For a long time, cosmologists could not peer beyond the starting point — the infinitely dense singularity of the Big Bang, where the equations of general relativity by Einstein lose their predictive power. The solution came from condensed-matter physics: the Chaplygin gas — an exotic substance that, under certain conditions, mimics dark energy, the mysterious force driving the Universe's accelerated expansion. Imagine a tightly stretched trampoline: when something falls onto it, it bends, then sharply flings the object back up. Similarly, when the contracting Universe reaches a critical density, it 'sinks into' this medium, deforming it, and like a spring, it pushes spacetime into a new phase of expansion.

At the moment of this bounce, something counterintuitive happens: the clock that measures cosmic age (that is, time itself, inextricably linked to the geometry of space) runs backward. For matter inside the Universe, time starts flowing in reverse, and this is not just a mathematical curiosity: from outside, the bounce looks like a moment of silence, a pause between contraction and expansion, but inside that pause lies an entire epoch of reversed flow. The Universe does not annihilate in a singularity; instead, it undergoes a 'big bounce.'

Remarkably, the mechanism remains stable even under extreme conditions where sound waves in the Chaplygin gas travel faster than light. Our everyday intuition rebels against such a violation of causality, but the mathematical framework remains consistent: the equations keep working. Moreover, adding any other forms of matter (radiation, ordinary matter) does not destroy the bounce. This striking robustness makes the model attractive for explaining what might have happened before the Big Bang.