Towards understanding fundamental properties of our universe from quantum gravity
Marc Schiffer
Heidelberg University
in collaboration with Gustavo de Brito, Astrid Eichhorn, Alessia Platania
In this poster I discuss different routes towards understanding fundamental properties of our universe from quantum gravity. Specifically, I present first steps towards investigating, whether the interplay of quantum gravity and matter might explain certain aspects of our universe, such as its dimensionality, its matter content or its symmetries at low energies.
Perfect discretization as gateway to partition functions for 4D linearized gravity
Seth K. Asante
Perimeter Institute
in collaboration with Bianca Dittrich
Discretizations are used to regularize and define path integrals for quantization. However, for general relativity, discretizations break diffeomorphism symmetry. Introducing perfect discretizations—a family of discretizations whose members encode the same predictions as in the continuum— can restore the broken symmetries. These perfect discretizations also appear as fixed points of a renormalization flow equations defined via some iterative procedures. Here, we use perfect discretizations to compute the partition function of a 4D linearized gravity with graviton degrees of freedom.
The theory of gravity wasn’t successfully quantised yet. The asymptotic safety approach (and the functional renormalisation group) let us treat gravity non-perturbatively, which cannot be achieved by means of standard quantisation techniques. By means of this calculations one can show that General Relativity and its extensions can be tested with future particle accelerators and future gravitational waves experiments. However, connecting the scattering observables with the the effective average action calculations seems to be troublesome. Application of the Weinberg's conditions of asymptotic safety to amplitudes and not to couplings of the effective action can help to uniquely define the running in the UV and avoid some of the asymptotic safety program problems. The idea is illustrated by the 4-graviton amplitude in string theory and its symmetries.
The formalism presented on this poster allows for the perturbative derivation of the Extended Uncertainty Principle for arbitrary spatial curvature models. The leading curvature induced correction is proportional to the Ricci scalar evaluated at the expectation value of the position operator. By Born reciprocity this method can be equivalently applied in curved momentum space allowing for a general uncertainty principle or curved momentum space quantum mechanics.
Experimental Observation of Acceleration-Induced Thermality
Morgan H. Lynch
Technion
in collaboration with Eliahu Cohen, Yaron Hadad, Ido Kaminer
We examine the radiation emitted by high energy positrons channeled into silicon crystal samples. The positrons are modeled as semiclassical vector currents coupled to an Unruh-DeWitt detector to incorporate any local change in the energy of the positron. In the subsequent accelerated QED analysis, we discover a Larmor formula and power spectrum that are both thermalized by the acceleration. As such, these systems will explicitly exhibit thermalization of the detector energy
gap at the celebrated Fulling-Davies-Unruh (FDU) temperature. Our derived power spectrum, with a nonzero energy gap, is then shown to have an excellent statistical agreement with high energy channeling experiments and also provides a method to directly measure the FDU temperature. We also investigate the Rindler horizon dynamics and confirm the Bekenstein-Hawking area-entropy law is also satisfied in these experiments. As such, we present the evidence for the first observation
of acceleration-induced thermality in a non-analogue system.
On this poster, I discuss how singularities and singularity avoidance in particular can be studied using the low energy effective action for quantum gravity.
Quantum fluctuations pay a crucial role in primordial cosmology, providing a seed for the large-scale structure formation after inflation.
As it has been suggested, the quantum fluctuations are responsible for the quantum scale symmetry, they are also the reason why the intrinsic scales are erased at the fixed points.
However, the flatness of the inflationary potential provided by the scale invariance enables quantum fluctuations to run eternal inflation.
Exact Renormalization Group, Entanglement Entropy, and Black Hole Entropy
João Lucas Miqueleto Reis
Federal University of ABC
in collaboration with André G. S. Landulfo
The study of black hole physics revealed a fundamental connection between thermodynamics, quantum mechanics, and gravity. Today, it is known that black holes are thermodynamical objects with well-defined temperature and entropy. Although black hole radiance gives us the mechanism from which we can associate a well-defined temperature to the black hole, the origin of its entropy remains a mystery. Here we investigate how the quantum fluctuations from the fields that render the black hole its temperature contribute to its entropy. By using the exact renormalization group equation for a self-interacting real scalar field in a spacetime possessing a bifurcate Killing horizon, we find the renormalization group flow of the total gravitational entropy. We show that throughout the flow one can split the quantum field contribution to the entropy into a part coming from the entanglement between field degrees of freedom inside and outside the horizon and a part due to the quantum corrections to the Wald entropy coming from the Noether charge. The renormalized black hole entropy is shown to be constant throughout the flow while the balance between the effective black hole entropy at low energies and the infra-red entanglement entropy changes. A similar conclusion is valid for the Wald entropy part of the total entropy. Additionally, our calculations show that there is no mismatch between the renormalization of the coupling constants coming from the effective action or the gravitational entropy, solving an apparent "puzzle" that appeared to exist for interacting fields.
