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In cosmology, "dust" refers to a pressureless perfect fluid, which essentially means a continuum of nonrelativistic material particles, such as galaxies. This is a picturesque and unusual piece of terminology, and in western culture it has the inevitable connotation from Genesis: "In the sweat of thy face shalt thou eat bread, till thou return unto the ground; for out of it wast thou taken: for dust thou art, and unto dust shalt thou return."

Who invented this term? I notice that, for example, the title of van Stockum's 1937 paper describing what we now call "van Stockum dust," was "The gravitational field of a distribution of particles rotating around an axis of symmetry." So presumably the term didn't exist in 1937, or he would have used it in the title.

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    $\begingroup$ I personally find dust a rather obvious candidate for denoting a collection of nonrelativistic particles that are distributed homogeneously and isotropically... But maybe that's besides the point. Also, I'm pretty sure the Biblical connotation isn't very well-known in Northern Europe (at least not in my experience!). $\endgroup$ – Danu May 15 '15 at 23:55
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    $\begingroup$ @Danu: Dust is a perfect fluid, so its stress-energy isotropic in its rest frame. I'm not aware of any connection to homogeneity. The van Stockum dust solution is anisotropic. Re the cultural stuff, I'm in the US, where although religion is on the retreat, it's still far more prevalent than in places like France. Also, the two black obelisks towering over the history of the English language are Shakespeare and the King James version of the bible, so the connotation may spring to mind more naturally to native English speakers (don't know if you are one). $\endgroup$ – Ben Crowell May 16 '15 at 0:16
  • $\begingroup$ Good points (and nope, I'm not a native speaker, although my English is arguably better than my Dutch by now). $\endgroup$ – Danu May 16 '15 at 7:05
  • $\begingroup$ I must agree with Danu in that the biblical reference sounds a little bit far-fetched. We've all seen suspended dust particles in the air, which seems as a more natural reason to call it like that. $\endgroup$ – hjhjhj57 May 16 '15 at 20:03
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The prime suspect for injecting Biblical references into cosmology would be Georges Lemaître, a cosmologist and... a Catholic priest. His "Cosmic Egg" and "Primeval Atom" lost to Hoyle's sarcastic "Big Bang" however, and it seems he passed up on introducing "dust". His 1927 Ph.D. thesis The Gravitational Field in a Fluid became the basis for the now famous 1927 paper Homogeneous Universe of Constant Mass and Growing Radius Accounting for the Radial Velocity of Extragalactic Nebulae, written in French, and self-translated into English in 1931 with minor edits. Lemaître talks about nebulae and introduces the usual assumption of negligible pressure, but does not call his fluid "dust" even descriptively.

So it seems the 'priority' goes to Tolman's Effect of Inhomogeneity on Cosmological Models (1934), who writes "the fluid in the models was taken as dust exerting negligible pressure", and repeats "dust" 16 times on 8 pages. In the assumptions on the particular form of stress-energy tensor he follows Lemaître:"we shall confine our attention to very simple models composed of dust particles (nebulae) which exert negligible pressure... This will permit us to employ expressions for the line element and its consequences which are equivalent to those recently developed by Lemaître for investigating the formation of nebulae". Some authors define "dust" more broadly, Sachs and Wu in General Relativity for Mathematicians (3.13) simply give a kind of divergence free condition on stress energy tensor, and note that pressureless perfect fluid satisfies it. The prototypical example seems to be nebulae, clouds of particles interacting only gravitationally, literally dust.

Although the association with stress energy tensors is clearly specific to relativity more loosely "nebulae" and "dust" were already used by Newtonian astronomers, and there seems to be continuity with Tolman's usage. The reference to nebulae reminds the nebular hypothesis, according to which a rotating dust cloud formed the solar system. It goes back to Kant (1755), and in a more detailed form to Laplace (1796), and was revived by Chamberlin and Moulton in 1901. Both van Stockum's and Gödel's early "dust solutions" to the Einstein equations involve rotating dust. The nebular hypothesis is (metaphorically) in line with the Biblical origin story, but nebular dust is also literally analogous to the ordinary dust, so we can only speculate what contributed to the spread of usage. As an alternative reference, Democritus envisioned a universe filled with "fine dust which is divided in the breeze and becomes visible in the sun's beam". Lucretius expressed it poetically in De Rerum Natura, well familiar to people with classical education.

There are other examples of classical usage. The paradox that in an infinite universe uniformly filled with stars the night sky would appear to be bright, stated most eloquently by Olbers in 1826 and named after him, was already pointed out by Digges back in 1576. Kepler took it as an argument for finite universe, but Halley in 1705, Cheseaux in 1744 and Olbers himself suggested interstellar dust obscuring distant stars as a resolution. In 1831 Herschel pointed out however that the dust would simply be heated up until it glowed as brightly as the stars (further reflection shows that the sky should in fact be infinitely bright). Finally, zodiacal light is explained by an interplanetary dust cloud, which attracted much attention at the turn of the centuries, because both Newcomb (1895) and Seeliger (1906) tried to use it to explain the anomalous precession of Mercury, before Einstein gave the general relativity explanation in 1915.

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  • $\begingroup$ Zwicky also wrote about interstellar dust. Many references. $\endgroup$ – Guido Jorg May 16 '15 at 18:05
  • $\begingroup$ @GuidoJorg: Wouldn't that be the more literal meaning of "dust," as opposed to the relativist's definition? $\endgroup$ – Ben Crowell May 16 '15 at 20:51
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    $\begingroup$ There's some interesting material in this answer, but most of it doesn't seem to address the question, which asks about the relativistic term "dust." The first sentence does show that the relativistic usage dates back at least to Tolman in 1934, which is nice. It doesn't seem like it would have been possible to define dust without the stress-energy tensor, which seems to date to around 1911. So I guess we have it narrowed down to 1911-1934. Lemaitre seems to have been making cosmological models as early as 1927. "Dust" would have been "poudre." Seems like we must have 1927-1934. $\endgroup$ – Ben Crowell May 17 '15 at 14:21
  • $\begingroup$ @Ben Crowell It wasn't Lemaître, see edit. The question is phrased as about dust in cosmology though, and mentions "nonrelativistic material particles". Even if it was about relativistic stress-energy tensors, motivation, Biblical or not, may well come from prior usage in cosmology. Pressureless perfect fluid is a classical concept, and even that is an overkill to define "dust". Prior to Cauchy derivatives couldn't be defined in terms of limits, but limits were intended to express the same idea, so he kept the name. $\endgroup$ – Conifold May 18 '15 at 1:22

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