General Relativity & Quantum Cosmology

Zoom link: https://pitp.zoom.us/j/96212372067?pwd=dWVaUFFFc3c5NTlVTDFHOGhCV2pXdz09

Experimental searches for new fundamental physics are increasingly adopting an Effective Field Theory (EFT) approach, in which the phenomenological effects of the underlying high-energy (UV) physics are parametrised by a series of EFT coefficients that can be readily compared with data.
While pragmatically useful, this begs the question: what UV information can be extracted from our measurements of these EFT coefficients?
In this talk, I will describe how scattering amplitudes techniques ("sum rules") can establish precise connections between EFT coefficients and the underlying UV physics.
In particular, I will focus on recent progress in applying these techniques in cosmology, where they have been used to connect our large-scale measurements of dark energy, gravitational waves and the CMB with properties of the underlying UV completion.

Zoom Link: https://pitp.zoom.us/j/99740767444?pwd=OTMxWlVDYitSTXdKdmlFRWxhdGl1dz09

https://pitp.zoom.us/j/96212372067?pwd=dWVaUFFFc3c5NTlVTDFHOGhCV2pXdz09

Do the postulates of quantum mechanics survive in quantum gravity? The probabilistic interpretation of amplitudes, enforced by the unitarity of time evolution, is not guaranteed within the path integral formulation and has to be checked. Leveraging the gravitational path integral, we find a non-perturbative mechanism whereby a sum over smooth geometries leads to isometric rather than unitary evolution, which we demonstrate in simple models of de Sitter quantum gravity. These models include Jackiw-Teitelboim gravity and a minisuperspace approximation to Einstein gravity with a positive cosmological constant. In these models we find that knowledge of bulk physics, even on arbitrarily large timescales, is insufficient to deduce the de Sitter S-matrix.

Zoom Link: https://pitp.zoom.us/j/99363952517?pwd=bm80NzhTRUtwLzQ0YjNBTkFpWVpOdz09

The formation of Primordial black holes is naturally enhanced during the quark-hadron phase transition, because of the softening of the equation of state: at a scale between 1 and 3 solar masses, the threshold is reduced of about 10% with a corresponding abundance of primordial black significantly increased by more than 100 times. Performing detailed numerical simulation we have computed the modified mass spectrum for such black holes. Making then a confutation with the LVK phenomenological models describing the GWTC-3 catalog, it is  shown that a sub-population of primordial black black holes formed in the solar mass range is compatible with the current observational constraint and could explain some of the interesting sources emitting gravitational waves detected by LIGO/VIRGO in the black hole mass gap, such as GW190814, and other light events.

Zoom Link:https://pitp.zoom.us/j/94805744664?pwd=Z3oyd05YWVlwenI5NFV6bFZOYUR0dz09

Canonical Quantum Gravity can be considered as a gauge theory of translations. Just like in other gauge theories this implies that physical observables need to be gauge-invariant. Hence, quantities like the metric cannot be observables. This poses new challenges, as this requires to rephrase in the quantum theory how to characterize physics. Moreover, such observables are usually composite. To determine them, the Fröhlich-Morchio-Strocchi mechanism from QFT can be borrowed, to have an augmented perturbative approach.

Zoom Link: https://pitp.zoom.us/j/97927004145?pwd=ekFJaUJSc21UUGdkcDZDWCtpSmdIUT09

Numerical relativity continues to play a crucial role in interpreting gravitational wave detections as well as the first multi-messenger detection of GW170817. More so, state-of-the-art models for kilonvae, gravitational waves, and more rely on the thousands of numerical relativity simulations that have taken place over more than 20 years.  Simulations of binary systems including neutron stars are particularly taxing due to the equation of state of matter being a significant unknown.  In spite of this fact, there exists vast amount of literature on the independent influence mass asymmetry or spin can have on the merger and post-merger dynamics of neutron star binaries across a wide array of possible equations of state.

In this talk I will extend this topic to extremal configurations consisting of binaries that are not only asymmetric, but include appreciable spins on the component neutron stars.  To do so I will give an introduction into the initial data problem for numerical relativity, it's complexities, and its importance to current and future research.  Furthermore, I will discuss a collection of results for extremal binary configurations including neutron stars and why this line of research is important to enable the next generation of multi-messenger models.

