redshift

The night sky is dark because the universe has a finite age, not because stars are few. How Olbers' paradox works, who solved it, and what it reveals.

Space telescopes do not look back in time by magic. They collect ancient light stretched by cosmic expansion. Here is how redshift reveals the early universe.

Hubble's 1929 redshift observation tied galaxy speed to distance. See what the 24-galaxy data showed, why his constant was off 7x, and the tension today.

In 1929, Edwin Hubble showed that galaxies recede from us at speeds proportional to their distance, the first direct evidence that the universe is expanding. The slope of that relation, the Hubble constant, is still debated: two precise methods disagree by about 5 sigma.