The hypothesis that the universe could be infinite in size is based on several lines of reasoning and observations. While the universe has been expanding for a finite time, the concept of infinity in this context refers to the possibility that the universe does not have a boundary or an edge, rather than referring to its age or the duration of expansion.
Here are a few reasons why scientists entertain the idea of an infinite universe:
Scale and Homogeneity: Observations of the large-scale structure of the universe indicate a remarkable degree of homogeneity and isotropy on very large scales. This suggests that the universe is similar in structure and properties across vast distances, implying that it extends indefinitely.
Cosmic Inflation: The theory of cosmic inflation proposes that in the earliest moments of the universe, there was a rapid exponential expansion, which caused the universe to grow by an enormous factor. If inflation occurred, it would have stretched the universe to a size much larger than the observable universe we can see today. This suggests that the observable universe may only be a tiny portion of a much larger, possibly infinite, universe.
Flat Geometry: According to current cosmological models, the overall geometry of the universe appears to be flat. A flat geometry indicates that space extends infinitely in all directions. While the observable universe is limited due to the finite speed of light and the age of the universe, this doesn't necessarily imply a finite size for the entire universe.
The Multiverse Hypothesis: Some theories and models in cosmology, such as the concept of a multiverse, propose the existence of an ensemble of universes with different properties. In such scenarios, the universe we observe could be just one of many, and the overall ensemble could be infinite.
It's important to note that these ideas are still speculative and subject to ongoing scientific investigation. The nature of the universe's size and geometry is a topic of active research, and scientists continue to gather observational evidence and develop new theoretical frameworks to better understand the true nature of our universe.