This series consists of talks in the area of Quantum Gravity.
In approaches to quantum gravity, where smooth spacetime is an emergent approximation of a discrete Planckian fundamental structure, any standard effective field theoretical description will miss part of the degrees of freedom and thus break unitarity. Here we show that these expectations can be made precise in loop quantum cosmology.
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.
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.
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.
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
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".