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.
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.