in collaboration with Sumati Surya and Yasaman K. Yazdi
de Sitter cosmological horizons are known to exhibit thermodynamic properties similar to black hole horizons. We study causal set de Sitter horizons, using Sorkin's spacetime entanglement entropy (SSEE) formula, for a conformally coupled quantum scalar field. We calculate the causal set SSEE for the Rindler-like wedge of a symmetric slab of 4d dS spacetime using the Sorkin-Johnston vacuum state. We find that the SSEE obeys an area law and satisfies complementarity when the spectrum of the Pauli-Jordan operator is appropriately truncated in both the de Sitter slab as well as its restriction to the Rindler-like wedge. Without this truncation, the SSEE satisfies a volume law. This is in agreement with Sorkin and Yazdi's calculations for the causal set SSEE for nested causal diamonds in M2, where they showed that an area law is obtained only after truncating the Pauli-Jordan spectrum.We explore different truncation schemes with the criterion that the SSEE so obtained obeys an area law as well as complementarity.
String corrections from cosmological RG flows and T-duality
Ivano Basile
University of Mons
in collaboration with Alessia Platania
String theory predicts higher-derivative corrections to its low-energy gravitational effective action. While they are difficult to compute in general, within a mini-superspace framework their form is dramatically constrained by T-duality, an intrinsically "stringy" symmetry. Motivated by these constraints, we apply FRG techniques to the corresponding restricted theory space, studying the resulting flow and its fixed points. Furthermore, we determine the complete set of corrections at leading order in the epsilon expansion.
University of Debrecen, Department of Theoretical Physics
in collaboration with Janos Polonyi, Imola Steib
The functional renormalization group method is applied for a scalar theory in Minkowski space-time. It is argued that the appropriate choice of the subtraction point is more important in Minkowski than in Euclidean space-time. The parameters of the cutoff theory, defined by a subtraction point in the quasi-particle domain, are complex due to the mass-shell contributions to the blocking and the renormalization group flow becomes more involved.
Entropy and the Link Action in the Causal Set Path-Sum
Abhishek Mathur
Raman Research Institute
in collaboration with Anup Anand Singh and Sumati Surya
In causal set theory the gravitational path integral is replaced by a path-sum over a sample space $\Omega_n$ of $n$-element causal sets. The contribution from non-manifold-like orders dominates $\Omega_n$ for large $n$ and therefore must be tamed by a suitable action in the low energy limit of the theory. We extend the work of Loomis and Carlip on the contribution of sub-dominant bilayer orders to the causal set path-sum and show that the ``link action" suppresses the dominant Kleitman-Rothschild orders for the same range of parameters.
Flowing in the unimodular theory space: Steps beyond perturbation theory
Gustavo Pazzini de Brito
CP3-Origins
in collaboration with Antônio Duarte Pereira
In this poster, I discuss the renormalization group flow of unimodular quantum gravity. The renormalization group flow is computed by taking into account symmetry-breaking terms induced by the coarse-graining procedure, as well as the graviton and Faddeev-Popov ghosts anomalous dimensions. A discussion on the equivalence of unimodular quantum gravity and a specific gauge choice in the standard diffeomorphism invariant theories is provided.
Functional renormalization group in TGFT - the cyclic-melonic potential approximation
Johannes Thürigen
WWU Münster
in collaboration with Andreas Pithis
In tensorial group field theory, continuous spacetime geometry is expected to emerge via phase transition. To understand the theory’s phase diagram we apply the functional renormalization group method. We derive the full flow equation restricting to a cyclic melonic potential and projecting to a constant field in group space. For a tensor field of rank r on U(1) we explicitly calculate beta functions and find equivalence with those of O(N) models but with an effective dimension flowing from r−1 to zero. Thus, fixed points describing a transition between a broken and unbroken phase do not persist and we find universal symmetry restoration.
Fixed Points of Quantum Gravity and the Dimensionality of the UV Critical Surface
Yannick Kluth
University of Sussex
in collaboration with Daniel Litim
We discuss the effects of Riemann tensor interactions for asymptotic safety using the functional renormalisation group. Results include interacting fixed points and their UV critical surface. Most notably, we discover that quantum-induced shifts of scaling dimensions can lead to a fourth relevant direction. Moreover, eigenvectors are studied by use of a new equal weight condition and we find a consistent picture in accordance with physical expectations.
