Events

# Program

## Program

### Program of the Workshop

All talks will be held in lecture hall F in the Physics Department.

Time Monday Tuesday Wednesday Thursday Friday
09:00-10:15 Ghazal Geshnizjani Kristina Giesel Tehseen Rug Lucas Lombriser
10:15-10:45 Coffee break Coffee break Coffee break Coffee break
10:45-12:00 Ivan Agullo Angnis Schmidt-May Niayesh Afshordi Steffen Gielen
12:00-13:30 Lunch break Lunch break Lunch break Discussion
until 13:00
13:30-14:45 Oriol Pujolas Camille Bonvin Tessa Baker
14:45-16:00 Edmund Copeland 15:00
until 16:15
Raphael Flauger Jean-Luc Lehners Edward Wilson-Ewing
16:00-16:30 Coffee break
16:15-16:45
Coffee break Coffee break Coffee break
16:30-17:45 Latham Boyle
16:45-18:00
Discussion Discussion Discussion
17:45-19:00 Abhay Ashtekar
18:00-19:15
19:15 Snacks & beer Foyer conference venue
20:00 Conference Dinner
Zen Erlangen

### Titles and Abstracts

#### Sepaker: Edmund Copeland

Title: Inflation in light of Planck
Abstract: I will discuss the Inflationary Universe scenario and the way recent Planck measurements have impacted on favoured models of inflation. I will then go on to briefly discuss the place of inflation in a number of approaches to Quantum Gravity, pointing out successes and problems as I see them. Hopefully this will lead to the start of a discussion in this interesting workshop concerning the cosmology associated with Quantum Gravity.

#### Speaker: Latham Boyle

Title: Rethinking Connes’ Approach to the Standard Model of Particle Physics via Non-Commutative Geometry
Abstract: Connes’ notion of non-commutative geometry (NCG) generalizes Riemannian geometry and yields a striking reinterpretation of the standard model of particle physics, coupled to Einstein gravity. I will start with a gentle introduction to his approach and the physical reasons to be interested in it. I then explain our recent reformulation, which has two key mathematical advantages: (i) it unifies many of the traditional NCG axioms into a single one; and (ii) it immediately generalizes from non-commutative to non-associative geometry. Strikingly, it also resolves a long-standing problem plaguing the NCG construction of the standard model, by precisely eliminating from the Lagrangian the collection of 7 problematic terms that previously had to be removed by an ad hoc assumption. Applying this new formulation to the NCG traditionally used to describe the standard model, we find that, instead, it actually describes a certain minimal extension of the standard model by an extra U(1)_{B-L} gauge symmetry, and a single extra complex scalar field sigma, which is a singlet under SU(3)_{C} x SU(2)_{L} x U(1)_{Y} but carries baryon-minus-lepton charge B-L=2. This extension has phenomenological and cosmological implications, and offers a new solution to the discrepancy between the observed Higgs mass of 125 GeV and the traditional NCG prediction of 170 GeV.
Based on: 1) Latham Boyle and Shane Farnsworth, “Non-commutative geometry, non-associative geometry and the standard model of particle physics,” New J. Phys. 16, 123027 (2014) [arXiv:1401.5083]. 2) Shane Farnsworth and Latham Boyle, “Rethinking Connes’ approach to the standard model of particle physics via non-commutative geometry,” New J. Phys. 17, 023021 (2015) [arXiv:1408.5367].

#### Speaker: Abhay Ashtekar

Title: Pre-inflationary dynamics in LQC: Interplay between theory and Observations
Abstract: In Loop Quantum Cosmology (LQC) the big-bang singularity of the Friedman-Lemaitre models is naturally resolved because of quantum geometry effects. Furthermore, one can systematically extend the cosmological perturbation theory from classical to quantum background geometries. This enables one to analyze the pre-inflationary dynamics in detail by facing the Planck regime squarely. Contrary to a wide-spread belief, this phase of dynamics can have observational consequences. In particular, it can lead to quantum states that differ from the Bunch Davies vacuum at the onset of inflation, with interesting consequences for the observable properties of the longest wave length (or low $\ell$) modes. I will discuss these and the resulting interplay between observations and the question of initial conditions in quantum gravity. This talk will also provide the background material for many more the phenomenological consequences of LQC that Ivan Agullo will discuss.
The results I will present were obtained jointly with Ivan Agullo, Alex Corichi, Brajesh Gupt, William Nelson, Tomasz Pawlowski and Param Singh. This work was supported in part by the NSF grant PHY-1205388 and the Eberly research funds of Penn State.

#### Speaker: Ghazal Geshnizjani

Title: Theoretical implications of detecting gravitational waves
Abstract: I will discuss in what sense detecting primordial gravitational waves is a smoking gun for inflation. I argue that a detectable tensor signal on large scales and sourced by vacuum fluctuations in a non-accelerating, sub-Planckian universe using cosmological perturbation theory leads to contradictory limits on cosmological dynamics. The contradiction implies that one or more of our axioms for early universe must have been broken. The bound from tensor perturbations will not only independent of, but also stronger than the one obtained from scalar power spectrum.

