Laboratory experiments here on Earth have determined that each element in the periodic table emits photons only at certain wavelengths. These photons are apparent to astronomers as either emission or absorption lines in the spectrum of an astronomical object, like stars in galaxies. Astronomers can, by measuring the position of these spectral lines, determine which elements are present in the star itself or along the line of sight. When astronomers perform this analysis, they note that for most astronomical bodies, the observed spectral lines are all shifted to longer (redder) wavelengths. This is known as cosmological redshift and is analogous to the Doppler redshift. What does this redshift indicate to us about the universe in general?
A) The age of the universe.
B) The universe is expanding.
C) How much longer the universe should exist.
D) The universe is contracting into a black hole.