Bulletin of the American Physical Society
APS April Meeting 2020
Volume 65, Number 2
Saturday–Tuesday, April 18–21, 2020; Washington D.C.
Session G15: Quantum Field Theory in a Curved SpacetimeLive

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Sponsoring Units: DGRAV Chair: Ivan Agullo, Louisiana State University Room: Virginia B 
Sunday, April 19, 2020 8:30AM  8:42AM Live 
G15.00001: Inflation with Preinflationary Radiation Dominated Era Taylor Ordines, Paul Anderson, Eric Carlson, Bradley Hicks We argue that if from the viewpoint of semiclassical gravity the Universe began with zero size, then there is a preferred initial vacuum state and that if the quantum fields are in any other homogeneous and isotropic state, then before inflation began the Universe, to a good approximation, was radiation dominated. We then study the effects of a preinflationary radiation dominated era that smoothly evolves into an inflationary era by numerically solving the mode equation for a massless, minimally coupled scalar field. We model this transition with a combination of classical radiation and a cosmological constant with the Universe asymptotically approaching de Sitter space. The resulting latetime power spectrum is free of enhancements at all scales, oscillating with peaks that never exceed the BunchDavies value. Comparisons with previous results in models in which the Universe began with a radiation dominated phase will be made. [Preview Abstract] 
Sunday, April 19, 2020 8:42AM  8:54AM Live 
G15.00002: Vacuum entanglement in the presence of gravitational waves Shadi Ali Ahmad, Qidong Xu, Alexander Smith A remarkable fact about the vacuum state of a quantum field theory is that it is entangled across spacelike separated regions. This entanglement is strong enough to violate a Bell inequality and is ultimately responsible for some of the most important predictions of quantum field theory on curved space, such as the Unruh and Hawking effects. In this talk, I will introduce the entanglement harvesting protocol as an operational way to probe vacuum entanglement. This protocol relies on two atoms, modeled by UnruhDeWitt detectors, that are initially unentangled. These atoms then interact locally with the field and become entangled. Because the atoms do not interact with one another, any entanglement between the atoms is a result of entanglement that is 'harvested' from the field, and thus quantifying this entanglement serves as a proxy for how entangled the field is across the regions in which the atoms interacted. Using this protocol, I will show that while the local statistics of each atom are unaffected by the presence of a gravitational wave, the entanglement between them depends sensitively on both the amplitude and frequency of the gravitational wave. This suggests that the entanglement signature left by a gravitational wave may be useful in characterizing its properties. [Preview Abstract] 
Sunday, April 19, 2020 8:54AM  9:06AM Live 
G15.00003: Averaged null energy condition and eternal inflation EleniAlexandra Kontou, Ken Olum Fluctuations in the scalar field driving inflation can lead to an increase in the expansion rate, decreasing the Hubble distance. A converging shell of geodesics just inside the old Hubble distance will be outside the new one and thus be carried away by the Hubble flow. This implies defocusing, which requires violation of the Averaged Null Energy Condition (ANEC). However, semiclassical proofs of ANEC suggest that such violations should not exist. In this talk I will argue that this paradox arises from considering the entire quantum state at early times but only a specific quantum state at late times. Examining a latetimes fast expanding state and tracing back its evolution, I show how ANEC is in fact obeyed in an eternally inflating spacetime. [Preview Abstract] 
Sunday, April 19, 2020 9:06AM  9:18AM Live 
G15.00004: Rapidly Expanding BEC Ring: Analog Cosmology in a Lab Ted Jacobson, Stephen Eckel, Avinash Kumar, Ian Spielman, Gretchen Campbell I will describe an experiment and some theory of an expanding, ringshaped BoseEinstein condensate. The expansion redshifts and damps long wavelength excitations, as in an expanding universe. After expansion, energy in the radial mode leads to the production of bulk topological excitationssolitons and vorticesdriving the production of a large number of azimuthal phonons and, at late times, causing stochastic persistent currents. These complex nonlinear dynamics, fueled by the energy stored coherently in one mode, are reminiscent of a type of "preheating" that may have taken place at the end of inflation. (Based on 10.1103/PhysRevX.8.021021.) [Preview Abstract] 
Sunday, April 19, 2020 9:18AM  9:30AM On Demand 
G15.00005: Validity of the Semiclassical Approximation for Quantum Electrodynamics in 1+1 Dimensions. I Ian Newsome, Paul R. Anderson, Robert S. Link, Silvia Pla, Jose NavarroSalas A derivation of the linear response equations for quantum electrodynamics in 1+1 dimensions will be presented for the case of a spatially homogeneous electric field coupled to a quantized scalar field and a quantized fermion field. It will be shown that these equations depend upon the two point correlation function derived from the currentcurrent commutator, $\langle [ j(t,x), j(t', x')] \rangle$. A criterion for the validity of the semiclassical approximation will be given that involves the stability of solutions to the linear response equations. This is an adaptation of criteria previously used for the validity of the semiclassical approximation for gravity and that for the preheating phase of chaotic inflation. A method of finding approximate solutions to the linear response equations will also be given. [Preview Abstract] 
