Gravitational Waves!

Billion years ago were two orbiting black holes

Closer they drew; they collided and merged through

Space-time fabric was warped; still it folds and unfolds

The colossal collision set off these gravitational waves

Stacking crest after trough, endlessly propagating, until time stays.

I wrote this poem to celebrate the historic day on which mankind first observed the gravitational waves directly! This poem - an acrostic - has more than the word formed by first line first letter, second line second letter and so forth. There’s another word in it. Can you find what that is?

Also, for reigniting my interest in astronomy and black holes in particular, many thanks to Usha Keshav!