When scientists say that the universe is expanding, they mean that the space between galaxies, clusters of galaxies, and other cosmic structures is growing over time. The concept of the universe's expansion is based on observations of the redshift of light from distant galaxies. The redshift is a phenomenon in which light waves are stretched as the objects emitting them move away, causing a shift towards longer wavelengths (redder light).
The prevailing cosmological model, known as the Lambda-Cold Dark Matter (ΛCDM) model, describes the expansion of the universe. According to this model, the expansion is driven by a property called dark energy, which is a form of energy that fills space and exerts a repulsive force, causing the acceleration of the expansion.
As for the future of the universe's expansion, current scientific understanding suggests that it will continue to expand indefinitely. The expansion is not expected to stop or reverse unless there are significant changes in the fundamental properties of dark energy, which is still not well understood.
There are a few possibilities that scientists have theorized for the ultimate fate of the universe:
If the dark energy remains constant or only changes slightly, the expansion will continue, and galaxies and cosmic structures will become increasingly distant from each other. This scenario is known as the "Big Freeze" or "Heat Death" of the universe.
If the dark energy evolves and its repulsive force becomes stronger over time, it could lead to a scenario called the "Big Rip." In this scenario, the expansion would accelerate to such an extent that it would eventually tear apart galaxies, stars, and even subatomic particles.
Alternatively, there is a possibility that the expansion could slow down and eventually stop if the repulsive force of dark energy weakens or if the attractive force of matter becomes dominant. This scenario is called the "Big Crunch," in which the universe would contract under gravity, potentially leading to a new Big Bang and a cycle of expansion and contraction.
At present, observations and measurements suggest that the universe's expansion is accelerating, favoring a scenario of continued expansion or a Big Rip outcome. However, our understanding of dark energy and the ultimate fate of the universe is an active area of research, and future discoveries could provide new insights into these questions.