Gravitational Physics with a Positive $\Lambda$ : Some surprises
In the positive cosmological constant sector, several basic questions in classical and quantum gravity whose answers we take for granted in the zero cosmological constant sector have remained open. For example, in asymptotically de Sitter space-times, we still do not have a satisfactory notion of gravitational radiation in exact GR, nor a positive energy theorem. Similarly, the standard constructions of `in’ and `out’ Hilbert spaces on scri that we routinely use (e.g. in the analysis of black hole evaporation) do not extend to asymptotically de Sitter space-times. The positive cosmological constant also has unforeseen effects in the classical and quantum analysis of mini-superspaces. In this talk I will present some illustrative examples of these surprises and suggest an approach to resolve the open issues.
Observational footprints of effective loop quantum gravity
In this talk I will review some recent aspects of effective loop quantum cosmology focusing on the interplay with observations. In particular I will present new results obtained in loop cosmology when both quantum corrections (holonomy and inverse-triad) are taken into account simultaneously. I will then consider consequences for the Cosmological Microwave Background. I will also mention interesting issues related to the BKL conjecture.
Effective evaluations of canonical quantum gravity
Canonical quantum gravity poses important questions not only regarding its mathematical formulation but also its physical evaluation. Sufficient control on properties of quantum-geometry states is required in order to obtain a reliable picture of the early universe and for robust phenomenology. This talk reviews recent progress on new methods by which effective descriptions of dynamical quantum states can be derived canonically, and it includes a discussion of potential applications to gravity.
Cosmology of multiscale spacetimes
The cosmological implications of various classes of recently introduced multiscale spacetimes are discussed. These spacetimes are non-Riemannian: the metric structure is accompanied by an independent measure-differential structure with the characteristics of a multi-fractal, namely, different dimensionality at different scales and, at ultra-short distances, a discrete symmetry known as discrete scale invariance. Under this minimal paradigm, five general features arise: (a) the big-bang singularity can be replaced by a finite bounce, (b) the cosmological constant problem is reinterpreted, since accelerating phases can be mimicked by the change of geometry with the time scale, without invoking a slowly rolling scalar field, (c) the discreteness of geometry at Planckian scales can leave an observable imprint of logarithmic oscillations in cosmological spectra and (d) give rise to an alternative mechanism to inflation or (e) to a fully analytic model of cyclic mild inflation, where near scale invariance of the perturbation spectrum can be produced without strong acceleration.
Post-inflationary string cosmology
In this talk I will analyse the post-inflationary evolution of cosmological models built in the framework of string compactifications. I will argue that reheating is generically driven by the decay of the lightest modulus which can produce, together with Standard Model particles, also non-thermal dark matter and light hidden sector degrees of freedom that behave as dark radiation.
Gravitational theories have to respect the special and general theories of relativity in appropriate limits, as well as the quantum field theory approach leading to the idea of gravitons. I will make the case that (a) space time must be discrete at the micro level, with a continuum view emerging via coarse graining; (b) holonomy is central to any foundational approach, inter alia because of the Aharanov-Bohm exeriments; (c) the true gravitational equations must be trace free, both to avoid the vacuum energy disaster and to respect the nature of gravitons; (d) this is related to many indications that gravity is essentially a conformal theory, with matter breaking the conformal symmetry; (e) at a fundamental level, the theory should probably be a relative theory, as initially pointed out by Weyl, but with outcomes depending on context, as pointed out by Mach; (f) at the emergent level, the focus should be on the Weyl tensor rather than the metric; (g) The past, present, and future are fundamentally different. Our theories must adequately take account both of the quantum measurement issue (`collapse of the wave function’), and the irreversible flow of time, as experienced in everyday physics, chemistry, and biology.
Cosmic bounces and the inflationary multiverse
I will review the properties of the inflationary multiverse and the measure problem. In particular, I will discuss the issue of thermal death due to approximate ergodicity in the transition rates. Then, I will consider the possibility of cosmic bounces following a contracting phase in AdS bubbles. The resolution of crunch singularities seems to provide a natural solution to the measure problem (which we call the watcher measure), where the threat of thermal death is avoided due to the non-ergodicity of vacuum transitions at the bounce.
