Sebastian Cespedes, Anne-Christine Davis, Scott Melville

Developing our understanding of how correlations evolve during inflation is crucial if we are to extract information about the early Universe from our late-time observables. To that end, we revisit the time evolution of scalar field correlators on de Sitter spacetime in the Schrodinger picture. By direct manipulation of the Schrodinger equation, we write down simple "equations of motion" for the coefficients which determine the wavefunction. Rather than specify a particular interaction Hamiltonian, we assume only very basic properties (unitarity, de Sitter invariance and locality) to derive general consequences for the wavefunction's evolution. In particular, we identify a number of "constants of motion": properties of the initial state which are conserved by any unitary dynamics. We further constrain the time evolution by deriving constraints from the de Sitter isometries and show that these reduce to the familiar conformal Ward identities at late times. Finally, we show how the evolution of a state from the conformal boundary into the bulk can be described via a number of "transfer functions" which are analytic outside the horizon for any local interaction. These objects exhibit divergences for particular values of the scalar mass, and we show how such divergences can be removed by a renormalisation of the boundary wavefunction - this is equivalent to performing a "Boundary Operator Expansion" which expresses the bulk operators in terms of regulated boundary operators. Altogether, this improved understanding of the wavefunction in the bulk of de Sitter complements recent advances from a purely boundary perspective, and reveals new structure in cosmological correlators.

Jennifer Rittenhouse West

The binding energy of diquarks may affect physical phenomena on vastly different scales, from nuclei to neutron stars. On nuclear scales, the ratio of nucleon structure functions for $A>3$ nuclei as compared to deuterium is distorted from theoretical predictions in the quark momentum fraction range $0.3 < x_{B} < 0.7$. A QCD model for structure function distorting short-range nucleon-nucleon interactions is given in terms of diquark degrees of freedom. A diquark can form in the attractive $\rm SU(3)_C$ channel acting on the valence quarks of two overlapping nucleons and its binding energy may be calculated from color hyperfine structure arguments. The most energetically favorable diquark configuration is a valence $u$ quark from one nucleon with a valence $d$ quark from the other in a spin-0 state bound together via single gluon exchange. Diquarks carry color charge, are in the antifundamental representation of $\rm SU(3)_C$ and can act as the QCD equivalent of $\rm U(1)_{\rm EM}$ Cooper pairs by breaking $\rm SU(3)_C$ for the duration of the diquark bond. In the quark-diquark model of baryon structure, the new diquark may effectively form a color-charged bosonic condensate in nuclear matter; a set of 3 QCD Cooper pairs in the two nucleon system. Formation of a new scalar isospin singlet $[ud]$ diquark with binding energy $\sim 150 ~\rm MeV$ between overlapping nucleons is proposed as the primary QCD foundation for short-range nucleon-nucleon correlation models of distorted structure functions in nuclei, with smaller contributions from the spin-1 isospin-1 triplet $(ud)$, $(uu)$ and $(dd)$ of binding energies $67-75~\rm MeV$. Diquark formation and binding energy release may provide an energetically favorable state for the core of neutron stars to collapse into.

Alessia Platania, Christof Wetterich

We discuss aspects of non-perturbative unitarity in quantum field theory. The additional ghost degrees of freedom arising in "truncations" of an effective action at a finite order in derivatives could be fictitious degrees of freedom. Their contributions to the fully-dressed propagator -- the residues of the corresponding ghost-like poles -- vanish once all operators compatible with the symmetry of the theory are included in the effective action. These "fake ghosts" do not indicate a violation of unitarity.

Peter Adshead, Yanou Cui, Andrew J. Long, Michael Shamma

We propose a method for testing the Dirac neutrino hypothesis by combining data from terrestrial neutrino experiments, such as tritium beta decay, with data from cosmological observations, such as the cosmic microwave background and large scale structure surveys. If the neutrinos are Dirac particles, and if the active neutrinos' sterile partners were once thermalized in the early universe, then this new cosmological relic would simultaneously contribute to the effective number of relativistic species, $N_\text{eff}$, and also lead to a mismatch between the cosmologically-measured effective neutrino mass sum $\Sigma m_\nu$ and the terrestrially-measured active neutrino mass sum $\Sigma_i m_i$. We point out that specifically correlated deviations in $N_\text{eff} \gtrsim 3$ and $\Sigma m_\nu \gtrsim \Sigma_i m_i$ above their standard predictions could be the harbinger revealing the Dirac nature of neutrinos. We provide several benchmark examples, including Dirac leptogenesis, that predict a thermal relic population of the sterile partners, and we discuss the relevant observational prospects with current and near-future experiments. This work provides a novel approach to probe an important possibility of the origin of neutrino mass.

Vitor Cardoso, Wen-di Guo, Caio F. B. Macedo, Paolo Pani

Gravitational-wave astronomy, together with precise pulsar timing and long baseline interferometry, is changing our ability to perform tests of fundamental physics with astrophysical observations. Some of these tests are based on electromagnetic probes or electrically charged bodies, and assume an empty universe. However, the cosmos is filled with plasma, a dilute medium which prevents the propagation of low-frequency, small-amplitude electromagnetic waves. We show that the plasma hinders our ability to perform some strong-field gravity tests, in particular: (i)~nonlinear plasma effects dramatically quench plasma-driven superradiant instabilities; (ii)~the contribution of electromagnetic emission to the inspiral of charged black hole binaries is strongly suppressed; (iii)~electromagnetic-driven secondary modes, although present in the spectrum of charged black holes, are excited to negligible amplitude in the gravitational-wave ringdown signal. The last two effects are relevant also in the case of massive fields that propagate in vacuum and can jeopardize tests of modified theories of gravity containing massive degrees of freedom.

Xian Chen

Many objects discovered by LIGO and Virgo are peculiar because they fall in a mass range which in the past was considered unpopulated by compact object. Given the significance of the astrophysical implications, it is important to first understand how their masses are measured from gravitational-wave signals. How accurate is the measurement? Are there elements missing in our current model which may result in a bias? This chapter is dedicated to these questions. In particular, we will highlight several astrophysical factors which are not included in the standard model of GW sources but could result in a significant bias in the estimation of the mass. These factors include strong gravitational lensing, the relative motion of the source, a nearby massive object, and a gaseous background.