4 research outputs found
Fat Euclidean Gravity with Small Cosmological Constant
The cosmological constant problem is usually considered an inevitable feature
of any effective theory capturing well-tested gravitational and matter physics,
without regard to the details of short-distance gravitational couplings. In
this paper, a subtle effective description avoiding the problem is presented in
a first quantized language, consistent with experiments and the Equivalence
Principle. First quantization allows a minimal domain of validity to be carved
out by cutting on the proper length of particle worldlines. This is facilitated
by working in (locally) Euclidean spacetime, although considerations of
unitarity are still addressed by analytic continuation from Lorentzian
spacetime. The new effective description demonstrates that the cosmological
constant problem {\it is} sensitive to short-distance details of gravity, which
can be probed experimentally. ``Fat Gravity'' toy models are presented,
illustrating how gravity might shut off at short but testable distances, in a
generally covariant manner that suppresses the cosmological constant. This
paper improves on previous work by allowing generalizations to massless matter,
non-trivial spins, non-perturbative phenomena, and multiple (metastable) vacua.Comment: 31 pages, LaTe
Hidden Quantum Gravity in 4d Feynman diagrams: Emergence of spin foams
We show how Feynman amplitudes of standard QFT on flat and homogeneous space
can naturally be recast as the evaluation of observables for a specific spin
foam model, which provides dynamics for the background geometry. We identify
the symmetries of this Feynman graph spin foam model and give the gauge-fixing
prescriptions. We also show that the gauge-fixed partition function is
invariant under Pachner moves of the triangulation, and thus defines an
invariant of four-dimensional manifolds. Finally, we investigate the algebraic
structure of the model, and discuss its relation with a quantization of 4d
gravity in the limit where the Newton constant goes to zero.Comment: 28 pages (RevTeX4), 7 figures, references adde