by David Mattingly (University of New Hampshire)

28 September 2021, 15:00 CEST (GMT +2)

Quantum gravity phenomenology incorporates a tremendous spread of experimental approaches - from kilometer scale astrophysical laboratories sensitive to classical imprints of Planck scale quantum spacetime to micron sized mechanical systems aiming to prove the quantum nature of weak field general relativity. The proliferation of dramatically different techniques is due to two related difficulties: there is no single current experimental approach that can directly probe the quantum gravitational regime and there are numerous theoretical quantum gravity models that give different subtle modifications to low energy physics. In this talk I give a survey of various phenomenological approaches, relating each to the underlying relevant theoretical frameworks, and detail how each approach says something about quantum gravity, although none can give us a direct picture.

by Markus Aspelmeyer (University of Vienna) and Sougato Bose (University College London)

22 October 2021, 17:00 CEST (GMT +2)

Markus Aspelmeyer: No experiment today provides evidence that gravity requires a quantum description. Two type of table-top searches have been suggested to provide answers: experiments that test low-energy consequences of quantum theories of gravity, and experiments that directly probe the phenomenology of quantum states of the metric generated by a quantum source mass (in the spirit of a quantum-Cavendish experiment). The latter requires to bridge the gap between two different realms: precision measurements of gravity with microscopic source masses (currently 10^21 atoms) and quantum state preparations of massive solid state objects (currently 10^9 atoms). I will review the current status in the lab and the challenges to be overcome for future experiments.

Sougato Bose: There is no empirical evidence yet as to “whether” gravity has a quantum mechanical origin. Motivated by this, I will present a potentially feasible idea for testing the quantum origin of the Newtonian interaction based on the simple fact that two objects cannot be entangled without a quantum mediator. I will clarify the assumptions underpinning the above proposal such as a reasonable definition of “classicality”, as well as the "locality" of physical interactions. Further, I will show that despite its weakness, gravity can detectably entangle two adjacent micron sized test masses held in quantum superpositions even when they are placed far apart enough to keep Casimir-Polder forces at bay. A prescription for witnessing this entanglement through spin correlations is also provided. Further, I will state a few ideas about screening EM forces and Inertial noise reduction.

by Giulia Gubitosi (University of Naples) and Nikolaos Mavromatos (King's College London)

19 November 2021, 18:00 CET (GMT+1)

In search for a consistent theory of Quantum Gravity, I explore the possibility that Riemannian geometry might be abandoned, at the expense of introducing torsion as a fundamental quantum field. In string inspired theories, such a quantum torsion is linked to spin-one antisymmetric-tensor fields in the massless string gravitational multiplet, which in (3+1)-dimensions, after extra-dimensional compactification, corresponds to a fully dynamical massless pseudoscalar (axion-like) field. The latter can couple to gravitational anomalies in a way consistent with general covariance. In other settings, such as Loop Quantum Gravity, these axions may be thought of as somewhat analogous to field theoretic extensions of the Barbero-Immirzi parameter, which accompany torsional topological invariants in the corresponding effective gravitational actions.

In such modified theories of gravity, which could be viewed as effective low-energy theories of mathematically consistent quantum gravity models, including, but also going beyond, string theory, there are solutions for the axion background field which violate spontaneously Lorentz symmetry, as a result of condensation of gravitational anomalies induced by primordial gravitational waves. The low-energy theory with matter resembles then the Standard Model Extension (SME) with terms that Violate Lorentz (LV) and CPT symmetries (CPTV). Apart from theoretical considerations, the talk will also explore the phenomenology of the resulting cosmological models, paying particular attention to demonstrating a link with a running-vacuum-model inflationary cosmology, the emergence of matter antimatter asymmetry as a result of the aforesaid LV and CPTV, and the connection of the torsion-induced axion field to dark matter, upon appropriate mass generation at post inflationary epochs.

by Scott Melville (University of Cambridge) and Irene Valenzuela (Harvard University)

