What fundamental constant dictates that observing distant galaxies is looking backward through cosmic time?

Answer

The finite speed of light

The ability to study cosmic history by observing distant galaxies relies entirely on the fact that light travels at a finite, constant speed. Because light requires a significant amount of time to traverse billions of light-years, the light received today from a galaxy billions of light-years away began its journey billions of years ago. This finite travel time establishes a direct, measurable relationship between the distance to an object and the age of the universe captured in that observation. This relationship is the underpinning of extragalactic archaeology, allowing researchers to assemble a chronological sequence of galactic evolution, effectively peering into the universe's infancy by collecting photons emitted long ago.

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cosmology galaxy evolution galaxy formation distant galaxies cosmic time