Puzzles in the Quantum Gravity Landscape: viewpoints from different approaches

Oct 23, 2023, 8:30 a.m. → Oct 27, 2023, 5:00 p.m. America/Toronto

PI/1-100 - Theatre (Perimeter Institute for Theoretical Physics)

Alessia Platania, Marc Schiffer

Unraveling the quantum nature of gravity is one of the most pressing problems of theoretical physics. Several ideas have been put forward and resulted in a number of theories of quantum gravity. While these theories have explored different facets of the “quantum gravity landscape”, all viable approaches should ultimately make contact with observations, and answer exciting questions in cosmology and black-hole physics.

Sharing knowledge, exchanging ideas, and building a dictionary between different theories are crucial steps toward answering these questions, efficiently contrasting different theories, and ultimately reaching a deeper understanding of our Universe.

This conference will contribute to these goals by bringing together leading experts in different approaches to quantum gravity, gravitational effective field theory, black-hole physics, and cosmology. We will focus on specific puzzles in quantum gravity and their resolutions within different approaches. The conference will be highly interactive, with plenty of time to discuss common problems, understand the big picture, and develop novel connections between fields.

---

Registration: Registration is now open, and both in-person and virtual participation is welcome. Online participants will be able to interact on an equal footing in question sessions and discussions. In-person attendance is limited and will be approved on a first-come, first-served basis. Talks are by invitation only, but in-person participants are encouraged to apply to present a poster.

Spam warning: There is an increasing number of scam agencies reaching out to conference speakers and attendees. Perimeter Institute does not use third-party agencies. We advise speakers and attendees to ignore emails and not to provide any details to anyone who is not from Perimeter Institute.  

Confirmed Speakers and Panelists:

Abhay Ashtekar (Penn State University) Robert Brandenberger (McGill University) Luca Buoninfante (Nordita) Xavier Calmet (University of Sussex) Francesco di Filippo (Kyoto University) Bianca Dittrich (Perimeter Institute) John Donoghue (University of Massachusetts) Astrid Eichhorn (CP3-origins) Johanna Erdmenger (Würzburg University) Ghazal Geshnizjani (Perimeter Institute) Ruth Gregory (King's College) Lavinia Heisenberg (Heidelberg University) Bob Holdom (University of Toronto) Benjamin Knorr (Nordita) Renate Loll (Radboud University Nijmegen) Miguel Montero (IFT Madrid) Rob Myers (Perimeter Institute) Sabrina Pasterski (Perimeter Institute) Fernando Quevedo (Cambridge University) Lisa Randall (Harvard University) Kasia Rejzner (York University) Mairi Sakellariadou (King's College) Lee Smolin (Perimeter Institute) Kellogg Stelle (Imperial College) Sumati Surya (Raman Research Institute) Andrew Tolley (Imperial College) Neil Turok (University of Edinburgh) Pedro Vieira (Perimeter Institute) Yasaman Yazdi (Imperial College)

Territorial Land Acknowledgement

Perimeter Institute acknowledges that it is situated on the traditional territory of the Anishinaabe, Haudenosaunee, and Neutral peoples.

Perimeter Institute is located on the Haldimand Tract. After the American Revolution, the tract was granted by the British to the Six Nations of the Grand River and the Mississaugas of the Credit First Nation as compensation for their role in the war and for the loss of their traditional lands in upstate New York. Of the 950,000 acres granted to the Haudenosaunee, less than 5 percent remains Six Nations land. Only 6,100 acres remain Mississaugas of the Credit land. 

We thank the Anishinaabe, Haudenosaunee, and Neutral peoples for hosting us on their land.

poster.pdf

Monday, October 23

1 Opening Remarks

2 Lessons of the Effective Field Theory Treatment of General Relativity

I will briefly review the key concepts underlying the Effective Field Theory of General Relativity, and give a couple of examples of how it works. Then I will describe seven lessons which can be extracted from the theory. Finally I discuss some of the limitations of the EFT framework.

Speaker: John Donoghue

Donoghue - PI Puzzles.pdf

3 Positivity Bounds and Effective Fields Theories (A Review)

I will briefly review recent progress on how causality/analyticity and unitarity can put powerful constraints on both gravitational and non-gravitational EFTs that admit consistent UV completions.