18 January 2022, 17:00 CET (GMT+1)

Scott Melville: One of the greatest barriers to understanding quantum gravity is that its characteristic (Planck) energy scale is well beyond our experimental reach. Instead, we can only access a low-energy regime in which essentially all quantum gravity approaches must reduce to General Relativity (/Standard Model) plus small corrections. In this talk, I will describe how to connect these (observable!) low-energy corrections to concrete properties of the underlying quantum gravity theory, and in particular how "positivity bounds" on effective field theory scattering amplitudes can be used to diagnose whether the high-energy completion is unitary, causal and local. As an example, I will show how observations of dark energy fluctuations on cosmological scales and the low-energy speed of gravitational waves can be used to distinguish between different kinds of quantum gravity.

Irene Valenzuela: Consistency with quantum gravity can impose non-trivial constraints at low energies, even if the Planck scale is at very high energy. The Swampland program aims to determine the constraints that an effective field theory must satisfy to be consistent with a UV embedding in a quantum gravity theory. This has led to new quantum gravity constraints, motivated by black hole physics and string theory, that can be used as new guiding principles to construct beyond standard models of Particle Physics and Cosmology. They might also provide the missing piece to solve the long-standing naturalness issues observed in our universe. In this talk, I will review the most important Swampland conjectures and recent developments regarding their connections and phenomenological implications.

by Sachiko Kuroyanagi (IFT Madrid)

24 February 2022, 17:00 CET (GMT+1)

Primordial gravitational waves provide one of the few opportunities for learning the physics of the early universe and for testing quantum gravity. Among all cosmological quantum-gravity or quantum-gravity-inspired scenarios, only very few predict a blue-tilted primordial tensor spectrum. In this talk, first, I will review the potential of future gravitational wave observations to test the early universe. Then, in the second part, I will present the results of my recent paper arXiv:2012.00170 [JCAP 03, 019 (2021)], where we explored quantum-gravity-inspired scenarios and checked whether they can generate a stochastic gravitational-wave background detectable by present and future interferometers.

by Kostas Skenderis (University of Southampton) and Edward Wilson-Ewing (University of New Brunswick)

20 April 2022, 17:00 CET (GMT +1)

Kostas Skenderis: I will provide an introduction to holographic cosmology, outlining how this new framework addresses long standing issues in cosmology, like the origin of the arrow of time and the resolution of the Big Bang singularity. I will then present holographic models describing a non-geometric very early universe and discuss how they their predictions compare with CMB data and discuss possible observational signatures of the resolution of the Big Bang singularity.

Edward Wilson-Ewing: I will give an introduction to loop quantum cosmology, first explaining the main ideas underlying the theory, then briefly describing the quantum theory, and showing how the initial big-bang singularity is replaced by a bounce. I will also discuss cosmological perturbation theory in loop quantum cosmology, and point out some possible observational signatures from loop quantum cosmology for inflation, and for other cosmological scenarios.

by Steve Giddings (UC Santa Barbara) and Hal Haggard (Bard College)

26 May 2022, 18:00 CEST

Steve Giddings: Attempts to reconcile black holes with quantum evolution have produced a crisis, which may yield observational opportunities arising from new quantum gravitational effects. After briefly summarizing this crisis, I’ll overview some leading proposed resolutions. A common theme of many of these is new dynamics on the horizon scale; combining this with our newfound access to both electromagnetic and gravitational wave observations of this scale suggests seeking observational signatures. I’ll particularly outline a scenario with `minimal’ new quantum effects, and its possible observational windows.

Hal Haggard: One of the most striking inroads into observations of quantum phenomena is through statistical predictions, such as Planck's prediction of black-body radiation. I will describe how to use the Bekenstein-Hawking entropy formula and general-relativistic statistical mechanics to determine the probability distribution of random geometries uniformly sampled in phase space. This statistics (in the limit hbar → 0) is relevant to large curvature perturbations, resulting in a population of primordial black holes with zero natal spin. In principle, the identification of such a population at LIGO, Virgo, and future gravitational wave observatories could provide the first observational evidence for the statistical nature of black hole entropy.