#### Speaker: Ivan Agullo

Title: Phenomenological consequences of LQC
Abstract: Loop quantum cosmology has become a robust framework to describe the highest curvature regime of the early universe. This talk will describe explore the phenomenology of this framework. We will discuss the parameter space of the theory, both for the background space-time geometry and for metric perturbations. We will then explore the corrections to the inflationary predictions for the primordial spectrum of cosmological perturbations that the pre-inflationary, quantum gravity phase of the universe introduces. Non-Gaussianity and its relation to large scale anomalies in the CMB will be discussed.

#### Speaker: Oriol Pujolas

Title: Status of Non-Relativistic Quantum Gravity
Abstract: I will give my personal view on the present status of Horava’s proposal for Quantum Gravity. The low-energy limit of the theory is rather well behaved but it requires new ingredients in order to overcome the most serious low-energy issue — the Lorentz fine tuning problem or why the matter sector is Lorentz invariant to such a great precision. I will present the recent progress made in this direction. I will also review other aspects, including Black Hole physics and cosmology.

#### Speaker: Raphael Flauger

Title: Planck, BICEP, and the Early Universe”
Abstract: The cosmic microwave background contains a wealth of information about cosmology as well as high energy physics. It tells us about the composition and geometry of the universe, the properties of neutrinos, dark matter, and even about the conditions in our universe long before the cosmic microwave background was emitted. After a brief review of what we may hope to learn from studies of the cosmic microwave background about the early universe, I will review measurements of the angular power spectrum of temperature perturbations from the first 15.5 months of Planck data by the Planck collaboration and by Renee Hlozek, David Spergel and myself. I will then discuss the implications for the early universe of the recently released Planck full mission data as well as the joint analysis between BICEP/KeckArray and Planck.

#### Speaker: Kristina Giesel

Title: Loop Quantum Gravity as a Motivation for Manifestly Gauge Invariant Perturbation Theory
Abstract: The underlying theoretical framework of loop quantum cosmology is loop quantum gravity, which will be briefly introduced in this talk with a particular focus on the assumptions and choices one makes when passing from general relativity to its corresponding quantum theory. It will be discussed how a reduced phase space approach to loop quantum gravity can be used to formulate a manifestly gauge invariant version of general relativistic perturbation theory. The latter allows to disentangle the construction of gauge invariant quantities (observables) from the perturbation theory. The application of this approach to cosmological perturbation theory is analysed and compared to the results one obtains in standard cosmological perturbation theory.

#### Speaker: Angnis Schmidt-May

Title: Consistent Spin-2 Field Theories
Abstract: Cosmological and astrophysical observations suggest the presence of dark energy and dark matter, indicating that known physics may not suffice to completely describe our universe. There are many different possibilities to go beyond the standard models of particle physics and gravity in order to remedy deficiencies of the currently known descriptions. In this talk, I will introduce models that involve massless and massive spin-2 fields instead of only a massless graviton. The only theories of this type which avoid an unacceptable ghost instability are ghost-free nonlinear massive gravity and its generalization to bimetric theory. Ghost-free bimetric theory possesses self-accelerating cosmological solutions and can reproduce the expansion history of the universe. Around such solutions, the linear perturbation theory can also be in accordance with observational data. Its interesting mathematical structure and its viable cosmology make bimetric theory a promising alternative to general relativity.

#### Speaker: Camille Bonvin

Title: Relativistic effects in large-scale structure
Abstract: A key challenge in cosmology is to understand the current accelerated expansion of the universe. The distribution of galaxies, as well as their observed shape and size provide a powerful way to probe dark energy and modified gravity models. In order to exploit these observables properly it is necessary to understand what we are really measuring when we look at the distribution, sizes and shapes of galaxies. Since our universe is not completely homogeneous and isotropic, we only see a distorted picture of our sky. In this talk, I will discuss the various relativistic effects that distort our observations. I will show that even though these effects complicate the interpretation of galaxy surveys, they are very useful since they contain information on the dynamic of the universe and can therefore be regarded as a new opportunity for future surveys.

#### Speaker: Jean-Luc Lehners

Title: Inflationary and ekpyrotic universes in the no-boundary wavefunction
Abstract: I will review the no-boundary proposal, paying particular emphasis on the conditions for obtaining a classical spacetime in the no-boundary wavefunction. This analysis singles out inflationary and ekpyrotic cosmologies, and so I will discuss the implications for both types of cosmologies. The talk will also include a discussion of the current observational status of these two theories, and of open issues that remain to be understood.

