Buchdahl's theorem

Evolution of central pressure against compactness (radius over mass) for a uniform density 'star'. This central pressure diverges at the Buchdahl bound.

In general relativity, Buchdahl's theorem, named after Hans Adolf Buchdahl, makes more precise the notion that there is a maximal sustainable density for ordinary gravitating matter. It gives an inequality between the mass and radius that must be satisfied for static, spherically symmetric matter configurations under certain conditions. In particular, for areal radius $R$, the mass $M$ must satisfy

$M<{\frac {4Rc^{2}}{9G}}$

where $G$ is the gravitational constant and $c$ is the speed of light. This inequality is often referred to as Buchdahl's bound. The bound has historically also been called Schwarzschild's limit as it was first noted by Karl Schwarzschild to exist in the special case of a constant density fluid. However, this terminology should not be confused with the Schwarzschild radius which is notably smaller than the radius at the Buchdahl bound.

Theorem

Given a static, spherically symmetric solution to the Einstein equations (without cosmological constant) with matter confined to areal radius $R$ that behaves as a perfect fluid with a density that does not increase outwards. Assumes in addition that the density and pressure cannot be negative. The mass of this solution must satisfy

$M<{\frac {4Rc^{2}}{9G}}$

For his proof of the theorem, Buchdahl uses the Tolman-Oppenheimer-Volkoff (TOV) equation.

Special Cases

Incompressible fluid

The special case of the incompressible fluid or constant density, $\rho (r)=\rho _{*}$ for $r<R$, is a historically important example as, in 1916, Schwarzschild noted for the first time that the mass could not exceed a the value ${\frac {4Rc^{2}}{9G}}$ for a given radius $R$ or the central pressure would become infinite. It is also a particularly tractable example. Within the star one finds

$m(r)={\frac {4}{3}}\pi r^{3}\rho _{*}$

and using the TOV-equation

$p(r)=\rho _{*}c^{2}{\frac {R{\sqrt {R-2GM/c^{2}}}-{\sqrt {R^{3}-2GMr^{2}/c^{2}}}}{{\sqrt {R^{3}-2GMr^{2}/c^{2}}}-3R{\sqrt {R-2GM/c^{2}}}}}$

such that the central pressure, $p(0)$, diverges as $R\to 9GM/4c^{2}$.

Extensions

Extensions to Buchdahl's theorem generally either relax assumptions on the matter or on the symmetry of the problem. For instance, by introducing anistropic matter or rotation. In addition one can also consider analogues of Buchdahl's theorem in other theories of gravity.

References

Buchdahl, H.A. (15 November 1959), "General relativisitc fluid spheres", Physical Review, 116 (4): 1027–1034, doi:10.1103/PhysRev.116.1027

Carroll, Sean M. (2004), Spacetime and Geometry: An Introduction to General Relativity, San Francisco: Addison-Wesley, ISBN 978-0-8053-8732-2