Confronting the Cosmological Principle: How secure are the foundations of the cosmos?

I will present our recent work using quasars and radio galaxies to confront the cosmological principle, the idea that sits at the heart of modern cosmological theories. Whilst the cosmological principle states that the universe is isotropic and homogeneous, our observations of the Cosmic Microwave Background (CMB) reveal a pronounced dipole, interpreted as our kinematic departure from the local Hubble flow. This motion should be imprinted on other cosmological observations, but recent studies are in significant tension with the kinematic expectation, bringing the cosmological principle into doubt. Exploring large-scale surveys of cosmological sources with Bayesian-based approaches, I show that a dipole aligned with the CMB is present. However, the amplitude of the dipole is still in tension with our expectations, bringing the validity of the cosmological principle into question. I will conclude with some ideas about the source of these tensions and whether a cosmological rethink is on the horizon.