Quantum Gravity

Cosmological perturbation theory has a long tradition for describing the early phases of the Universe. As the observations of the CMB radiation suggest, it is reasonable, at least as a first approximation, to implement cosmological inhomogeneities as small perturbations around homogeneous and isotropic FRW solutions. In these approaches, backreactions between the inhomogeneities and the background are usually neglected. There is an ongoing debate about how and to which extend these backreactions affect the large scale structure of the Universe. Even at a purely classical level, there is no conclusive answer to this question yet.

In my talk, I am going to present a new systematic formalism for implementing backreactions in cosmological perturbation theory, in which both, the perturbations and the homogeneous degrees of freedom are considered as quantum degrees of freedom. As a more realistic theory of quantum fields on quantum cosmological space times, it can help to close the gap between a full theory of quantum gravity and symmetry-reduced models of quantum cosmology, and to confront these theories with observations. Our results show that quantum backreactions imply non-trivial corrections that are potentially phenomenologically significant.

I will explain how dark energy in cosmology could arise from the

noisy diffusion of energy from the low energy degrees of freedom of matter (described in terms of QFT)

to the fundamental Planckian granularity (expected from quantum gravity). This

perspective leads to a natural model resolving the fine tuning problem associated to the small

value of the cosmological constant. However, recent observations suggest that the dark energy

component in our universe might not be constant and should  instead have grown from the recombination time to the present.

I will present new ideas suggesting a possible explanation within the same framework.

It has long been argued that combining the uncertainty principle with gravity will lead to an effective minimum length at the Planck scale. A particular challenge is to model the presence of a smallest length scale in a manner which respects continuous spacetime symmetries. One path for deriving low-energy descriptions of an invariant minimum length in quantum field theory is based on generalized uncertainty principles. Here I will consider the question how this approach enables one to retain Euclidean or even Lorentzian symmetries. The Euclidean case yields a ultraviolet cutoff in the form of a bandlimit, and this then allows one to apply the powerful Shannon sampling theorem of classical information theory which establishes the equivalence between continuous and discrete representations of information. As a consequence, one obtains discrete representations of fields which are more subtle than a simple discretization of space, and are in fact equivalent to a continuum representation. Quantum fields in this model exhibit a finite density of information and a corresponding regularization of the entanglement of the vacuum, as I will demonstrate in detail. We then examine the Lorentzian symmetry generalization. This case leads to a Lorentz-invariant analogue of bandlimitation, and we discuss the nature of the corresponding sampling theory.

We study dimensionally restricted non-perturbative causal set quantum dynamics in two and three spacetime dimensions with non-trivial global spatial topology.  The causal set sample space is generated from causal embeddings into latticisations of flat background spacetimes with global spatial topology  and  in two and three dimensions, respectively. The quantum gravity partition function over these sample spaces is studied using Markov Chain Monte Carlo (MCMC) simulations via an analytic continuation of a parameter  analogous to an inverse temperature. In both two and three dimensions we find a phase transition that separates the dominance of the action from that of the entropy. The action dominated phase is characterised by "layered" posets with a high degree of connectivity, while the causal sets in the entropy dominated phase are manifold-like. These results are similar in character to those obtained for topologically trivial causal set dynamics over the sample space of 2-orders. The current simulations use a newly developed framework for causal set MCMC calculations, and provide the first implementation of a three-dimensional causal set dynamics.

arXiv paper: https://arxiv.org/abs/1908.11647

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. I will discuss the ongoing efforts of understanding these questions in the covariant phase space formalism.

Albert Einstein predicted a century ago the existence of gravitational waves – ripples in the fabric of spacetime moving at the speed of light. It was believed that these ripples were so faint that no experiment would ever be precise enough to detect them. But in September 2015, LIGO did exactly that. The teams working with the Laser Interferometer Gravitational-wave Observatory (LIGO) detectors in Louisiana and Washington measured a loud gravitational wave signal as it traveled through the Earth after a billion-year journey from the violent merger of two black holes.

Since that first detection, scientists have measured many more gravitational waves, including a signal produced by colliding neutron stars captured by LIGO and the Virgo detector in Europe in 2017. That cataclysm also generated electromagnetic waves – light – detected by numerous other telescopes, and helped scientists understand how gold is created in deep space.

In a special public lecture webcast at Perimeter Institute on October 23, 2019, Gabriela González will provide a first-hand account of LIGO’s century-in-the-making breakthrough, and explain observations made as recently as this year. González, a professor of physics and astronomy at Louisiana State University and former spokesperson of the LIGO collaboration, will take the audience on a journey to some of the universe’s most violent places, and explain how such distant events can lead to a very bright future here on Earth.

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. Finally, we will draw the path for a full LQG amplitude computation that could lead to observational predictions.

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. My talk will discuss recent findings showing various features of the brain's temporal dynamics and how these are relevant in constituting mental features like consciousness and self as well as their changes in psychiatric disorders. 

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". We examine the Hamiltonian structure of the 2 branches (parity compliant and parity violating) and conclude that they must correspond to different theories, with different numbers of degrees of freedom. Parity violation may save these models phenomenologically, giving observational relevance to the Pontryagin invariant (and possibly the Immirzi parameter) in cosmology.