Birth of de Sitter universe from a time crystal universe
We show that a simple sub-class of Horndeski theory can describe a time crystal Universe. The time crystal Universe can be regarded as a baby Universe nucleated from a flat space, which is mediated by an extension of Giddings-Strominger instanton in a 2-form theory dual to the Horndeski theory. Remarkably, when a cosmological constant is included, de Sitter Universe can be created by tunneling from the time crystal Universe. It gives rise to a past completion of an inflationary Universe.