Quantum Theory and the Nature of Gravitation

This is an essay sketching the line of thinking which has led the present author to propose the constituent or atomic model of gravitation more than a decade ago. It turns out that viewing the problem of gravitation as a quantum many body problem could be quite useful when addressing some old unsolved problems such as the cosmological constant problem. I have applied this idea in 1996 to the problem of the largest cold gravitating system, the finite Universe itself. The result was the prediction of a small, positive vacuum energy density, now known, after its experimental discovery in 1998, as `dark energy'. The smallness of this quantity was understood as the finite size effect in the cold quantum many body system, and I quote here from \cite{Ma96}, {\it `` The smallness of the cosmological constant in natural Planck units is a result of an almost perfect thermodynamical limit. This is to say that the smallness of the cosmological constant is an effect due to an enormous number $N$ of hypothetical \textsf{gravitational} \textsf{atoms}. The present upper bound on the cosmological constant $\Lambda$ allows us to draw the conclusion about the lower bound on a number of \textsf{gravitational} \textsf{atoms} in the observed Universe, $N\sim 10^{122}$. ...''}. The old cosmological constant problem is a man-made problem because the vacuum energy density has nothing to do with the quartic divergences (zero point energies) in the interacting relativistic quantum field theories (i.e. in the Standard Model of elementary particles). The actual value of the vacuum energy density of the vacuum finite size de Sitter Universe is $\frac{E_{0}}{V}$, where $E_{0}$ is the ground state energy of $N$ \textsf{gravitational} \textsf{atoms}, which are spin-zero bosons of mass $M\sim M_{_{Pl}}$, contained in the finite volume $V$.