Speaker: Andrew Tolley

Tolley - Perimeter 2023.pdf

4 Holography and its implications for quantum gravity - VIRTUAL

I will review implications of the AdS/CFT correspondence for quantum gravity, paying particular attention to holographic RG flows and the processing of information. Deformed hyperbolic geometries correspond to boundary field theory RG flows. Conversely, as I will exemplify using the recent approach of discrete holography, boundary RG flows may be used to reconstruct the bulk spacetime, for instance via tensor networks. I will discuss recent approaches towards including gravity dynamics into this reconstruction process.

The presenter will be joining via Zoom for this talk.

Speaker: Johanna Erdmenger

11:00 a.m. Coffee Break

5 A hike through the Swamp

At first sight, it seems that almost any QFT, seen as a low-energy effective field theory (EFT), can be coupled to gravity -- simply add an Einstein-Hilbert term, change all derivatives by covariant ones, and you're done. However, a growing body of evidence coming from String Theory compactifications, general properties of black hole evaporation, and rigorous results in holography, suggests that the opposite is true. We can work out the hidden constraints that quantum gravity consistency imposes in the matter sector systematically, an approach that receives the colourful name of "The Swampland Program". I will briefly review the program, its motivation, current results, and long-term goals.

Speaker: Miguel Montero

Montero - Puzzles-talk-Montero.pdf

6 Piecing Together a Flat Hologram

The Celestial Holography program encompasses recent efforts to understand the flat space hologram in terms of a CFT living on the celestial sphere. A key development instigating these efforts came from understanding how soft limits of scattering encode infinite dimensional symmetry enhancements corresponding to the asymptotic symmetry group of the bulk spacetime. Historically, the construction of the bulk-boundary dual pair has followed bottom up approach matching symmetries on both sides. Recently, however, there has been exciting progress in formulating top down descriptions using insights from twisted holography. We review salient aspects of the celestial construction, the status of the dictionary, and active research directions.

Speaker: Sabrina Pasterski

Pasterski - PI_Puzzles.pdf

12:45 p.m. Lunch

7 Open Discussion with today's speakers (Donoghue, Erdmenger, Montero, Pasterski, Tolley)

Speakers: John Donoghue, Johanna Erdmenger, Miguel Montero, Sabrina Pasterski, Andrew Tolley

3:15 p.m. Coffee Break

8 Panel Discussion - Strengths and limitations of EFT (Donoghue, Knorr, Montero, Quevedo, Tolley)

Speakers: John Donoghue, Miguel Montero, Fernando Quevedo, Carlo Rovelli, Andrew Tolley

5:15 p.m. Poster Session and Welcome Reception

Tuesday, October 24

9 Status, perspective and three challenges in the asymptotic-safety paradigm for quantum gravity - VIRTUAL

In my talk, I'll review the key idea underlying asymptotically safe quantum gravity, before giving an overview of the current status and open questions. I'll then spotlight three challenges. The first challenge is how to test quantum gravity and I'll advocate that by linking quantum gravity to particle physics and cosmology, we can probe quantum gravity at all scales. The second challenge is how to connect to other ideas about quantum spacetime and I'll discuss effective asymptotic safety as a way to link to, e.g., string theory. The third challenge is the status of global symmetries and I'll discuss what we know about the preservation of global symmetries in asymptotic safety and what this implies for a broader perspective on the question.

The presenter will be joining via Zoom for this talk.

Speaker: Astrid Eichhorn

10 CDT as computational gateway to nonperturbative quantum gravity

Based on quantitative results on quantum observables in a Planckian regime, I will argue that using dynamical lattices `a la CDT to construct and explore the nonperturbative path integral over Lorentzian geometries is a bona-fide quantum field-theoretic "method" rather than an "approach" to quantum gravity. Its most important features are universality, unitarity and the presence of powerful numerical tools to directly simulate and measure a chunk of quantum spacetime of about 20 Planck lengths across. This small observational window is a preferred place to look for clues to what quantum gravity is all about. Despite the diffeomorphism-invariant and nonlocal character of the observables, qualitatively new and surprising UV properties have already been discovered, and compatibility checks with semiclassical expectations for several cosmological observables ("classical limit") have been performed successfully. The underlying new mathematics is that of random geometry and beyond-Riemannian geometry.

