Final answer:
The universe is considered an isolated system because it does not exchange energy or matter with anything outside of it, fitting the definition of an isolated system and supporting the first law of thermodynamics.
Step-by-step explanation:
The universe could be considered an isolated system because, by definition, an isolated system does not allow a transfer of energy or matter into or out of the system. While there are no perfectly isolated systems within the universe, as every system can exchange energy and/or matter with its surroundings, the universe as a whole is self-contained and does not exchange energy or matter with anything outside because there is nothing outside of it. As such, it fits the description of an isolated system.
Moreover, the first law of thermodynamics, which states that energy cannot be created or destroyed, only transformed, supports the idea of the universe as a closed system. This principle applies to the universe, suggesting that all energy within the universe remains constant. This would not be possible unless the universe itself operated as an isolated system.
The concept of a multiverse suggests a potentially infinite and eternal collection of distinct universes, but since we cannot observe or interact with other universes, for all intents and purposes, our universe can be treated as an isolated system in the context of our current understanding of physics.