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## Main Question or Discussion Point

In the Newtonian theory of cosmological perturbations, a density excess ##\delta\rho## in a localised region of spacetime leads to the equation of motion ##\ddot{\delta\rho} \sim G\delta\rho##. I can see that this follows directly from Newton's gravitational law.

Why does this equation lead to an exponential instability of flat spacetime to the development of fluctuations?

Why does this equation lead to an exponential instability of flat spacetime to the development of fluctuations?