The expansion of the universe does not require an external source of energy. The total energy of the universe, including matter, radiation, and even dark energy, is considered to be conserved. According to the law of conservation of energy, energy cannot be created or destroyed but can only be transformed from one form to another.
The expansion of the universe is driven by the properties of space itself, particularly the effects of dark energy. Dark energy is a theoretical form of energy that is thought to exist uniformly throughout space and is associated with a negative pressure. This negative pressure causes a repulsive gravitational effect, leading to the accelerated expansion of the universe.
The exact nature of dark energy is not yet fully understood, and it remains an area of active research and investigation. Its origin and the underlying physics behind its properties are subjects of ongoing scientific inquiry.
It's important to note that the total energy of the universe, including both visible and dark matter and energy, is presumed to be zero within the context of general relativity. This concept arises from the fact that the positive energy associated with matter and radiation is offset by the negative energy associated with gravity. Therefore, the expansion of the universe does not violate the conservation of energy principle.
While the concept of energy in cosmology can be complex, the expansion of the universe is not a process that requires an external input of energy. The current understanding is that it is driven by the properties of space itself, with dark energy playing a significant role in the accelerated expansion observed in recent cosmological observations.