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.
as written by the authors arXiv:2606.30818
Bouncing cosmologies offer an alternative to the standard $$\Lambda$$CDM model by avoiding the problem of the initial cosmological singularity by construction. In these models, the universe undergoes a contraction phase that begins in a nearly flat and dilute state, followed by a bounce, after which the universe transitions into the expanding phase described by the $$\Lambda$$CDM model. During contraction, most large-scale structures are expected to be erased. Black holes, however, as shown by several previous investigations, may persist through the bounce. The goal of this work is to analyze the evolution of a black hole population throughout the contraction, bounce, and expansion phases. Additionally, we investigate how the presence of black holes influences the properties of the background cosmological fluid. To this end, we develop a cosmological model involving two interacting fluids. Our findings indicate that the cosmological fluid alters its properties in a spacetime with fixed geometry.
If our Sun were compressed into a black hole, its event horizon would be only about 6 kilometers in diameter — smaller than many cities.