According to quantum field theory, the vacuum is not a state of "pure nothingness" but rather a state with underlying energy fluctuations. This concept is often referred to as vacuum energy or zero-point energy.
Quantum field theory describes particles and fields as excitations of underlying quantum fields that permeate space. Even in the absence of particles, these fields are subject to quantum fluctuations. These fluctuations give rise to virtual particle-antiparticle pairs continuously popping in and out of existence. Each virtual particle has associated energy, and when they annihilate, they contribute to the overall energy content of the vacuum.
The vacuum energy is not directly observable in the usual sense, as it represents the lowest possible energy state of the quantum fields. However, its effects can be indirectly observed through phenomena such as the Lamb shift, the Casimir effect, and the observed small but non-zero cosmological constant.
It's important to note that the concept of vacuum energy is different from the energy of matter or particles that are present within the vacuum. The vacuum energy refers to the energy associated with the empty space itself.
The borrowed energy explanation of quantum tunneling, also known as the "Feynman's sum over histories" approach, involves considering all possible paths that a particle can take when tunneling through a barrier. It allows for the consideration of virtual particle-antiparticle pairs being created and annihilated in the vicinity of the barrier, which can contribute to the tunneling process. This approach does not directly relate to the concept of vacuum energy but rather involves the dynamics of the quantum fields in the vicinity of the barrier.
In summary, according to quantum field theory, the vacuum is not devoid of energy but instead exhibits underlying fluctuations known as vacuum energy. These fluctuations arise from the inherent quantum nature of the fields permeating space and have observable consequences.