Cosmology

I'll describe my recent work on black hole seeded vacuum decay, and proposals for testing seeded decay in cold atom experiments. I'll conclude with speculations on seeking insight into quantum black holes via experimentally constructing quantised analog black holes.

After a short introduction to the stochastic GW background I will highlight how one uses currently available LIGO/Virgo/Kagra data not only to learn about compact binaries and the large-scale-structure of our universe, but also to constrain particle physics models beyond the Standard Model, modified gravity proposals, and even quantum gravity theories.

I will discuss a number of ongoing efforts to understand quantum field properties in a manifestly spacetime framework. Entanglement entropy and causal set theory are among the topics that I will especially touch on.

Within a histories-framework for quantum field theory, the condition of \bold{persistence of zero} (PoZ for short) tries to capture (a part of) the elusive idea that no cause can act outside its future lightcone. The PoZ condition, however, does not easily carry over to theories like gravity where the causal structure is not only dynamical but indefinite (subject to quantum fluctuations). Despite this, I will suggest how to give PoZ meaning in the causal set context, and I will raise the hope that the resulting causality-criterion can lead us to a well-defined dynamcs for quantum causal sets.

There are three natural currents for Maxwell theory on a non-dynamical background: the stress, Noether and canonical current. Their associated fluxes across null infinity differ by boundary terms for asymptotically flat spacetimes. These boundary terms do not only quantitatively change the behavior of the flux associated with an asymptotic Lorentz symmetry, but also qualitatively: the stress flux contains both radiative and Coulombic information, whereas Noether and canonical ones are purely radiative. While all methods are equally valid and have their own range of usefulness, it is reasonable to ask if one definition is more natural than the other. In order to answer this question, we turn to general relativity. With Maxwell theory coupled to gravity, we use the Wald-Zoupas formalism to obtain an expression for the flux of angular momentum and find that it is purely radiative. When the gravitational field is ``frozen'', the Wald-Zoupas flux reduces to the Noether flux.

Linking quantum gravity approaches could be important to make progress in quantum gravity. In my talk, I will try to make this case using asymptotically safe gravity as an example. I will briefly review the status of the approach and highlight the open questions, and discuss proposed ideas how the link to other approaches could be useful to tackle these. Finally, I will emphasize the need for universality in quantum gravity, and argue that there might be universal features from quantum gravity in black-hole shadows.

According to general relativity, the coalescence of a compact binary system creates a gravitational wave signal generically described by an inspiral-merger-ringdown waveform. The recent observations of gravitational waves by LIGO allow us to test our theory of gravity in the strong field regime. In binary black hole detections, the ringdown portion of the wave can provide tests of the no-hair theorem, the most stringent proof of the existence of astrophysical black holes and even possible hints of quantum gravity. I will present the current status of these observations and discuss future prospects.

In physics, every observation is made with respect to a frame of reference. Although reference frames are usually not considered as degrees of freedom, in all practical situations it is a physical system which constitutes a reference frame. Can a quantum system be considered as a reference frame and, if so, which description would it give of the world? In the first part of my talk, I will introduce a general method to quantise reference frame transformations within a Galilean-relativistic setting, which generalises the usual reference frame transformation to a “superposition of coordinate transformations”. We describe states, measurement, and dynamical evolution in different quantum reference frames, without appealing to an external, absolute reference frame, and find that entanglement and superposition are frame-dependent features. The transformation also leads to a generalisation of the notion of covariance of dynamical physical laws. In the second part of my talk, I will show how these ideas can be used to operationally define the localization of events with respect to quantum clocks, each of which identifies a “time reference frame". In particular, I will consider clocks that i) are quantum mechanical, and ii) interact, gravitationally or otherwise, with other quantum systems. We find that, when gravitational effects are important, the time localisability of events becomes a relative concept, depending on the time reference frame. We discuss the physical significance of "jumping" onto a time reference frame with respect to which specific events are localised, in the context of indefinite causal structures arising from the interplay between quantum mechanics and gravity.

A proposal is made for a fundamental theory, which is hypothesized to be a completion of both quantum mechanics and general relativity, in which the history of the universe is constituted of diverse views of itself. Views are attributes of events, and the theory’s only be-ables; they comprise information about energy and momentum transferred to an event from its causal past. A dynamics is proposed for a universe constituted of views of events, which combines the energetic causal set dynamics with a potential energy based on a measure of the distinctiveness of the views, called the variety. As in the real ensemble formulation of quantum mechanics, quantum pure states are associated to ensembles of similar events; the quantum potential of Bohm then arises from the variety. This theory brings together results from two lines of development: energetic causal sets, developed with Marina Cortes, and the real ensemble formulation of quantum mechanics.