509 research outputs found
Perturbative quantum damping of cosmological expansion
Perturbative quantum gravity in the framework of the Schwinger–Keldysh formalism is applied to compute lowest-order corrections to expansion of the Universe described in terms of the spatially flat Friedman–Lemaître–Robertson–Walker solution. The classical metric is approximated by a third degree polynomial perturbation around the Minkowski metric. It is shown that quantum contribution to the classical expansion, though extremely small, damps, i.e. slows down, the expansion (phenomenon of quantum friction)
Quantum gravity at a large number of dimensions
We consider the large- limit of Einstein gravity. It is observed that a
consistent leading large- graph limit exists, and that it is built up by a
subclass of planar diagrams. The graphs in the effective field theory extension
of Einstein gravity are investigated in the same context, and it is seen that
an effective field theory extension of the basic Einstein-Hilbert theory will
not upset the latter leading large- graph limit, {\it i.e.}, the same
subclass of planar diagrams will dominate at large- in the effective field
theory. The effective field theory description of large- quantum gravity
limit will be renormalizable, and the resulting theory will thus be completely
well defined up to the Planck scale at GeV. The
expansion in gravity is compared to the successful expansion in
gauge theory (the planar diagram limit), and dissimilarities and parallels of
the two expansions are discussed. We consider the expansion of the effective
field theory terms and we make some remarks on explicit calculations of
-point functions.Comment: 18 pages, 23 figures (75 files), format RevTex4, typos corrected,
references adde
Gauge Amplitude Identities by On-shell Recursion Relation in S-matrix Program
Using only the Britto-Cachazo-Feng-Witten(BCFW) on-shell recursion relation
we prove color-order reversed relation, -decoupling relation,
Kleiss-Kuijf(KK) relation and Bern-Carrasco-Johansson(BCJ) relation for
color-ordered gauge amplitude in the framework of S-matrix program without
relying on Lagrangian description. Our derivation is the first pure field
theory proof of the new discovered BCJ identity, which substantially reduces
the color ordered basis from to . Our proof gives also its
physical interpretation as the mysterious bonus relation with
behavior under suitable on-shell deformation for no adjacent pair.Comment: 4 Pages. No Figures. Rewriting of Introductio
New Identities among Gauge Theory Amplitudes
Color-ordered amplitudes in gauge theories satisfy non-linear identities
involving amplitude products of different helicity configurations. We consider
the origin of such identities and connect them to the Kawai-Lewellen-Tye (KLT)
relations between gravity and gauge theory amplitudes. Extensions are made to
one-loop order of the full N=4 super Yang-Mills multiplet.Comment: 7 page
The Complete KLT-Map Between Gravity and Gauge Theories
We present the complete map of any pair of super Yang-Mills theories to
supergravity theories as dictated by the KLT relations in four dimensions.
Symmetries and the full set of associated vanishing identities are derived. A
graphical method is introduced which simplifies counting of states, and helps
in identifying the relevant set of symmetries.Comment: 41 pages, 16 figures, published version, typos corrected, references
adde
Monodromy and Kawai-Lewellen-Tye Relations for Gravity Amplitudes
We are still learning intriguing new facets of the string theory motivated
Kawai-Lewellen-Tye (KLT) relations linking products of amplitudes in Yang-Mills
theories and amplitudes in gravity. This is very clearly displayed in
computations of N=8 supergravity where the perturbative expansion show a vast
number of similarities to that of N=4 super-Yang-Mills. We will here
investigate how identities based on monodromy relations for Yang-Mills
amplitudes can be very useful for organizing and further streamlining the KLT
relations yielding even more compact results for gravity amplitudes.Comment: 6 pages, 12th Marcel Grossman meeting 200
Proof of Gravity and Yang-Mills Amplitude Relations
Using BCFW on-shell recursion techniques, we prove a sequence of explicit
n-point Kawai-Lewellen-Tye relations between gravity and Yang-Mills amplitudes
at tree level.Comment: 17 pages, no figures, JHE
Explicit Cancellation of Triangles in One-loop Gravity Amplitudes
We analyse one-loop graviton amplitudes in the field theory limit of a
genus-one string theory computation. The considered amplitudes can be
dimensionally reduced to lower dimensions preserving maximal supersymmetry. The
particular case of the one-loop five-graviton amplitude is worked out in detail
and explicitly features no triangle contributions. Based on a recursive form of
the one-loop amplitude we investigate the contributions that will occur at
n-point order in relation to the ``no-triangle'' hypothesis of N=8
supergravity. We argue that the origin of unexpected cancellations observed in
gravity scattering amplitudes is linked to general coordinate invariance of the
gravitational action and the summation over all orderings of external legs.
Such cancellations are instrumental in the extraordinary good ultra-violet
behaviour of N=8 supergravity amplitudes and will play a central role in
improving the high-energy behaviour of gravity amplitudes at more than one
loop.Comment: 25 pages. 2 eps pictures, harvmac format. v2: version to appear in
JHEP. Equations (3.9), (3.12) and minor typos correcte
Absence of Triangles in Maximal Supergravity Amplitudes
From general arguments, we show that one-loop n-point amplitudes in
colourless theories satisfy a new type of reduction formula. These lead to the
existence of cancellations beyond supersymmetry. Using such reduction relations
we prove the no-triangle hypothesis in maximal supergravity by showing that in
four dimensions the n-point graviton amplitude contain only scalar box integral
functions. We also discuss the reduction formulas in the context of gravity
amplitudes with less and no supersymmetry.Comment: 23 pages, RevTeX4 format. v2: Expanded version with a new section
providing some extra background material and an overview of the general
arguments. Minors typos have been corrected. Version to be publishe
- …