Cosmological Correlators

7 Sept 2020, 10:00 → 9 Sept 2020, 21:15 Europe/Zurich

CERN

This small, interactive, hands-on virtual workshop gathers a group of
international experts on theoretical aspects of primordial cosmological
correlators. The goal is to critically review recent progress attained in
computing correlators in (quasi) de Sitter spacetime both from the bulk and
from the boundary perspective. The workshop will take place over 3 days.
The format will prioritize discussions, interactions and will aim at
generating new concrete ideas and collaborations. There will be no more
than two or three talks per day followed by organized as well as
spontaneous discussion groups.

Slack workplace with discussion channels

YouTube channel with recorded talks

Zoom link to all live talks and discussions

Monday, 7 September

1 Introductory Remarks

Speakers: Dr Enrico Pajer (University of Cambridge), Guilherme Pimentel

Recording

2 Live Talk 1: Building a Boostless Bootstrap [Enrico Pajer]

Abstract: Cosmological surveys are believed to measure the future value of quantum correlators in a theory of gravity around quasi de Sitter spacetime. The goal of this talk is to develop a cosmological bootstrap, namely understand how general physical principles such as unitarity, locality and symmetries are encoded in these observables without a reference to the unobservable time evolution that generates them. 
I'll show that in single-clock inflation, the only correlators of curvature perturbations that are fully de Sitter isometric are those of the free theory. This provides a strong motivation to develop a "Boostless" Bootstrap that does not assume invariance under de Sitter boosts (generally broken during inflation). I'll show how all tree-level bispectra for a massless scalar and graviton follow from a set of simple boundary Bootstrap Rules. 
To move beyond the bispectrum, I'll derive a Cosmological Optical Theorem (see also Goodhew's talk), which expresses the consequences of unitary time evolution for the spatial correlations dictated by the wavefunction of the universe. This provides a very useful relation between the bispectrum and the (exchange) trispectrum.

Building a Boostless Bootstrap - Enrico Pajer.pdf

Recorded Talk

3 Discussion of talks [Guilherme Pimentel]

Harry Goodhew: The cosmological Optical Theorem
Dong-Gang Wang: Excursion in curved field space
Giovanni Cabass: Graviton non-Gaussianity in the EFT of I
Toshifumi Noumi: Country map of the EFT landscape

Recording of discussion

15:45 Break

4 Live Talk 2: Multipoint Correlators in Multifield Cosmology: formal structure and applications [Eva Silverstein]

Abstract: Connected N-point amplitudes in quantum field theory are enhanced by a factor of N! in appropriate regimes of kinematics and couplings, but the non-perturbative analysis of this for collider physics applications is subtle. We will start by briefly reviewing this effect and related developments in large-charge sectors of quantum field theory. We then resolve the question in the affirmative for N-point correlation functions of cosmological perturbations in multifield inflation, and comment on its application to primordial non-Gaussianity. We find that they are calculably N!-enhanced using a simple model for the mixing of the field sectors which leads to a convolution of their probability distributions. This effect leads to model-dependent but interesting prospects for enhanced observational sensitivity. Additional spinoffs include applications to primordial black hole production and a new inflationary model with a controlled resummation of derivative interactions.

Recorded Talk

5 Discussion of talks [Austin Joyce]

Spyros Sypsas: Primordial statistics from Multifields
Siyi Zhou: Classical String solutions on dS
Leonardo Senatore: Starting inflation from inhomogeneous universe
Kurt Hinterbichler: Shift symmetries on dS

Recording of discussion

17:45 Break

6 Discussion day 1 [Paolo Creminelli]

Recording of discussion

Tuesday, 8 September

7 Live Talk 3: Spinning Cosmological Bootstrap [Hayden Lee]

Abstract: In this talk, I will describe recent developments in the bootstrap approach to cosmological correlators, extended to correlators of massless spinning particles. Interactions of massless particles are highly constrained by gauge invariance, which manifests itself as current conservation on the boundary of de Sitter space. I will show that boundary spinning correlators satisfy current conservation only if different channels have correlated couplings to each other, allowing us to rediscover bulk facts such as charge conservation and the equivalence principle from a purely boundary perspective. Moreover, the constraints on the allowed singularities of massless spinning correlators and their residues are often strong enough to fix the answers completely. Based on this, I will describe an efficient way to construct spinning cosmological correlators in terms of lower-point building blocks.