Aspects of quantum gravity and the standard model from nonnoetherian spacetime
Charlie Beil
University of Graz
Nonnoetherian spacetime is a Lorentzian manifold that is modified by defining the worldline of a fundamental (classical) particle to be a causal curve with no distinct interior points. Such a 1-dimensional event is called a 'pointal curve', and has no tangent space. This new geometry was recently introduced to describe algebraic varieties with nonnoetherian coordinate rings in algebraic geometry. Our primary motivation for incorporating this framework into general relativity is to give a spacetime description of quantum nonlocality, which is based on the identity of indiscernibles applied to time.
FRG analysis of a multi-matrix model for 3d Lorentzian quantum gravity
Andreas G. A. Pithis
University of Heidelberg, SISSA
in collaboration with Astrid Eichhorn, Antonio Duarte Pereira
At criticality, discrete quantum gravity models are expected to give rise to continuum spacetime.
Recent progress has established the functional Renormalization Group method in the context of such models as a practical tool to study their critical properties and to chart their phase diagrams. Here, we apply these techniques to the multi-matrix model with ABAB-interaction potentially relevant for Lorentzian quantum gravity in 3 dimensions. We characterize the fixed-point structure and phase diagram of this model, paving the way for functional RG studies of more general multi-matrix or tensor models encoding causality.
Safety in Darkness: Towards Dark Matter in Asymptotic Safety
Martin Pauly
ITP Heidelberg
in collaboration with Astrid Eichhorn
Asymptotic safety might have significant predictive implications for dark matter models. We illustrate this in a toy model for the visible Higgs-Yukawa sector of the Standard Model, coupled to a dark sector through a portal coupling. This model provides a very first example for a model that i) could feature a fixed point at finite portal coupling, ii) might predict the value of all interactions in the matter sector. The second property might give rise to calculable relations between the masses of the dark particles, their self-interactions, and the portal coupling.
Curvature bound from gravitational catalysis in a thermal background
Abdol Sabor Salek
FSU Jena
in collaboration with Holger Gies
Any suitable candidate for a UV-complete quantum theory of gravity needs to be compatible with the existence of chiral fermions. Studies of quantum fields in curved spacetime have shown that an external, gravitational background field with negative curvature spontaneously breaks chiral symmetry and leads to the existence of massive fermions on the order of the curvature scale - an effect described as gravitational catalysis. Recent studies came to the conclusion that the inclusion of a Renormalization Group scale can restore chiral symmetry if the ratio of the average curvature of local patches of spacetime to the energy scale obeys a certain bound. In this project, we extend these calculations to finite temperature and analyze how thermal fluctuations affect this bound. By applying these results to the Asymptotic Safety scenario of quantum gravity, we obtain an upper bound for the fermionic matter content of the particle physics sector of our universe.
Quantum Error Correction in Loop Quantum Gravity
Deepak Vaid
National Institute of Technology Karnataka
Research in AdS/CFT has shown that quantum error correction must play an important role in the emergence of macroscopic spacetime from entanglement between some, as yet undetermined, fundamental, "pre-geometric" degrees of freedom. One approach to quantum gravity is loop quantum gravity (LQG) in which these pre-geometric degrees of freedom take the form of spin network states. I show that by considering the action of diffeomorphisms on spin-networks one arrives at the Yang-Baxter equation. Solutions of this equation correspond to quantum circuits which can generate qutrit states (of the form $ |000\rangle + |111\rangle $) which are the fundamental ingredient of certain well known quantum error correction schemes. This provides support for spin networks as a physically plausible candidate for the "atoms" of spacetime and also suggests avenues for connecting LQG with the AdS/CFT conjecture.
Everpresent Lambda. III. In the Closed Universe
Muhammad Bilal Azam
Lahore University of Management Sciences (LUMS), Pakistan
in collaboration with Dr. Maqbool Ahmed
A variety of observations indicate that the universe is accelerating, and dark energy is thought to be the simplest candidate of cosmological constant $(\Lambda)$, responsible for the positive acceleration, but the problems of fine-tuning and coincidence are associated with it. To solve these problems, a fluctuating and time-dependent cosmological constant of the right order of magnitude was predicted by R. D. Sorkin $(1991)$ using the ideas from causal set theory and a more detailed phenomenological model was simulated numerically by M. Ahmed $(2004)$ for the spatially flat universe. This study is the continuation of the model for a closed universe, and its sensitivity is analyzed under different orders of the radii of curvature.