Speaker: Renate Loll

Loll - PI_QGconf2023 - For website.pdf

11 UV complete 4-derivative scalar field theory

A scalar field theory with 4-derivative kinetic terms and 4-derivative cubic and quartic couplings is presented as a proxy for quadratic gravity. Unitarity and positivity appear as the key issues in the scalar field theory, just as they do in quadratic gravity. We have extended some calculations to show how these issues are resolved in the high energy limit of the theory. The results also show how it is that differential cross sections can have good high energy behaviour.

Speaker: Bob Holdom

03_Holdom.pdf

10:45 a.m. Coffee Break

12 The Case for Renormalizable Quantum Gravity: from local to nonlocal approaches (and back!)

In the context of perturbative quantum field theory (QFT), the addition of quadratic-curvature invariants to the Einstein-Hilbert action makes it possible to achieve strict renormalizability in four dimensions. This theory exhibits unusual features due to an additional massive spin-2 ghost which, in general, may cause instabilities. In the first part of this talk, we focus on the possibility of giving up locality as a way to avoid ghost-like degrees of freedom and provide a critical assessment on open questions in nonlocal theories of gravity, such as the uniqueness problem. In the second part of the talk, we take a step back and argue that, despite the presence of the ghost and actually thanks to it, Quadratic Gravity can still provide a consistent local perturbative QFT description of the gravitational interaction and explain new physics beyond Einstein's general relativity, e.g., it offers a natural explanation for the inflationary phase. Finally, we argue that a type of nonlocality in gravity can still occur non-perturbatively and show that a new lower bound on scattering amplitudes indicates that the gravitational interaction is intrinsically nonlocal if black holes form.

Speaker: Luca Buoninfante

Buoninfante - PI-puzzlesQG_Buoninfante - For website.pdf

13 Form Factors - a unifying language for Quantum Gravity

QFT's remarkable success lies in its ability to yield precise predictions through e.g. scattering amplitudes, capturing the essence of running couplings in a gauge-invariant manner. However, the introduction of gravity disrupts this straightforward notion of momentum dependence, posing a significant challenge to our theoretical framework.
Enter form factors—an ingenious concept that extends the idea of momentum dependence to the curved spacetime of quantum gravity. They form the central ingredient when discussing quantum gravity in a QFT language. Furthermore, within an effective action, form factors offer a unifying language, allowing us to compare and contrast different quantum gravity approaches apples-to-apples.
I will give an overview of the ideas underlying form factors, their application to scattering problems, and their potential to put different approaches on the same footing.

Speaker: Benjamin Knorr

05_Knorr.pdf

12:30 p.m. Lunch

14 Open Discussion with today's speakers (Buoninfante, Eichhorn, Holdom, Knorr, Loll)

Speakers: Luca Buoninfante, Astrid Eichhorn, Bob Holdom, Benjamin Knorr, Renate Loll

3:15 p.m. Coffee Break

15 Panel Discussion - QFT vs beyond-QFT approaches (Buoninfante, Rejzner, Smolin, Vieira)

Speakers: Luca Buoninfante, Benjamin Knorr, Kasia Rejzner, Lee Smolin, Pedro Vieira

5:15 p.m. Poster Session

6:30 p.m. Pizza & Career: Panel for young researchers (Francesco di Filippo, Yasaman Yazdi, Miguel Montero, Kasia Rejzner)

Wednesday, October 25

16 Glimpses into Loop Quantum Gravity

The organizers requested an overview of Loop Quantum Gravity (LQG). However, this is an impossible task, especially for a 30 minute talk, given that the subject is over three decades old with tens of thousands of papers. Instead, I will outline some of the key distinguishing features of LQG and discuss an illustrative application. Hopefully these topics will complement those that will be covered by Bianca Dittrich and Lee Smolin, without too much of an overlap.

