Zero-point quantum fluctuations and dark energy

Understand

In the Hamiltonian formulation of General Relativity the energy associated to an asymptotically flat space-time with metric $g_{\mu\nu}$ is related to the Hamiltonian $H_{GR}$ by $E=H_{GR}[g_{\mu\nu}]-H_{\rm GR}[\eta_{\mu\nu}]$, where the subtraction of the flat-space contribution is necessary to get rid of an otherwise divergent boundary term.

• This classic result indicates that the energy associated to flat space does not gravitate.
• We apply the same principle to study the effect of zero-point fluctuations of quantum fields in cosmology, proposing that their contribution to the cosmic expansion is obtained computing the vacuum energy of quantum fields in a FRW space-time with Hubble parameter $H(t)$ and subtracting from it the flat-space contribution.
• [...] After renormalization, this produces a renormalized vacuum energy density $\sim M^2H^2(t)$, where $M$ is the scale where quantum gravity sets is, so for $M$ of order of the Planck mass a vacuum energy density of the order of the critical density can be obtained without any fine tuning.