Are spiral arms long-lived density waves or recurrent transients?
In plain words
Spiral arms could be a fixed pattern that stars pass through, like a traffic jam, or short-lived patterns that keep reforming. Simulations mostly give short-lived arms, while classic theory assumes lasting ones.
Precise statement
Decide whether spiral structure in isolated, unbarred disk galaxies is a quasi-steady density wave with a single pattern speed lasting many rotations (Lin and Shu 1964) or a sequence of transient, swing-amplified modes each lasting a few rotations. Observational discriminants include pattern speed versus radius, azimuthal age gradients across arms, and Gaia kinematic structure.
What would settle it
Measured pattern speeds as a function of radius in a sample of grand-design and flocculent spirals plus Milky Way arm kinematics from Gaia.
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