DHOST bounce

Abstract

We present a new class of nonsingular bounce cosmology free from\ninstabilities, using a single scalar field coupled to gravity within the\nframework of the Degenerate Higher-Order Scalar-Tensor (DHOST) theories. In\nthis type of scenarios, the gradient instability that widely exists in\nnonsingular bounce cosmologies in the framework of scalar-tensor and\nHorndeski/Galileon theories is removed by the effects of new operators\nintroduced by the DHOST, due to the modification that they later bring about to\nthe dispersion relation of perturbations. Hence, our results demonstrate that\nthere is indeed a loophole for this type of bounce scenarios to be free from\npathologies when primordial perturbations evolve through the bounce phase, and\nthus the theoretical {\\it no-go} theorem for nonsingular bounce cosmology of\nHorndeski/Galileon theories can be delicately evaded in DHOST extensions.\n