Zoom Link: https://pitp.zoom.us/j/99895521696?pwd=T1VtN0RGbjZrVTNleXB3V0FtQjhldz09

We show that if a massive (or charged) body is put in a quantum superposition of spatially separated states in the vicinity of any (Killing) the mere presence of the horizon will eventually destroy the coherence of the superposition. This occurs because, in effect, the long-range fields sourced by the superposition registers on the horizon which forces the radiation of entangling soft gravitons/photons through the horizon. This allows the horizon to harvest “which path” information about the superposition. The electromagnetic decoherence arises only when the superposed particle carries electric charge. However, since all matter sources gravity, the quantum gravitational decoherence applies to all superpositions. We provide estimates of the decoherence time for such quantum superpositions. Additionally, we show that this decoherence is distinct from--and larger than--the decoherence resulting from the presence of thermal radiation from the horizon (i.e. Hawking/Bunch-Davies/Unruh radiation). We believe that the fact that Killing horizons will eventually decohere any quantum superposition may be of fundamental significance for our understanding of the nature of black holes and horizons in quantum gravity. (Based on arXiv:2205.06279 and arXiv: 2301.00026).

Zoom link:  https://pitp.zoom.us/j/95263116767?pwd=amZ6SkROV1lxckVpdzhNbFhYc1ZiQT09

Tidal interactions in coalescing binary neutron stars modify the dynamics of the inspiral, and hence imprint a signature on their gravitational-wave (GW) signals in the form of an extra phase shift. We need accurate models for the tidal phase shift in order to constrain the supranuclear equation of state from observations. In previous studies, GW waveform models were typically constructed by treating the tide as a linear response to a perturbing tidal field. In this work, we incorporate non-linear corrections due to hydrodynamic three- and four-mode interactions and show how they can improve the accuracy and explanatory power of waveform models. We set up and numerically solve the coupled differential equations for the orbit and the modes, and analytically derive solutions of the system's equilibrium configuration. Our analytical solutions agree well with the numerical ones up to the merger and involve only algebraic relations, allowing for fast phase shift and waveform evaluations for different equations of state over a large parameter space. We find that, at Newtonian order, nonlinear fluid effects can enhance the tidal phase shift by  >~ 1 radian at a GW frequency of 1000 Hz, corresponding to a  10−20%  correction to the linear theory. The scale of the additional phase shift near the merger is consistent with the difference between numerical relativity and theoretical predictions that account only for the linear tide. Nonlinear fluid effects are thus important when interpreting the results of numerical relativity, and in the construction of waveform models for current and future GW detectors.

Zoom link:  https://pitp.zoom.us/j/91730134841?pwd=U0JXNHhFbWdQYno3aUpDWHRxWEtkUT09

Understanding gravity in the framework of quantum mechanics is one of the great challenges in modern physics. Along this line, a prime question is to find whether gravity is a quantum entity subject to the rules of quantum mechanics. It is fair to say that there are no feasible ideas yet to test the quantum coherent behaviour of gravity directly in a laboratory experiment. I will introduce an idea for such a test based on the principle that two objects cannot be entangled without a quantum mediator.  I will show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. I will provide a prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, through simple correlation measurements between two spins: one embedded in each test mass. Fundamentally, the above entanglement is shown to certify the presence of non-zero off-diagonal terms in the coherent state basis of the gravitational field modes.

Zoom link:  https://pitp.zoom.us/j/99584743899?pwd=aHl1cVlpK29ZVDkrdFZyM01GemJJdz09

Extensions of asymptotic phase spaces and their corresponding asymptotic symmetry groups have become a topic of increasing interest in recent years, due to results that connect them with a wide spectrum of areas, such as symmetries of the S-matrix, soft theorems, corner symmetries, double copy maps and celestial holography. The study of these extensions aims to characterize the degrees of freedom of the physical theories at the classical level, gathering information on how their symmetries can be upgraded to the quantum theories. In this talk, I will describe extensions of phase spaces at null infinity for gravity and gauge theories, such that the charges are consistent with tree-level soft (graviton, gluon or photon) theorems and act canonically. First, as motivation, I will show how the Geroch group for cylindrically symmetric general relativity can be upgraded as a quantum symmetry, exploiting the integrability of the system (arXiv:1906.04856 [gr-qc]). Second, I will review the construction of an extended phase space where the generalized BMS symmetry group acts canonically (arXiv:2002.06691 [gr-qc]). I will show that this construction is consistent with the extended corner symmetry approach (via the embedding maps). Finally, I will show that a similar approach can be done in Yang Mills, by extending the phase space with "Goldstone modes" that transform inhomogeneously under linearized O(r) symmetries (arXiv:2111.00973 [hep-th]). Some preliminary results regarding the extension to higher order O(r^n) will be discussed (arXiv:2211.12991 [hep-th]).

Zoom link:  https://pitp.zoom.us/j/95866794894?pwd=UEtQYlBpV0NCM3cyeWpDcTI2WVp6UT09