Physics Group Meeting

The vacuum state reduced to a region of the space gives a density matrix which can be written in a thermal-like form. In this "thermal" density matrix the role of the Hamiltonian is played by the so called entanglement or modular Hamiltonian. Surprisingly, this modular Hamiltonian has a local universal expression in terms of the stress tensor for a large class of regions with boundary on a null plane. We will show that this property, together with strong subadditivity of entanglement entropy, gives place to an entropic version of the a-theorem about irreversibility of the renormalization group in d=4.