Past exam of the mathematics course of the University of Cambridge / 2015 / iii / Paper 64 / 2 / c / Solution

For nonzero real wavenumber $k$, seek a Fourier mode of the Newtonian gravitational potential in the form ${\Phi }^{\prime }=f(z)e^{ikx}$. Away from the razor-thin sheet, the Poisson equation for Newtonian gravity becomes $f^{\prime \prime }-k^{2}f=0$. Requiring the perturbation to decay on both sides and remain continuous gives $f=A{e}^{-|k||z|}$.

Integrating the Poisson equation for Newtonian gravity through $z=0$ fixes the derivative jump:Thus the perturbing gravitational potential isAt the midplane this reduces to the razor-thin disk Poisson kernel. The observable real disturbance is the real part. Writing the perturbation amplitude as ${\Sigma }_a$ instead of ${\Sigma }_0$ gives the same formula with ${\Sigma }_a$; linearity makes it independent of the background density. The $k=0$ disturbance is a uniformly changed sheet and has potential proportional to $|z|$, rather than a decaying nonzero-wavenumber solution.