Exotic ${\mathbb{R}}^4$'s as unions of two thickened circles

We give an infinite family of non-diffeomorphic exotic ${\mathbb{R}}^4$’s, each of which is the union of two copies of a standard open tubular neighborhood of the circle, which intersect in a standard open tubular neighborhood of the wedge of two circles. This gives the simplest known decomposition of an exotic ${\mathbb{R}}^4$ into two open sets, making progress on Question 4.43 (A) of Kirby’s 1995 problem list. The decompositions are obtained by studying fibered slice disk complements in certain Casson-Gordon homotopy 4-balls, for which we provide similar decompositions. We distinguish the ${\mathbb{R}}^4$’s using end Floer homology.