Usually, the story of the universe goes like this: there was nothing, then a bang, and ever since everything has been flying apart. But what if there was no 'nothing'? A group of physicists has shown that the universe could have existed forever: it just sometimes contracts and then expands again, like a spring.
In their model, space is closed in on itself, like the surface of a huge sphere. When such a universe contracts to tiny sizes, curvature (the bending of space) prevents it from collapsing to a point: it bounces back. This bounce works without any exotic matter: a simple field filling space with energy is enough. After the bounce, the universe smoothly transitions into an inflationary stage that stretches it to the scales we know and gives birth to galaxies.
Calculations showed that this scenario does not contradict what we see in the sky. The predicted ripples in the cosmic microwave background (ancient light that has reached us from the edges of the visible world) fall within the measured range. Moreover, the model avoids violating key energy conditions — matter behaves 'decently' even at the moment of the bounce.
This gives rise to a picture of an eternal, pulsating universe. It did not emerge from a point but has always been: it is just that sometimes its rhythm accelerates, and we call that the Big Bang.
as written by the authors arXiv:2604.27103
The early universe need not be described by an incomplete inflationary phase connected to a separate, more exotic prehistory. Recent results show that, within non-static FRW cosmology, only positive spatial curvature permits a nonsingular, geodesically complete universe with ANEC-respecting matter. We construct a geodesically complete closed $$k=+1$$ bounce-plus-inflation cosmology in ordinary general relativity, sourced by a single canonical scalar field with a positive vacuum offset. The bounce is supported by curvature rather than exotic stress energy: the matter content satisfies the NEC throughout and violates only the strong energy condition, as in any accelerated expansion. The solved branch remains sub-Planckian and evolves onto a curvature-diluted slow-roll phase with inflationary observables consistent with current constraints. The pivot-scale predictions are $$n_s=0.9617$$, $$r=0.0045$$ at $$N_*=55$$ and $$n_s=0.9650$$, $$r=0.0037$$ at $$N_*=60$$. Direct evolution of closed-universe infrared perturbations shows regular tensor and scalar propagation through the bounce and inflationary era, with the physical curvature perturbation freezing in the standard way. This gives a minimal explicit realization of a complete early-universe cosmology in the closed FRW branch selected by completeness and ANEC compatibility.
If our universe were closed, a beam of light sent in one direction could travel around it and come back from the opposite side, like a round-the-world trip, but without borders.
In Paul Anderson's novel 'Tau Zero', a starship survives the contraction and re-expansion of the universe, a conceptual predecessor of bounce models.