A few weeks ago I was at a conference about galaxy evolution. In the titles of many talks was the puzzling phrase, “secular evolution.” Secular? As opposed to religious? So secular evolution is galaxy evolution that’s not in the context of religion? Surely not. I stopped listening to the talks and googled “secular.” It’s Latin, meaning “belonging to a certain age,” as opposed to “infinite.” Not helping. I opted for the extreme measure of waiting for the coffee break and asking an astronomer.
“Secular evolution” in galaxies turns out to require a little context. Years ago when I started writing about the origin and evolution of the universe, “galaxy evolution” was a matter of connecting some pretty dicey dots. Cosmologists looked at nearby galaxies, at more distant galaxies, at the galaxies so far away you nearly couldn’t see them. And assuming that most distant = farthest back in time = youngest, then those populations of nearby galaxies were grownups, the more distant were adolescents, and the far-away, babies.
Cosmologists arranged these populations into an evolution: Galaxies began as little blue messes, spun up into sparkly spirals, collided and merged into unchanging ellipticals. Galaxy evolution was interesting partly because it showed the universe growing up. The universe that formed those galaxies was aging with them.