re: Density waves
This up and down motion of our system through the galaxy
is one reason why some scientist think we get our planet
bombarded by comets every so many millions of years:
When we hit a dense region, the comets of the Oort Cloud
are disrupted, which causes a greater number than usual to
plunge towards the Sun, where some of them hit Earth.
This is supposed to happen either every 26 or 65 million years.
At least we have a space infrastructure that could allow us
to either stop or escape such a disaster, which the dinosaurs
and the trilobites sadly lacked.
Or did they....