#### Speaker: Tehseen Rug

Title: Quantum Bound States – From QCD to Gravity
Abstract: After reviewing some basics of gravity and QFT in curved space-time, I will explain how to approach gravity from a field theorists perspective. I will try to motivate that it is natural to consider large gravitating sources, such as planets or black holes, as bound states of microscopic degrees of freedom such as gravitons. After discussing some qualitative, kinematic aspects of this picture and its implications, I will introduce a theoretical framework that allows one to deal with these gravitational bound states. This formalism is very close to that used in other field theories such as QCD. Using this framework, I will present results for observables such as number density and energy density inside gravitational bound states in pure gravity. I will explain how these observables can be extracted by an outside observer doing scattering experiments. I will conclude with a summary and an outlook.

#### Speaker: Niayesh Afshordi

Title: The Cosmological Non-Constant Problem: the case for TeV scale quantum gravity
Abstract: We study the influence of the fluctuations of a Lorentz invariant and conserved vacuum on cosmological metric perturbations, and show that they generically blow up in the IR. We compute this effect using, i) a toy model of Poisson sprinkling in the phase space, ii) Kallen-Lehmann spectral representation of stress correlators, and iii) holographic bound on entanglement entropy, all leading to an IR cut-off that scales as the fifth power of the UV scale (in Planck units). The leading effect comes from anisotropic vacuum stresses; consistency with cosmological observations limits the highest UV scale of the quantum field theory to be < 20 TeV- 2 PeV, which is also independently motivated by the Higgs hierarchy problem, and one may interpret as the scale of quantum gravity. Our findings can be viewed as an extension of the cosmological constant (CC) problem which demonstrates the non-trivial UV-IR coupling and (yet another) failure of effective field theory in gravity. However, it is more severe than the old CC problem, as vacuum fluctuations cannot be tuned to cancel, due to the positivity of the covariance of the stress tensor.

#### Speaker: Tessa Baker

Title: Testing Gravity with Cosmology
Abstract: I’ll begin by giving an overview of the space of gravity theories currently under consideration by the cosmology community, constructed as candidate explanations for the accelerating expansion rate of the universe. I’ll talk briefly about sensible strategies for testing these theories. I’ll then move to the main focus of this talk, which is the construction of a unified parameter space for tests of General Relativity. This seeks to draw together, in an intuitive way, constraints from laboratory, astrophysical and cosmological systems. I’ll point out some interesting observations that emerge from the construction of this parameter space, including an untested regime of gravitational fields which deserves further investigation.

#### Speaker: Edward Wilson-Ewing

Title: A ΛCDM Bounce Scenario
Abstract: We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. Assuming that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, we calculate the spectrum of scalar and tensor perturbations after the bounce, assuming initial quantum vacuum fluctuations. We find that the modes that exit the (sound) Hubble radius during matter-domination when the effective equation of state is slightly negative due to the cosmological constant will be nearly scale-invariant with a slight red tilt, in agreement with observations. The tensor perturbations are also nearly scale-invariant, and the predicted tensor-to-scalar ratio is small.

#### Speaker: Lucas Lombriser

Title: Observational signatures of screening mechanisms in scalar-tensor gravity
Abstract: Scalar fields can serve as alternative to a cosmological constant driving cosmic acceleration, plausibly representing the effective low-energy limit of a more fundamental theory of gravity. In case of nonminimal coupling, the new field modifies the gravitational interactions. These modifications must, however, be suppressed in the Solar System, where gravity is well tested. Hence, viable scalar-tensor theories employ screening mechanisms to satisfy the local constraints, but they leave observable signatures on cosmological and galactic scales. I will discuss some of these effects and constraints that can be inferred from them. Finally, I will present a new, linear shielding mechanism, where gravitational modifications may be limited to scales near the cosmological horizon.

#### Speaker: Steffen Gielen

Title: A homogeneous universe as a quantum gravity condensate
Abstract: Quantum gravity provides the opportunity to answer questions of modern cosmology while avoiding some of the theoretical issues of inflation. The challenge then is to describe a cosmologically relevant, i.e. time-dependent and nearly (but not quite) homogeneous, universe within a given approach to quantum gravity, in order to extract cosmological predictions. I will outline a new approach addressing this challenge in the group field theory (GFT) approach to quantum gravity, closely related to loop quantum gravity (LQG). In our approach, a homogeneous universe is described, in a first approximation, as a gas’ of weakly interacting discrete building blocks (atoms’) of space. The gas is characterised by an order parameter describing the collective state of many coherent degrees of freedom, and mathematically similar to a Bose-Einstein condensate in condensed matter physics. Continuum spacetime, together with a spatially homogeneous metric, arises as an approximate hydrodynamic description of a particular phase (given by this condensate’) of a fundamentally discrete quantum gravity theory. I will present the analogy between these GFT condensates’ and real condensates in some detail. The analogue in GFT of the Gross-Pitaevskii equation governing the dynamics of a real condensate is interpreted as an effective quantum cosmology model from which possible phenomenology can be extracted. I will then outline several further possibilities for the phenomenology of GFT condensates, including the description of inhomogeneities, a possible transition between geometric’ and non-geometric’ phases in the very early universe, and potential unexpected consequences for issues such as the cosmological constant problem.