The universe hasn't always been expanding. In some models, it first contracted, nearly empty and flat, and then, upon reaching a limit, bounced back and started growing, like the world we know. This scenario is called bouncing cosmology. It sidesteps the problem of the initial singularity (the point where the laws of physics break down), because no infinite density arises.
The researchers built a model of two interacting "fluids." One fluid describes the ordinary cosmic medium, the other describes a population of black holes. They traced the evolution of this mixture through contraction, bounce, and re-expansion. It turned out that black holes can survive the very moment of the bounce — consistent with earlier work. But the main discovery is different: even when the geometry of space is fixed, the presence of black holes changes the properties of the background medium. The medium becomes different from what it would be without them.
This gives rise to a picture of a cyclic universe where black holes are bridges between epochs. What exactly do they carry through the bounce? Mass, spin, perhaps some information — the question remains open. But now it's clear: these objects are not just witnesses but active participants, altering the cosmic background around them.
If our Sun were compressed into a black hole, its event horizon would be only about 6 kilometers in diameter — smaller than many cities.