The first term in Equation (28) is the cosmological constant, which is dropped in what follows since the observed size of the universe implies is extremely small. There is, of course, no real theoretical understanding why the cosmological constant should be this small (a comprehensive review of the cosmological constant problem is given in ; for a recent suggestion in the spirit of effective field theories see [3, 29]). Once the cosmological term is dropped, the leading term in the derivative expansion is the one linear in , which is the usual Einstein–Hilbert action of general relativity. Its coefficient defines Newton’s constant (and so also the Planck mass, ).
The explicit mass scales, and , are introduced to ensure that the remaining constants , , , and appearing in Equation (28) are dimensionless. Since it appears in the denominator, the mass scale can be considered as the smallest microscopic scale to have been integrated out to obtain Equation (28). For definiteness we might take the electron mass, , for when considering applications at energies below the masses of all elementary particles. (Notice that contributions like or could also exist, but these are completely negligible compared to the terms displayed in Equation (28).)
As discussed in the previous section, some of the interactions in the Lagrangian (28) may be redundant, in the sense that they do not contribute independently to physical observables (like graviton scattering amplitudes about some fixed geometry, say). To eliminate these we are free to drop any terms which are either total derivatives or which vanish when evaluated at solutions to the lower-order equations of motion.
The freedom to drop total derivatives allows us to set the couplings and to zero. We can drop because , and we can drop because the quantity
The freedom to perform field redefinitions allows further simplification (just as was found for the toy model in earlier sections). To see how this works, consider the infinitesimal field redefinition , under which the leading term in undergoes the variation
Since the lowest-order equations of motion for pure gravity (without a cosmological constant) imply , we see that all of the interactions beyond the Einstein–Hilbert term which are explicitly written in Equation (28) can be removed in one of these two ways. The first interaction which can have physical effects (for pure gravity with no cosmological constant) in this low-energy expansion is therefore proportional to the cube of the Riemann tensor.
This last conclusion changes if matter or a cosmological constant are present, however, since then the lowest-order field equations become for some nonzero tensor . Then terms like or no longer vanish when evaluated at the solutions to the equations of motion, but are instead equivalent to interactions of the form , , or . Since some of our later applications of are to the gravitational potential energy of various localized energy sources, we shall find that these terms can generate contact interactions amongst these sources.
© Max Planck Society and the author(s)