Published online by Cambridge University Press: 30 June 2014
We study the distribution of 2-Selmer ranks in the family of quadratic twists of an elliptic curve $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}E$ over an arbitrary number field
$K$. Under the assumption that
${\rm Gal}(K(E[2])/K) \ {\cong }\ S_3$, we show that the density (counted in a nonstandard way) of twists with Selmer rank
$r$ exists for all positive integers
$r$, and is given via an equilibrium distribution, depending only on a single parameter (the ‘disparity’), of a certain Markov process that is itself independent of
$E$ and
$K$. More generally, our results also apply to
$p$-Selmer ranks of twists of two-dimensional self-dual
${\bf F}_p$-representations of the absolute Galois group of
$K$ by characters of order
$p$.