is measurable and proportional to the distance of the galaxy from us, a concept central to Hubble's law.
A Hubble graph plots the velocity of galaxies against their distance, showing that the further away a galaxy is, the faster it is moving away from us. This is used to support the cosmological model of an expanding universe since the Big Bang. To convert redshift into distance, astronomers use the Hubble constant along with models of the universe's expansion. By knowing how far back we are looking in time, we can infer various properties of the universe's early history and composition, including the characteristics of dark matter.