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I came across an article on something called gray dust, a hypothetical substance explored theoretically during the early 2000s. It attempted to explain cosmological supernova observations by postulating the existence of "self-replenishing dust", ejected into intergalactic space, that absorbed light without substantial reddening - unlike the normal dust we're familiar with. Further measurements appear to have ruled out gray dust, however, and dark energy is the more accepted explanation for the supernova observations.

What I'm trying to find out is how gray dust works, microscopically. I did a literature search that turned up a paper from 1999 proposing that thin carbon needles could produce the necessary extinction curve, and that extinction depends on the ratio of needle length to diameter. However, it didn't give a more detailed explanation of why this ratio is important, pointing instead to an older paper that was beyond me, for the most part.

Essentially, why should carbon needles produce opacity curves that don't vary with wavelength, instead of the typical curves from normal dust?

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