Ask Ethan: How can ultra-distant galaxies move so fast?

Key Points:

• Edwin Hubble’s 1920s observations established that distant galaxies are receding from us, leading to Hubble’s Law which relates a galaxy’s recession speed to its distance, implying very distant galaxies can appear to recede faster than light.
• This apparent superluminal recession does not violate physics because it arises from the expansion of spacetime itself, not from galaxies moving through space at speeds exceeding light, a distinction explained by general relativity rather than special relativity.
• Measurements of angular diameter and apparent brightness of distant galaxies show deviations from simple inverse-distance relationships expected in special relativity, confirming that the Universe’s expansion and spacetime curvature must be described by general relativity.
• Using standard candles like Type Ia supernovae, astronomers construct redshift-distance diagrams that reveal the Universe’s expansion history, demonstrating that high redshifts primarily result from cosmic expansion rather than galaxy motion through space.
• The expanding Universe requires no infinite energy input to accelerate galaxies; instead, general relativity dictates that a uniform matter-energy filled Universe cannot remain static and must expand or contract, consistent with observations since the 1920s.