Dust solution

A pressureless perfect fluid can be interpreted as a model of a configuration of dust particles that locally move in concert and interact with each other only gravitationally, from which the name is derived. For this reason, dust models are often employed in cosmology as models of a toy universe, in which the dust particles are considered as highly idealized models of galaxies, clusters, or superclusters. In astrophysics, dust models have been employed as models of gravitational collapse. Dust solutions can also be used to model finite rotating disks of dust grains; some examples are listed below. If superimposed somehow on a stellar model comprising a ball of fluid surrounded by vacuum, a dust solution could be used to model an accretion disk around a massive object; however, no such exact solutions that model rotating accretion disks are yet known due to the extreme mathematical difficulty of constructing them.

The stress–energy tensor of a relativistic pressureless fluid can be written in the simple form

${\displaystyle T^{\mu \nu }=\rho U^{\mu }U^{\nu }}$

Here

• the world lines of the dust particles are the integral curves of the four-velocity ${\displaystyle U^{\mu }}$,
• the matter density is given by the scalar function ${\displaystyle \rho }$.

Because the stress-energy tensor is a rank-one matrix, a short computation shows that the characteristic polynomial

${\displaystyle \chi (\lambda )=\lambda ^{4}+a_{3}\,\lambda ^{3}+a_{2}\,\lambda ^{2}+a_{1}\,\lambda +a_{0}}$

of the Einstein tensor in a dust solution will have the form

${\displaystyle \chi (\lambda )=\left(\lambda -8\pi \mu \right)\,\lambda ^{3}}$

Multiplying out this product, we find that the coefficients must satisfy the following three algebraically independent (and invariant) conditions:

${\displaystyle a_{0}\,=a_{1}=a_{2}=0}$

Using Newton's identities, in terms of the sums of the powers of the roots (eigenvalues), which are also the traces of the powers of the Einstein tensor itself, these conditions become:

${\displaystyle t_{2}=t_{1}^{2},\;\;t_{3}=t_{1}^{3},\;\;t_{4}=t_{1}^{4}}$

In tensor index notation, this can be written using the Ricci scalar as:

${\displaystyle {G^{a}}_{a}=-R}$

${\displaystyle {G^{a}}_{b}\,{G^{b}}_{a}=R^{2}}$

${\displaystyle {G^{a}}_{b}\,{G^{b}}_{c}\,{G^{c}}_{a}=-R^{3}}$

${\displaystyle {G^{a}}_{b}\,{G^{b}}_{c}\,{G^{c}}_{d}\,{G^{d}}_{a}=R^{4}}$

This eigenvalue criterion is sometimes useful in searching for dust solutions, since it shows that very few Lorentzian manifolds could possibly admit an interpretation, in general relativity, as a dust solution.

• Lorentz group

• Schutz, Bernard F. (2009), "4. Perfect fluids in special relativity", A first course in general relativity (2 ed.), Cambridge University Press, ISBN 978-0-521-88705-2
• Stephani, H.; Kramer, D.; MacCallum, M.; Hoenselaers, C.; Herlt, E. (2003). Exact Solutions of Einstein's Field Equations (2nd edn.). Cambridge: Cambridge University Press. ISBN 0-521-46136-7. Gives many examples of exact dust solutions.