David Kordahl at The New Atlantis:
Why was the direction of cosmology altered so decisively when two radio engineers observed the cosmic microwave background? It wasn’t because of just one thing. The “hot big bang” scenario predicted the cosmic microwave background, since matter and light were presumed to be in thermal equilibrium at the beginning. As the universe expanded and cooled, the matter was thought to clump together gravitationally, becoming stars and galaxies, while the light was thought to zip around diffusely, becoming the cosmic microwave background. But the hot big bang also solved other problems. Steady-state theorists posited that hydrogen was being created ex nihilo, while heavier elements could be forged by nuclear fusion in the stars. But their numbers didn’t work out. Steady-state theorists found that helium was more abundant in stars than stellar fusion alone would predict. But in a hot big bang, hydrogen could also be fused into helium at the very beginning.