Spiral Galaxy Upset

In 1964, C. C. Lin and Frank Shu looked at the galaxy’s curvaceous arms and said, “You are my density.”

The density-wave theory of spiral arm formation was married to galactic astronomy for nearly a half century. Now, however, we are back to the future, where theories do not always fulfill their destiny. An upstart postgrad told the Royal Astronomical Society has debunked the old theory and proposed a new one.

The press release from the Royal Astronomical Society says, “A study of spiral patterns found in galaxies like our Milky Way could overturn the theory of how the spiral arm features form and evolve.” Robert Grand found it “impossible to reproduce” spiral arms in simulations using the density-wave model of Lin and Shu.

Since 1960s, the most widely accepted explanation has been that the spiral arm features move like a Mexican wave in a crowd, passing through a population of stars that then return to their original position. Instead, computer simulations run by Grand and his colleagues at University College London’s Mullard Space Science Laboratory (MSSL) suggest that the stars actually rotate with the arms. In addition, rather than being permanent features the arms are transient, breaking up and new arms forming over a period of about 80-100 million years.

A statement about this new idea has already found its way into the Wikipedia entry on spiral galaxies. Grand said of his model, “As well as helping us understand the evolution of our own galaxy, it may have applications for regions of star formation.”

Whether or not Grand’s new theory will survive a half century, the take-home lesson from the story is that a theory can be “widely accepted” by experts for decades, only to be found “impossible” later.

The density-wave model was favored partly because it answered the galaxy wind-up problem – the conundrum that after billions of years, spiral arms would be wound up so tightly as to be undiscernible (see Jason Lisle’s explanation at Answers in Genesis under “Spiral Galaxies”).

Grand was careful to tweak his model to make sure spiral arms are transient and can re-evolve as often as necessary. Whether or not his computer simulations have anything to do with external reality is the question.