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Register a new userAmplitudes, Gravity and Classical Discontinuities
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On-shell methods have revitalized interest in scattering amplitudes which have, in turn, shed some much needed light on the structure of quantum field theories. These developments have been warmly embraced by the particle physics community. Less so in the astrophyical and cosmological contexts. As part of an effort to address this imbalance, we illustrate these methods by revisiting two classic problems in gravity: gravitational light-bending and the vDVZ discontinuity of massive gravity.
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We derive new positivity bounds for scattering amplitudes in theories with a massless graviton in the spectrum in four spacetime dimensions, of relevance for the weak gravity conjecture and modified gravity theories. The bounds imply that extremal black holes are self-repulsive, $M/|Q|<1$ in suitable units, and that they are unstable to decay to smaller extremal black holes, providing an S-matrix proof of the weak gravity conjecture. We also present other applications of our bounds to the effective field theory of axions, $P(X)$ theories, weakly broken galileons, and curved spacetimes.
Conformal supergravity amplitudes are obtained from the double-copy construction using gauge-theory amplitudes, and compared to direct calculations starting from conformal supergravity Lagrangians. We consider several different theories: minimal ${cal N}=4$ conformal supergravity, non-minimal ${cal N}=4$ Berkovits-Witten conformal supergravity, mass-deform
The analytic structures of scattering amplitudes in gauge theory and gravity are examined on the celestial sphere. The celestial amplitudes in the two theories - computed by employing a regulated Mellin transform - can be compared at low multiplicity. It is established by direct computation that up to five external particles, the double copy relations of Kawai, Lewellen and Tye continue to hold identically, modulo certain multiplicative factors which are explicitly determined. Supersymmetric representations of the amplitudes are utilized throughout, manifesting the double copy structure between $mathcal{N}=4$ super Yang-Mills and $mathcal{N}=8$ supergravity on the celestial sphere.
A nonperturbative regularization of UV-divergencies, caused by finite discontinuities in the field configuration, is discussed in the context of 1+1-dimensional kink models. The relationship between this procedure and the appearance of quantum copies of classical kink solutions is studied in detail and confirmed by conventional methods of soliton quantization.
We reanalyze and expand upon models proposed in 2015 for linear dilaton black holes, and use them to test several speculative ideas about black hole physics. We examine ideas based on the definition of quantum extremal surfaces in quantum field theory in curved space-time. The low energy effective field theory of our model is the large N CGHS model, which includes the one loop effects that are taken into account in the island proposal for understanding the Page curve. Contrary to the results of the island analysis, that solution leads to a singular geometry for the evaporated black hole. If the singularity obeys Cosmic Censorship then Hawking evaporation leaves behind a remnant object with a finite fraction of the black hole entropy. If the singularity becomes naked at some point, boundary conditions on a time-like line emanating from that point can produce a sensible model where we expect a Page curve. We show that the fully UV complete model gives a correct Page curve, as it must since the model is manifestly unitary. Recent result on replicawormholes suggest that the island formula, which appears to involve only one loop computations, in fact encodes non-perturbative contributions to the gravitational path integral. The question of why Euclidean gravity computations can capture information about microscopic states of quantum gravity remains mysterious. In a speculative coda to the paper we suggest that the proper way of understanding the relation between Euclidean gravity path integrals and quantum spectra is via a statistical approach to Jacobsons interpretation of general relativistic field equations as the hydrodynamic equations of the area law for the maximal entropy of causal diamonds.
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