Information Problem in Black Holes and Cosmology and Ghosts in Quadratic Gravity

Abstract

Black hole information problem is the question about unitarity of the evolution operator during the collapse and evaporation of the black hole. One can ask the same question about unitarity of quantum and inflationary cosmology. In this paper we argue that in both cases, for black holes and for cosmology, the answer is negative and we face non-unitarity. Such a question can not be addressed by using the fixed classical gravitational background since one has to take into account the backreaction. To his end one uses the semi-classical gravity, which includes the expectation value of the energy - momentum tensor operator of the matter fields. One has to renormalize the energy-momentum tensor and one gets an effective action which contains quadratic terms in scalar curvature and Ricci tensor. Such quadratic gravity contains ghosts which in fact lead to violation of unitarity in black holes and cosmology. We discuss the question whether black holes will emit ghosts. One can try to restrict ourselves to the $f(R)$ gravity that seems is a good approximation to the semi-classical gravity and widely used in cosmology. The black hole entropy in $f(R)$ gravity is different from the Bekenstein-Hawking entropy and from entanglement island entropy. The black hole entropy in $R+R^2$ gravity goes to a constant during the evaporation process. This can be interpreted as another indication to the possible non-unitarity in black holes and cosmology