Sunday, April 19, 2020 9:30AM  9:42AM On Demand 
G15.00006: Validity of the Semiclassical Approximation for Quantum Electrodynamics in 1+1 Dimensions. II Silvia Pla García, Jose Navarro Salas, Paul R. Anderson, Ian Newsome, Robert S. Link Numerical techniques will be discussed that were used to solve the semiclassical backreaction equations for a quantized scalar field and a quantized fermion field interacting with a homogeneous classical electric field in 1+1 dimensions. These include using adiabatic regularization to renormalize the expectation value of the current, $\langle j_\mu \rangle$, and initial conditions for the backreaction and mode equations. Results of numerical calculations will be shown and their implications for the validity of the semiclassical approximation will be discussed. [Preview Abstract] 
Sunday, April 19, 2020 9:42AM  9:54AM On Demand 
G15.00007: Method to compute the stressenergy tensor of a scalar field outside of a Schwarzschild black hole that forms from the collapse of null shell. Shohreh Gholizadeh Siahmazgi, Paul R. Anderson, Raymond D. Clark, Alessandro Fabbri A method will be discussed which allows for the numerical computation of the semiclassical stressenergy tensor, $\left\langle in \middle T_{\mu\nu} \middle in \right\rangle$, associated with a quantized massless minimally coupled scalar field in the region outside the event horizon of a (3+1)D Schwarzschild black hole that forms from the collapse of a null shell. This method is based on the idea that one can expand the inmodes in terms of a complete set of solutions to the mode equation in the exact Schwarzschild geometry. Applying this method, a full numerical computation of the renormalized stressenergy tensor for the (1+1)D case is done and shown to be equal to the known solution. In (3+1)D, the presence of an effective potential in the mode equation causes scattering effects that make the matching more difficult. Tests that check the validity of the 4D matching method will be discussed. [Preview Abstract] 
Sunday, April 19, 2020 9:54AM  10:06AM On Demand 
G15.00008: Compensator fields in dimensional reduction and compactification without truncation Michael Schulz In dimensionally reduced theories with gauge (or diffeomorphism) invariance, consistency of the KaluzaKlein ansatz requires the introduction of compensator fields. The compensator fields project gaugevariant field fluctuations to their horizontal components, which in turn determine the gaugeinvariant moduli space metric that appears in kinetic terms of the lower dimensional theory. The compensator fields must be introduced "by hand" in dimensional reduction (i.e., compactification truncated to zero modes), but arise automatically in the full untruncated theory. For the U(1), YangMills theory, and pure Einstein gravity compactified on an arbitrary compact manifold, we reexpress the full untruncated parent theory in lower dimensional language, and identify the compensator fields. One of their known geometric interpretations features prominently. The moduli space of gauge fields (or metrics) on the compact manifold can be regarded as a principal bundle whose fiber is the space of gauge transformations. The compensator fields arise as a repackaging of the connection on this bundle. [Preview Abstract] 
Sunday, April 19, 2020 10:06AM  10:18AM On Demand 
G15.00009: Local and Covariant Flow Relations for OPE Coefficients in Curved Spacetime Mark Klehfoth, Robert Wald The $n$point functions of (perturbativelyrenormalizable) quantum field theories are known to satisfy asymptotic relations called operator product expansions (OPEs) in the limit that all their spacetime points coincide. The coefficients of these expansions are stateindependent and contain essential information about the quantum field theory itself. In (flat) Euclidean spacetime, Hollands et al. have derived novel ``flow equations'' which govern how OPE coefficients depend on the QFT's interaction parameters. Although proven to hold orderbyorder in perturbation theory, these flow equations have been proposed as a potential avenue for defining OPE coefficients nonperturbatively. However, serious obstacles arise if one attempts to generalize the Hollands flow equations to curved Lorentzian spacetimes in a local and covariant manner. In this talk, I will describe these issues and sketch our resolutions for a solvable toy model: KleinGordon theory on curved spacetime with the (squared) mass, $m^2$, treated as an ``interaction parameter''. The strategies I describe for generalizing this toy model's flow relations in a local and covariant way are expected to be applicable, more generally, to QFTs with nonlinear interactions in curved Lorentzian spacetimes. [Preview Abstract] 
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