Physics Reports Chern–Simons modified general relativity

S. Alexander, N. Yunes / Physics Reports 480 (2009) 1–55 3Theory does suggest that the coupling constant in front of the CS correction should be suppressed, at least at the electroweakscale level, or even the Planck scale level. Indeed, if this were the case, the CS correction would be completely undetectableby any future experiments or observations.Other quantities exist, however, that String Theory predicts should also be related to the Planck scale, yet severalindependent measurements and observations suggest this is not the case. One example of this is the cosmological constant,which, according to String Theory, should be induced by supersymmetry breaking. If this breakage occurs at the electroweakscale, then the value of the cosmological constant should be approximately 10 45 eV 4 , while if it occurs at the Planck scale itshould be about 10 112 eV 4 . We know today that the value of the cosmological constant is close to 10 −3 eV 4 , nowhere nearthe String Theory prediction.A healthy, interdisciplinary rapport has since developed between the cosmology and particle communities. On the onehand, the astrophysicists continue to make more precise and independent measurements of the cosmological constant. Onthe other hand, the particle and String Theory communities are now searching for new and exciting ways to explain suchan observable value, thus pushing their models in interesting directions.Similarly, a healthy attitude, perhaps, is to view the CS correction as a model-independent avenue to investigate parityviolation, its signatures and potential detectability, regardless of whether some models expect this correction to be Plancksuppressed. In fact, there are other models that suggest the CS correction could be enhanced, due to non-perturbative instantoncorrections [5], interactions with fermions [6], large intrinsic curvatures [7] or small string couplings at late times [8–17].After mathematically defining the effective theory [Section 2], we shall discuss its emergence in the Standard Model,String Theory and Loop Quantum Gravity [Section 3]. We begin by reviewing how the CS gravitational term arises fromthe computation of the chiral anomaly in the Standard Model coupled to GR. While this anomaly is cancelled in theStandard Model gauge group, we shall see that it persists in generic Yang–Mills gravitational theories. We then present theGreen–Schwarz anomaly canceling mechanism and show how CS theory arises from String Theory, leading to a Pontryagincorrection to the action in four-dimensional gravity coupled to Yang–Mills theory. The emergence of CS gravity in LoopQuantum Gravity is then reviewed, as a consequence of the scalarization of the Barbero–Immirzi parameter in the presenceof fermions.Once the effective model has been introduced and motivated, we shall concentrate on exact and approximate solutionsof the theory both in vacuum and in the presence of fermions [Sections 4–6]. We shall see that, although sphericallysymmetric spacetimes that are solutions in GR remain solutions in CS modified gravity, axially-symmetric solutions do nothave the same fate. In the far field, we shall see that the gravitational field of a spinning source is CS modified, leadingto a correction to frame-dragging. Moreover, the propagation of gravitational waves is also CS corrected, leading to anexponential enhancement/suppression of left/right-polarized waves that depends on wave-number, distance travelled andthe entire integrated history of the CS coupling.With such exact and approximate solutions, we shall review the myriads of astrophysical and cosmological tests of CSmodified gravity [Sections 7 and 8]. We shall discuss Solar-System tests, including anomalous gyroscopic precession, whichhas led to the first experimental constraint of the model. We shall then continue with a discussion of cosmological tests,including anomalous circular polarization of the CMBR. We conclude with a brief summary of leptogenesis in the earlyuniverse as explained by the effective theory.This review paper consists of a summary of fascinating results produced by several different authors. Overall, we followmostly the conventions of [18,19], which are the same as those of [20,21], unless otherwise specified. In particular, Latinletters at the beginning of the alphabet a, b, . . . , h correspond to spacetime indices, while those at the end of the alphabeti, j, . . . , z stand for spatial indices only. Sometimes i, j, . . . , z will instead stand for indices representing the angular sectorin a 2 + 2 decomposition of the spacetime metric, but when such is the case the notation will be clear by context. Covariantderivatives in four (three) dimensions are denoted by ∇ a (D i ) and partial derivatives by ∂ a (∂ i ). The Levi-Civita tensor isdenoted by ɛ abcd , while ˜ɛ abcd is the Levi-Civita symbol, with convention ˜ɛ 0123 = +1. The notation [A] stands for the units ofA and L stands for the unit of length, while the notation O(A) stands for a term of order A. Our metric signature is (−, +, +, +)and we shall mostly employ geometric with G = c = 1, except for a few sections where natural units shall be moreconvenient h = c = 1.2. The ABC of Chern–Simons and its tools2.1. FormulationCS modified gravity is a 4-dimensional deformation of GR, postulated by Jackiw and Pi [22]. 1 The modified theory can bedefined in terms of its action:S := S EH + S CS + S ϑ + S mat , (1)1 Similar versions of this theory were previously suggested in the context of string theory [23,24], and three-dimensional topological massivegravity [25,26].