If you create an expanding universe inside another universe, it would essentially result in a nested or multiverse scenario. The behavior and consequences of such a configuration would depend on the specific nature and properties of the two universes involved.
In general, if the outer universe is unaffected by the inner universe's expansion, it might observe the inner universe as a separate region that expands within its boundaries. This could be analogous to observing a bubble forming and expanding within a liquid.
However, if the expansion of the inner universe affects the outer universe, the consequences could be more complex. The interaction between the two universes could potentially lead to gravitational forces, energy transfers, or even disturbances in the fabric of spacetime. These effects would heavily depend on the underlying laws of physics governing each universe and how they interact.
It is important to note that our current understanding of physics and cosmology is limited, and the idea of creating or observing multiple universes is purely speculative at this point. The concept of nested universes or a multiverse is still a topic of ongoing research and theoretical exploration.