Dust grain size and the history of dust production in the universe – Nature
For a simple estimate of the effect of porosity P on the grain opacity, we may use the Maxwell-Garnett effective medium theory, which leads to a reduction of the grain opacity by a factor of roughly (1−P) for P < 0.5. For example, for a porosity of 30%, the opacity is reduced by a factor of 0.7. The effect of porosity becomes more complicated when the grain size is larger than the wavelength, because of the scattering effect. … We find that in the early universe, large grains with low porosity are necessary to explain the observed extinction curves of high-redshift quasars and GRBs. The dust grains in the local universe are found to be more porous and smaller than those in the early universe. This suggests that the dust production mechanism has changed over cosmic time. In the early universe, dust grains were mainly produced by supernovae, while in the local universe, AGB stars are the dominant dust producers.
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