Final answer:
In a collapsing universe, light from galaxies would experience a cosmological blueshift due to the decrease in wavelength as they move closer together. This is opposite to the current redshift observed due to universal expansion. Discovering an unknown cause for the redshift would have a profound effect on cosmology, as it would challenge the Big Bang theory.
Step-by-step explanation:
If the universe were to begin collapsing, the light emitted from clusters and superclusters of galaxies would be affected by a blueshift. This is because, in a collapsing universe scenario, galaxies would be moving closer to each other rather than away, which is the opposite of the current expansion observed. This phenomenon of cosmological blueshift would result from the decrease in the wavelengths of light as galaxies accelerate towards each other, just as the light from galaxies currently experiences a redshift from the expansion of space that stretches the wavelengths, making them appear redder.
In the context of cosmological observations, if an unknown cause of the redshift—like light becoming 'tired' from traversing vast stretches of space—were discovered, it would have a profound effect on cosmology. The current understanding of the redshift, as evidence of the cosmic expansion and an integral part of the Big Bang theory, would need to be reconsidered, potentially upending many established concepts.