Speaker: Abhay Ashtekar

Ashtekar - 23PI.pdf

17 Puzzles, Resolutions and Open Questions in Causal Set Theory - VIRTUAL

I will begin with a broad overview of the main ingredients of causal set theory, and then discuss the partial resolution of a long-standing "puzzle" in the causal set path sum. I will then discuss other causal set curiousities, ending with a brief overview of the challenges in constructing a realistic fully non-perturbative quantum dynamics.

The presenter will be joining via Zoom for this talk.

Speaker: Sumati Surya

18 Where is String Theory

Speaker: Pedro Vieira

10:45 a.m. Coffee Break

19 Panel Discussion - Cosmological Puzzles (Ashtekar, Brandenberger, Geshnizjani, Sakellariadou, Yazdi)

Speakers: Abhay Ashtekar, Robert Brandenberger, Ghazal Geshnizjani, Mairi Sakellariadou, Yasaman Yazdi

12:45 p.m. Lunch

20 Open Discussion with today's speakers (Ashtekar, Surya, Vieira)

Speakers: Abhay Ashtekar, Sumati Surya, Pedro Vieira

2:30 p.m. Group Photo - meet in the Atrium

2:35 p.m. Coffee Break

7:00 p.m. Livestream: Secrets of the Universe – Hiding in Plain Sight? Neil Turok Public Lecture

PI is pleased to be hosting a Public Lecture in collaboration with the Puzzles in the Quantum Gravity Landscape conference.

Advance tickets are required to attend the live Public Lecture in the Theatre and this event is sold out.

If you would like to watch the live webcast, please join the waiting line in the Atrium no later than 6:45 pm and you will be taken to the Time Room prior to the start to the talk.

Alternatively, the talk will be livestreamed via YouTube (https://www.youtube.com/@PIOutreach/featured) for your convenience.

Thursday, October 26

21 Quantum hair and information loss paradox - VIRTUAL

In this talk I present our solution to the information paradox published in Phys.Rev.Lett. 128 (2022) 11, 111301 and Phys.Lett.B 827 (2022) 136995 (see EPL 139 (2022) 4, 49001 for a review). Long wavelength quantum gravitational effects allow the interior state of the black hole to influence Hawking radiation, leading to unitary evaporation. I explain why the Mathur theorem is evaded due to the complex nature of the Hawking radiation superposition state.

The presenter will be joining via Zoom for this talk.

Speaker: Xavier Calmet

Calmet - information paradox 2023 PI.pdf

22 Hearts of Darkness: Nonsingular Black Holes Beyond General Relativity

Black holes contain, deep in their interior, theoretical evidence of the failure of general relativity. A series of fundamental results, starting from the 1965 Penrose singularity theorem, proved that physically realistic initial conditions will unavoidably produce a singular black hole spacetime. It is generally expected that a full theory of quantum gravity should remove the singularities that appear in general relativity. However, the lack of proper understanding of the dynamical laws dictating the evolution of spacetime and matter in these extreme situations hinders the extraction of predictions in specific models. I will discuss, in a model-independent manner, the different possibilities that singularity regularization may open. I will then focus on fundamental open issues that need to be addressed to obtain viable nonsingular black hole candidates.

Speaker: Francesco di Filippo

Di Filippo - Slides Di Filippo Website.pdf

23 Holographic Thermodynamics: C-ing a Black Hole

One of the fascinating things about Black Hole Thermodynamics is how we often approach it in a very classical fashion, yet - if real - it is an intrinsically quantum phenomenon. I'll use the illustration of thermodynamics of an accelerating black hole to highlight some of these issues, and I'd like to raise questions on how we derive the First Law, and how unique this derivation is.

Speaker: Ruth Gregory

Gregory.pdf

10:45 a.m. Coffee Break

24 Open Discussion with today's speakers (Calmet, di Filippo, Gregory)

Speakers: Xavier Calmet, Francesco di Filippo, Ruth Gregory

12:00 p.m. Lunch

25 Poster Prize Talk

26 Panel Discussion - Black hole puzzles (Di Filippo, Gregory, Holdom, Myers, Stelle)

Speakers: Francesco di Filippo, Ruth Gregory, Bob Holdom, Robert Myers, Kelly Stelle

3:00 p.m. Coffee Break

27 Colloquium - Insights from Warped Extra Dimensions - VIRTUAL

Warped extra dimensions were originally introduced as a way of addressing the hierarchy problem. But insights from these scenarios have extended to black hole physics, cosmology, and even mechanisms for addressing issues in purely four dimensions. We investigate this scenario and some compelling lessons.