Recorded Talk

Spinning Cosmological Bootstrap - Hayden Lee.pdf

8 Discussion of talks [Paul McFadden]

Austin Joyce: Spinning correlators from the boundary
Carlos Duaso Pueyo: Weight-shifting operators
Paul McFadden: Conformal correlators as simplex in momentum space
Aaron Hilmann: Symbols in the Sky

Recording of discussion

15:45 Break

9 Live Talk 4: Steinmann Relations, Causality and the Wavefunction of the Universe [Paolo Benincasa]

Abstract: How consistent causal time evolution is encoded in the late
time wavefuntions of the universe or correlation functions? This is one
of the open question which is crucial for our understanding of the
fundamental physics encoded in quantum mechanical observables in
cosmology. In this talk I will report on a recent progress in addressing
such a question showing how the wavefunction of the universe, with
ab-initio description given by the cosmological polytopes, has to
satisfy the so-called Steinmann relations, which in the flat space case
are an avatar of causality. I will show that the Steinmann relations for
both the S-matrix and the wavefunction of the universe arise via the
same mechanism and rely on the intrinsic definition of the cosmological
polytopes themselves. Finally, such relations constitutes further
constrains which can be used in the bootstrap program.

Recorded talk

Steinmann relations, Causality and the Wavefunction of the Universe - Paolo Benincasa.pdf

10 Discussion of talks [Leonardo Senatore]

Dmitri Ponomarev: Spinor helicity formalism in AdS
Hiroshi Isono: Cosmological Witten diagrams
Yasha Neiman: Scattering amplitudes for dS observers
Massimo Taronna: From CFT bootstrap to correlators

Recording of discussion

17:45 Break

11 Discussion day 2 [Kostas Skenderis]

Recording of discussion

Wednesday, 9 September

12 Live Talk 5: Alternatives to inflation as cosmological particle scanners [Xingang Chen]

Abstract: We show that, while inflation can be regarded as a cosmological collider with fixed energy, possible alternative scenarios to inflation may be viewed as particle detectors with scanning energies. They scan through a tower of massive particles in the primordial universe and leave a series of bursts of oscillatory signals as scale-dependent features in the density perturbations. Besides particle contents, these signals directly record the time evolution of the scale factor of the primordial universe. We discuss observational aspects of these signals.

Alternatives to inflation as cosmological particle scanners - Xingang Chen.pdf

Recorded talk

13 Discussion of talks [Daniel Green]

Yi Wang: Non-standard quantum clocks
Soubhik Kumar: extending the read of the Cosmo Collider
Mark Wise: Galaxy Bispectrum and Conformal Fermi Coordinates
Sebastien Reneaux-Petel: A manifestly covariant theory of multifield stochastic inflation

Recording of discussion

15:45 Break

14 Live Talk 6: De Sitter Diagrammar and the Resummation of Time [Matthew Baumgart]

Abstract: Light scalars in inflationary spacetimes suffer from infrared logarithmic divergences at every order in perturbation theory. This corresponds to the scalar field values in different Hubble patches undergoing a random walk of quantum fluctuations, leading to a simple toy landscape'' on superhorizon scales, in which we can explore questions relevant to eternal inflation. However, for a sufficiently long period of inflation, the infrared divergences appear to spoil computability. Some form of renormalization group approach is thus motivated to resum the log divergences of conformal time. Such a resummation may provide insight into De Sitter holography. We present here a novel diagrammatic analysis of these infrared divergences and their resummation. Basic graph theory observations and momentum power counting for the in-in propagators allow a simple and insightful determination of the leading-log infrared divergences. One thus sees diagrammatically how the superhorizon sector consists of a semiclassical theory with quantum noise evolved by a first-order, interacting classical equation of motion. This rigorously leads to theStochastic Inflation'' ansatz developed by Starobinsky to cure the scalar infrared pathology nonperturbatively. Our approach is a controlled approximation of the underlying quantum field theory and is systematically improvable.

Recorded Talk

15 Discussion of talks [Garrett Goon]

Dan Green: Soft dS EFT
Victor Gorbenko: IR dynamics of QFT in dS
Mehrdad Mirbabayi: Loss of memory in dS
Skenderis: Inflationary correlators and the solution of conformal Ward identities
Bzowski: TripleK: A Mathematica package for evaluating triple-K integrals and conformal correlation functions

Recording of discussion

17:45 Break

16 Discussion [Nima Arkani-Hamed]

Recording of discussion

17 Conclusion