Title: Testing modified gravity: 4D Einstein-Gauss-Bonnet on cosmological scales

Abstract: 4D Einstein-Gauss-Bonnet gravity (4DEGB) is a modified gravity theory that has garnered significant attention in the last few years as a phenomenological competitor to general relativity. Although some parameters in this theory have been highly constrained, we look at the behaviour of 4DEGB in a previously untested regime, which has a ΛCDM background with an additional dark relativistic degree of freedom and modified scalar perturbations. In this talk, we will look at a range of interesting behaviours displayed by this theory both in the very early universe (at inflationary times) and at later times. We will constrain its characteristic parameter using CMB data and investigate its effects on large-scale structure and the matter power spectrum.