This series consists of talks in the area of Quantum Gravity.
The asymptotic symmetry charge algebra of certain class of spacetimes could have a nontrivial central extension, which measures the non-equivariance of the charges of the large gauge transformations. The Cardy formula of the dual CFT has been famously used to derive black hole entropy. However, it remains obscure from the point of view of gravity why such a constant on the gravitational phase space could encode the information about the density of black hole micro-states, and what the degrees of freedom accounting for the black hole entropy truly are.
Biological evolution is a complex blend of ever changing structural stability, variability and emergence of new phenotypes, niches, ecosystems. We wish to argue that the evolution of life marks the end of a physics world view of law entailed dynamics.
What is the ultimate fate of black holes? Since the discovery of Hawking evaporation process, the issue has been much discussed. Loop quantum gravity suggests that black hole could ultimately turn into white holes. In this talk, we investigate several possible mixed scenario where black holes first evaporate to a Planckian size before tunnelling to white holes. We build various spacetime models, taking Hawking backreaction into account, and we discuss some aspects of the expected phenomenology.
Time is one of the most basic features of nature which has been extensively discussed in philosophy and physics. In contrast, time is rather neglected in neuroscience; here time is only conceived in terms of our perception and cognition of time. That leaves open the relevance of time itself, that is, how the brain constitutes its own temporal dynamics and how that is relevant for, for instance, consciousness and other mental features like self.
We review recent efforts to turn the cosmological constant into a dynamical variable without an ungainly proliferation of free parameters. In a cosmological setting where parity invariance is imposed (along with homogeneity and isotropy) this leads to phenomenological disaster. However, in this theory it is possible to construct parity violating Friedman models due to torsion, a re-enactment of "Cartan's spiral staircase".
I will discuss the problem of an observer's S-matrix in de Sitter space, i.e. the mapping between fields on the initial and final horizons of a de Sitter static patch. I will show how the S-matrix of free massless fields can be packaged in a spinor-helicity language. This involves “cheating” the static patch’s painfully low symmetry, by relating each horizon separately to global, de Sitter-invariant data.
If General Relativity emerges from quantum gravity, then general covariance, the gauge invariance of GR, will emerge with it. We can ask, within any approach to the problem of quantum gravity, what is the “precursor” principle or precept that will give rise to — or manifest itself as — general covariance in the large scale semi-classical approximation?
Given how important the understanding of general covariance (or lack of it!) was in the development of GR we might expect that thinking about this question will be similarly important in the development of quantum gravity.
In this talk I will report on recent progress in building QFT models on causal sets. The framework I'm using is that of perturbative algebraic quantum field theory (pAQFT). It was developed for rigorous study of perturbative QFT in the continuum, but can also be applied in the situation where spacetime is replaced by a discrete structure. Causality plays a key role in pAQFT, so it is natural to apply it to causets.
We derive an effective Hamiltonian constraint for the Schwarzschild geometry starting from the full loop quantum gravity Hamiltonian constraint and computing its expectation value on coherent states sharply peaked around a spherically symmetric geometry. We use this effective Hamiltonian to study the interior region of a Schwarzschild black hole, where a homogeneous foliation is available.
Trisections were introduced by Gay and Kirby in 2013 as a way to study 4-manifolds. They are similar in spirit to a common tool in a lower dimension: Heegaard splittings of 3-manifolds. In both cases, one understands a manifold by examining the ways that standard building blocks can be put together. They both also have the advantage of changing problems about manifolds into problems about diagrams of curves on surfaces. This talk will be a relaxed introduction to these decompositions.