If the universe is expanding, then what does it expand into?

The concept of the universe expanding does not involve it expanding into any external medium or space outside of itself. Here are some key points to clarify this:

Expansion Within Itself

The universe expands by increasing the distance between objects within it, such as galaxies. This expansion is described as the universe expanding "into itself" rather than into any external space245.

No External Space

There is no "outside" or "beyond" the universe in the context of our current understanding of cosmology. The Big Bang was not an event that occurred at a specific point in space but rather a moment in time when the universe began expanding everywhere simultaneously4.

Scale Factor and Spacetime

The expansion is quantified by the scale factor, which is a parameter of the Friedmann–Lemaître–Robertson–Walker (FLRW) metric. This scale factor describes how the average separation between objects changes over time, but it does not imply expansion into an external space13.

Homogeneity and Isotropy

The universe is observed to be homogeneous and isotropic on large scales, meaning it looks the same everywhere and in all directions. This consistency supports the idea that the expansion is a property of the universe itself, rather than an expansion into something external13.

In summary, the universe's expansion is a process where the distances between objects within the universe increase over time, but it does not expand into any external space or medium. Instead, it expands within its own fabric of spacetime.