The way to the Big Bang
We propose conformal invariance as a fundamental symmetry governing cosmological particle creation from vacuum fluctuations, employing a phenomenological approach with an ideal fluid action to address the long-standing back-reaction problem. We demonstrate that particle production cannot emerge from classical vacua but must originate from a quantum vacuum at zero scale factor, with the transition surface constituting a light-like rather than space-like hypersurface. This implies that particles are created on the light cone and remain causally connected, with their apparent simultaneity being illusory. Our model requires an open Universe ($k=0, -1$) and reconceptualizes the Big Bang as a detonation wave propagating through quantum vacuum at the speed of light.