Definition of Cosmological Redshift
Cosmological redshift is the phenomenon where light from distant galaxies appears 'redder' (shifted towards the red end of the electromagnetic spectrum) because the space between us and those galaxies is stretching. It's not due to the galaxies moving *through* space, but rather the space *itself* expanding, carrying the galaxies along with it.
How Space Expansion Causes Redshift
As light travels across an expanding universe, the wavelengths of the photons get stretched. Imagine a wave drawn on a rubber sheet; as the sheet expands, the wave also stretches, increasing its wavelength. For visible light, longer wavelengths correspond to the red end of the spectrum, hence the term 'redshift.'
Evidence for the Expanding Universe
The observation of cosmological redshift is one of the strongest pieces of evidence supporting the Big Bang theory and the expansion of the universe. Edwin Hubble first noted this relationship, demonstrating that the farther away a galaxy is, the greater its observed redshift, implying a faster rate of apparent recession.
Distinction from Doppler Redshift
It's important to distinguish cosmological redshift from Doppler redshift. Doppler redshift occurs when an object is moving away from an observer *through* space (like a receding siren's pitch lowering). Cosmological redshift, however, is caused by the expansion of space itself, stretching the light waves during their journey, independent of the galaxy's peculiar motion.