Quantum gravity at low energies

I provide a conceptually-focused presentation of 'low-energy quantum gravity' (LEQG), the effective quantum field theory obtained from general relativity and which provides a well-defined theory of quantum gravity at energies well below the Planck scale. I emphasize the extent to which some such theory is required by the abundant observational evidence in astrophysics and cosmology for situations which require a simultaneous treatment of quantum-mechanical and gravitational effects, contra the often-heard claim that all observed phenomena can be accounted for either by classical gravity or by non-gravitational quantum mechanics, and I give a detailed account of the way in which a treatment of the theory as fluctuations on a classical background emerges as an approximation to the underlying theory rather than being put in by hand. I discuss the search for a Planck scale quantum-gravity theory from the perspective of LEQG and give an introduction to the Cosmological Constant problem as it arises within LEQG.

Automated summary

The concept of 'low-energy quantum gravity' (LEQG) is presented, which is an effective quantum field theory derived from general relativity. It provides a well-defined theory for quantum gravity at energies below the Planck scale. The need for such a theory is emphasized based on observational evidence in astrophysics and cosmology, which require a simultaneous treatment of quantum-mechanical and gravitational effects. The article discusses the search for a quantum-gravity theory at the Planck scale from the perspective of LEQG and introduces the Cosmological Constant problem within LEQG.