As far as I can understand, the Level I Multiverse theory is based on the idea of eternal inflation. It states that considering an eternally inflating and expanding universe, an infinite amount of possibilities or parallel universes will pop up essentially.
But doesn't the 2nd Law of Thermodynamics limit our universe to the amount of energy it has? Entropy will increase no matter what. For an infinite amount of parallel universes, wouldn't you correspondingly need an infinite source of energy?