The presenter will be joining via Zoom for this talk.

Speaker: Lisa Randall

7:00 p.m. Fireside Chat & Career Q&A (Mairi Sakellariadou and Renate Loll)

Friday, October 27

28 Balance Laws as Test of Gravity - VIRTUAL

I will discuss how one can use balance laws in full non-linear general relativity in order to test waveform models (arXiv:2309.12505)

The presenter will be joining via Zoom for this talk.

Speaker: Lavinia Heisenberg

Dittrich.pdf

29 The simplicial Lorentzian path integral and spin foams

I will discuss two versions of the simplicial Lorentizian path integral, namely the (Lorentzian) quantum Regge and the spin foam version. I will do so in the simple context of de Sitter cosmology. This simple example will reveal the important role of light cone irregular configurations in the simplicial path integral — I will show that these can either lead to an exponentially enhanced or an exponentially suppressed amplitude.
I will then highlight an important difference between the spin foams and quantum Regge path integral, which affects the probability for the creation of the (de Sitter) universe.

Speaker: Bianca Dittrich

30 String Theory and the Cosmological Constant

An overview is given on moduli stabilization in string theory with emphasis in obtaining de Sitter space solutions. Recent work on vacuum transitions will be emphasised.

Speaker: Fernando Quevedo

Quevedo.pdf

10:45 a.m. Coffee Break

31 A minimal SM/LCDM cosmology

Recent observations point to a surprisingly economical description of the universe on both very small and very large scales. Stimulated by these findings, Boyle and I have proposed a new, potentially more complete theoretical framework than currently popular paradigms. Our search has so far led to 1) the simplest-yet explanation for the cosmic dark matter, soon to be tested by galaxy surveys, 2) a thermodynamic explanation for the large scale geometry of the cosmos, based on the concept of gravitational entropy à la Hawking, 3) a new account of the big bang singularity as a “mirror” enforcing CPT-symmetric boundary conditions, realising Penrose's "Weyl curvature hypothesis" and 4) a new mechanism for cancelling the divergent vacuum energy and the trace anomalies in the Standard Model (SM). The new mechanism successfully predicts the primordial density perturbations in terms of the SM’s gauge couplings. It also explains why there are 3 generations of elementary particles, each including a RH neutrino, one of which is stable and comprises the dark matter. I’ll outline the challenges the new picture faces and the opportunities it presents, ranging from solving the gauge hierarchy problem to an improved description of quantum gravity along with prospective observational tests.

Speaker: Neil Turok

32 Open discussion with today's speakers (Dittrich, Heisenberg, Quevedo, Turok)

Speakers: Bianca Dittrich, Lavinia Heisenberg, Fernando Quevedo, Neil Turok

12:50 p.m. Lunch

33 Panel Discussion - Future Directions in QG (Dittrich, Gregory, Loll, Sakellariadou, Surya)

Speakers: Bianca Dittrich, Ruth Gregory, Renate Loll, Mairi Sakellariadou, Sumati Surya

3:00 p.m. Break

34 Branes & Strings, Freedom & Safety and a bit of Cosmology -- Some current problems in Quantum Gravity

What relations are there between the various ways of wrangling with quantum gravity? String theory is now much more than just a theory of strings -- branes and braneworlds abound. Some kind of effective theory for the familiar world needs to emerge. Are there ways that one could glimpse underlying structure from aspects of an effective theory? That happens for pions -- is there anything like that for gravity? Effective theories also involve higher derivatives, and those can summon up spirits (i.e. ghosts) from the vasty deep. Do asymptotic freedom or asymptotic safety give ways to exorcise them? And what might the effective theory tell us about the earliest times?

Speaker: Kelly Stelle

Stelle_website.pdf

35 Closing Remarks