nLab gravity

Idea

A field configuration of the physical theory of gravity on a spacetime $X$ is equivalently

• a vielbein field, hence a reduction of the structure group of the tangent bundle along $\mathbf{B} O(n-1,1) \to \mathbf{B}GL(n)$, defining a pseudo-Riemannian metric;

• a connection that is locally a Lie algebra-valued 1-form with values in the Poincare Lie algebra.

  $$(E, \Omega) : T X \to \mathfrak{iso}(d-1,1)$$

  such that this is a Cartan connection.

(This parameterization of the gravitational field is called the first-order formulation of gravity.) The component $E$ of the connection is the vielbein that encodes a pseudo-Riemannian metric $g = E \cdot E$ on $X$ and makes $X$ a pseudo-Riemannian manifold. Its quanta are the gravitons.

The non-propagating field? $\Omega$ is the spin connection.

The action functional on the space of such connection which defines the classical field theory of gravity is the Einstein-Hilbert action.

More generally, supergravity is a gauge theory over a supermanifold $X$ for the super Poincare group. The field of supergravity is a Lie-algebra valued form with values in the super Poincare Lie algebra.

$$(E,\Omega, \Psi) : T X \to \mathfrak{siso}(d-1,1)$$

The additional fermionic field $\Psi$ is the gravitino field.

So the configuration space of gravity on some $X$ is essentially the moduli space of Riemannian metrics on $X$.

Details

for the moment see D'Auria-Fre formulation of supergravity for further details

Related concepts

• Einstein-Hilbert action, Einstein equation

• rubber-sheet analogy of gravity

• general covariance

• Penrose-Hawking theorem?, cosmic censorship hypothesis

• weak gravity conjecture

• first order formulation of gravity

  • Plebanski formulation of gravity, gravity as a BF-theory, teleparallel gravity

  • Chern-Simons gravity

• Einstein-Maxwell theory, Einstein-Yang-Mills theory

• higher order curvature corrections

• supergravity

• D=2 gravity

  • Liouville theory

  • Jackiw-Teitelboim gravity

• D=5 gravity

• spacetime

  • black hole

  • gravitational wave

• gravitational entropy

  • Bekenstein-Hawking entropy

  • generalized second law of thermodynamics

• energy condition

• positive energy theorem

• asymptotic safety

• cosmology

  • standard model of cosmology

• MOND

• computer experiment: gevolution

References

General

Textbooks include

• Charles Misner, Kip Thorne, John Wheeler, Gravitation, 1973

Lecture notes

• Matthias Blau, Lecture notes on general relativity (web)

• Emil T. Akhmedov, Lectures on General Theory of Relativity (arXiv:1601.04996)

• Pietro Menotti, Lectures on gravitation (arXiv:1703.05155)

See also

• Alan Coley, Mathematical General Relativity (arXiv:1807.08628)

Discussion of classical gravity via its perturbative quantum field theory:

• Günter Scharf, Quantum Gauge Theories – A True Ghost Story, Wiley 2001

• Gustav Uhre Jakobsen, General Relativity from Quantum Field Theory (arXiv:2010.08839)

This way the theory of gravity based on the standard Einstein-Hilbert action may be regarded as just an effective quantum field theory, which makes some of its notorious problems be non-problems:

• John Donoghue, Introduction to the Effective Field Theory Description of Gravity (arXiv:gr-qc/9512024)

See also the references at general relativity.

Covariant phase space

The (reduced) covariant phase space of gravity (presented for instance by its BV-BRST complex, see there fore more details) is discussed for instance in

• Romeo Brunetti, Klaus Fredenhagen, Towards a Background Independent Formulation of Perturbative Quantum Gravity (arXiv:gr-qc/0603079)

which is surveyed in

• Katarzyna Rejzner, The BV formalism applied to classical gravity (pdf)

Careful discussion of observables in gravity is in

• Igor Khavkine, Local and gauge invariant observables in gravity, talk at Operator and Geometric Analysis on Quantum Theory, preprint arXiv:1503.03754

Non-renormalizability

The result that gravity is not renormalizable is due to:

• Gerard 't Hooft, Martinus Veltman, One loop divergencies in the theory of gravitation, Ann. Inst. Poincar é 20 (1974) 69 (numdam:AIHPA_1974__20_1_69_0)

Review:

• Zvi Bern, Perturbative quantum gravity and its relation to gauge theory, Living reviews in relativity, www.livingreviews.org/Articles/Volume5/2002-5bern

  .

• Assaf Shomer, A pedagogical explanation for the non-renormalizability of gravity (arXiv:0709.3555)