Space as a quantum gravity condensate
We discuss the proposal that space itself could be described as a condensate of elementary geometric “building blocks”, and its concrete implementation in the group field theory (GFT) formalism for quantum gravity. In this scenario, the condensation of many quantum gravity degrees of freedom into the same microscopic state represents a spatially homogeneous metric geometry. We show how, using this idea of condensation, an effective cosmological dynamics can be computed directly from the microscopic dynamics of a proposed group field theory action (which itself arises from a second quantisation picture for spin foam models/loop quantum gravity), illustrating this rather general procedure with examples that point at the possible cosmological phenomenology of our approach.
Can effects of quantum gravity be observed in the cosmic microwave background?
In any approach to quantum gravity, it is crucial to look for observational effects. In my talk, I discuss how quantum gravitational contributions to the anisotropy spectrum of the cosmic microwave background arise in the framework of quantum geometrodynamics (Wheeler-DeWitt equation). I present in detail the observational constraints coming from the CMB measurements performed by the PLANCK satellite. I also compare these results with the predictions from loop quantum cosmology.
This talk will summarize the main characteristic properties of multi-field inflation and how these models are now constrained after Planck. I will also discuss the impact on cosmological perturbations of heavy scalar fields during inflation.
Quantum spacetime, the view from below
I will discuss how some aspects of quantum spacetime could be inferred from “low” energy data, i.e. quantum field theory. As a tool I use the noncommutative geometry spectral action. I will argue that the presence of a physical cutoff scale, within the spectral action point of view, gives hints of a spacetime in which points cease to have an operational meaning.
Cosmology from First Principles in Causal Dynamical Triangulations
I will explain, in a manner hopefully accessible to non-experts in quantum gravity, the essence of the nonperturbative approach of Causal Dynamical Triangulations, and how far we are in deriving aspects of (quantum) cosmology from this framework. This will include a comparison with a minisuperspace quantum de Sitter cosmology, emphasizing the role of nonperturbative contributions to the gravitational path integral to bring about this result.
Non-commutative and Non-Associative Geometries from Non-Geometric String Flux Vacua
In this talk we discuss the geometric and non-geometric faces of closed string vacua arising by T-duality from torus bundles with constant H-flux. The associated closed string geometries are described by new non-commutative as well as non-associative algebras, which can be characterized by certain 3-cocycles in Lie algebra cohomology. We also present an associated star-product algebra on functions in phase space. Finally we discuss some aspects of quantization and uncertainty relations, as well as the magnetic monopole analogue of
non-geometric flux vacua.
Inflation after Planck
In this talk, I discuss the implications for inflation of the recently released CMB Planck data.
The universal attractor of inflation
We discuss a novel non-minimal coupling between gravity and the inflaton sector, and analyse the consequences at weak and strong coupling. For large enough coupling, all models asymptote to a universal attractor, which is located in the `sweet spot’ of Planck’s recent results.
From quantum cosmology to Planck stars
Some features of singularity resolution in anisotropic and landscape scenarios
In recent years, quantization of cosmological models using techniques of loop quantum gravity have given various insights on the resolution of singularities. In this talk we will focus on two applications of these ideas using effective theory in loop quantum cosmology. We will first discuss the singularity resolution in Bianchi models. We will show that due to quantum gravitational effects, there are Kasner transitions in the Bianchi-I model which are absent in the classical theory, and, discuss the singularity resolution in the inflationary and Cyclic models. In the second application, we consider a toy model description of a landscape scenario and show that due to quantum gravitational effects, non-singular anti-deSitter to deSitter transitions can be successfully obtained.
Low-l CMB Power Loss and String Inflation
The PLANCK satellite may have observed a lack of power on large scales (l < 40). We argue that this putative feature can be explained by a phase of fast roll at the onset of inflation. We show that in the context of single field models what is required is an asymmetric inflection point model of which fibre inflation is a string motivated example. We study the ability of fibre inflation to generate a suppression of the CMB 2-point function power at low l, finding that the potential derived from string loops is almost but not quite steep enough for this purpose. We introduce a steeper contribution to the potential, that dominates away from the inflationary region, and show that if properly tuned it can indeed lead to a spectrum with lack of power at large scales.