The universal thermodynamic properties of Extremely Compact Objects

An extremely compact object (ECO) is defined as a quantum object without horizon, whose radius is just a small distance $s$ outside its Schwarzschild radius. We show that any ECO of mass $M$ in $d+1$ dimensions with $s\ll (M/m_p)^{2/(d-2)(d+1)}l_p$ must have (at leading order) the same thermodynamic properties -- temperature, entropy and radiation rates -- as the corresponding semiclassical black hole of mass $M$. An essential aspect of the argument involves showing that the Tolman-Oppenheimer-Volkoff equation has no consistent solution in the region just outside the ECO surface, unless this region is filled with radiation at the (appropriately blueshifted) Hawking temperature. In string theory it has been found that black hole microstates are fuzzballs -- objects with no horizon -- which are expected to have a radius that is only a little larger than the horizon radius. Thus the arguments of this paper provide a nice closure to the fuzzball paradigm: the absence of a horizon removes the information paradox, and the thermodynamic properties of the semiclassical hole are nonetheless recovered to an excellent approximation.