3 research outputs found
Low-energy general relativity with torsion: a systematic derivative expansion
We attempt to build systematically the low-energy effective Lagrangian for
the Einstein--Cartan formulation of gravity theory that generally includes the
torsion field. We list all invariant action terms in certain given order; some
of the invariants are new. We show that in the leading order the fermion action
with torsion possesses additional U(1)_L x U(1)_R gauge symmetry, with 4+4
components of the torsion (out of the general 24) playing the role of Abelian
gauge bosons. The bosonic action quadratic in torsion gives masses to those
gauge bosons. Integrating out torsion one obtains a point-like 4-fermion action
of a general form containing vector-vector, axial-vector and axial-axial
interactions. We present a quantum field-theoretic method to average the
4-fermion interaction over the fermion medium, and perform the explicit
averaging for free fermions with given chemical potential and temperature. The
result is different from that following from the "spin fluid" approach used
previously. On the whole, we arrive to rather pessimistic conclusions on the
possibility to observe effects of the torsion-induced 4-fermion interaction,
although under certain circumstances it may have cosmological consequences.Comment: 33 pages, 1 figure. A new section, discussion and references added.
Final (published) versio
Beyond Einstein-Cartan gravity: Quadratic torsion and curvature invariants with even and odd parity including all boundary terms
Recently, gravitational gauge theories with torsion have been discussed by an
increasing number of authors from a classical as well as from a quantum field
theoretical point of view. The Einstein-Cartan(-Sciama-Kibble) Lagrangian has
been enriched by the parity odd pseudoscalar curvature (Hojman, Mukku, and
Sayed) and by torsion square and curvature square pieces, likewise of even and
odd parity. (i) We show that the inverse of the so-called Barbero-Immirzi
parameter multiplying the pseudoscalar curvature, because of the topological
Nieh-Yan form, can only be appropriately discussed if torsion square pieces are
included. (ii) The quadratic gauge Lagrangian with both parities, proposed by
Obukhov et al. and Baekler et al., emerges also in the framework of Diakonov et
al.(2011). We establish the exact relations between both approaches by applying
the topological Euler and Pontryagin forms in a Riemann-Cartan space expressed
for the first time in terms of irreducible pieces of the curvature tensor.
(iii) Only in a Riemann-Cartan spacetime, that is, in a spacetime with torsion,
parity violating terms can be brought into the gravitational Lagrangian in a
straightforward and natural way. Accordingly, Riemann-Cartan spacetime is a
natural habitat for chiral fermionic matter fields.Comment: 12 page latex, as version 2 an old file was submitted by mistake,
this is now the real corrected fil
Barbero-Immirzi parameter, manifold invariants and Euclidean path integrals
The Barbero-Immirzi parameter appears in the \emph{real} connection
formulation of gravity in terms of the Ashtekar variables, and gives rise to a
one-parameter quantization ambiguity in Loop Quantum Gravity. In this paper we
investigate the conditions under which will have physical effects in
Euclidean Quantum Gravity. This is done by constructing a well-defined
Euclidean path integral for the Holst action with non-zero cosmological
constant on a manifold with boundary. We find that two general conditions must
be satisfied by the spacetime manifold in order for the Holst action and its
surface integral to be non-zero: (i) the metric has to be non-diagonalizable;
(ii) the Pontryagin number of the manifold has to be non-zero. The latter is a
strong topological condition, and rules out many of the known solutions to the
Einstein field equations. This result leads us to evaluate the on-shell
first-order Holst action and corresponding Euclidean partition function on the
Taub-NUT-ADS solution. We find that shows up as a finite rotation of
the on-shell partition function which corresponds to shifts in the energy and
entropy of the NUT charge. In an appendix we also evaluate the Holst action on
the Taub-NUT and Taub-bolt solutions in flat spacetime and find that in that
case as well shows up in the energy and entropy of the NUT and bolt
charges. We also present an example whereby the Euler characteristic of the
manifold has a non-trivial effect on black-hole mergers.Comment: 18 pages; v2: references added; to appear in Classical and Quantum
Gravity; v3: typos corrected; minor revisions to match published versio