Origins of the Universe Conference 2018

Origins of the Universe 2018 Meeting Writeup

Bouncing Cosmology (Steinhardt)

Anna Ijjas (Harvard), Frans Pretorius (Princeton) and Paul Steinhardt (Princeton), the three PIs of the Bouncing Cosmology component of the Origins of the Universe Initiative, together presented the leading results from the past year of research.

Steinhardt gave an introduction to bouncing cosmology that described the overall theoretical framework being explored: namely, a cosmology in which the big bang is replaced by a big bounce; there is no beginning or end to space and time; the fundamental large-scale features of the universe are set during a period of contraction leading up to the bounce; and the entire evolution of the universe is dominantly classical, described by deterministic equations of motion, with quantum physics only contributing small perturbations.

To illustrate the concept, he invoked “wedge diagrams,” a novel way of representing cosmological scenarios first introduced by Ijjas and Steinhardt during the past year as part of the Simons Foundation project.  The conclusion was that all the observed large-scale features of the universe can potentially be obtained during a long period of slow contraction (also known as an ekpyrotic phase) without requiring special initial conditions or producing a multiverse.

Ijjas followed with a detailed account of the progress that has been made by the collaboration during the past year in constructing classical bounce solutions that carry the conditions generated during the contracting phase into the expanding phase without encountering any period in which quantum effects dominate.  The bounce is based on modifications to Einstein gravity that become important for only a few instants during the bounce phase.  In particular, Ijjas described how Horndeski theory, a well-known scalar-tensor modification of Einstein gravity, produces an effect called “braiding” that makes the bounce possible.  She described her own research showing that the background solution is stable under perturbations of the scalar field and the curvature, an important milestone given that this safely circumvents certain well-known “no-go” theorems that had been claimed in earlier literature. The analysis utilized conventional lines of linear perturbative analysis familiar to cosmologists, though with proper attention to subtleties of gauge invariant variables that had been missed before.

Finally, Ijjas turned to a major new work with Pretorius and Steinhardt that sets the foundations for studying non-perturbative cosmology, a new area of study essential for analyzing theories in which the background solution is non-trivial (like a bounce) and the gravitational sector differs from Einstein gravity (like Horndeski theories).  The methodology combines aspects familiar to cosmologists and aspects familiar to mathematical relativists in a way that is novel to both groups.   There could very well be many applications as the methods become better known.  In the short run, though, Ijjas’ showed the results of applying this approach to classical bounce solutions described by Horndeski theory – namely, an analytic proof that the theories are linearly well-posed at and about homogeneous and isotropic backgrounds.  This is a key step since failure would mean that the bounce solutions are inherently badly-behaved classically.

Pretorius’ talk elaborated on this point and spoke about where the project moves next.  Pretorius is the leading pioneer in developing numerical methods to analyze fully non-linear general relativistic problems, such as black hole mergers. In his presentation, Pretorius described the first applications of these methods to cosmology (by Pretorius, Steinhardt and collaborators) beginning in 2007. He used the examples to emphasize the importance of the mathematical analysis that must precede any numerics, especially the critical step of going beyond standard cosmological perturbative analysis to consider the much more challenging problem of well-posedness, as described by Ijjas.  With this breakthrough, he argued, it was now clear that the Horndeski theory of bouncing solutions is sufficiently well-behaved at the linear level to justify the next step of full nonlinear numerical integration.  This is the next step in the program plans, and the key to determining the observational predictions of classical bouncing cosmologies.

The team invited scientists and mathematicians who work on diverse aspects of cosmology and mathematical relativity aside from the work described by Ijjas, Pretorius, and Steinhardt.  To take advantage of the opportunity, the team organized a satellite meeting at the Flatiron Institute to take place the following day (Saturday).  The program was intentionally designed to be eclectic.  Stephon Alexander (Brown) spoke about torsion in general relativity and earlier work by him exploring the idea of a torsion-mediated bounce.  V. (Slava) Mukhanov (LMU Munich) spoke about his research on “mimetic” quantum field theories that can describe dark matter and dark energy.  Robert Brandenberger (McGill) discussed the problems encountered in constructing  “matter bounce” models and how those problems may be overcome in the ekpyrotic models described by Steinhardt and Ijjas.  Mihalis Dafermos, a mathematical relativist at Princeton, described his foundational work on geodesic incompleteness of Kerr black holes in asymptotically flat and certain cosmological problems. Savdeep Sethi (Chicago) closed by discussing the ongoing debate about whether string theory is compatible with cosmology, in particular the cosmic accelerated expansion observed today and the hypothetical accelerated expansion required in inflationary cosmology.

Modern Inflation (Silverstein)

At the Origins of the Universe meeting in September 2018, the talks and discussions of the Modern Inflation group covered topics ranging from theoretical structures to large scale structure data analysis. Results that were reported included the genericity of the onset of inflation (using mean curvature flow techniques along with numerics), a new primordial black hole production mechanism and an analysis of strong non-Gaussian tails during inflation and in (p)reheating, as well as a careful analysis of the origin of stochastic methods from the full quantum field theory in de Sitter spacetime. The more theoretical topics included the derivation of the Gibbons-Hawking entropy as entanglement entropy in a holographic dual framework, and a careful analysis of multifield axion spectra within a class of string theory compactifications, in the context of a broader overview of large-field inflation. Finally, the talks reported progress on the characterization of large-scale structure and its use as a probe of cosmological parameters, including neutrino physics as well as tests of new physics such as non-Gaussianity and oscillatory features motivated by string theory.

The participant list consisted of a broad and very interesting set of people; intense discussions were frequently interrupted by the interesting talks; as such the meeting went very well both informally and formally. I recall fruitful cross talk among the groups on the question of alternative (beyond GR) gravity theories vis a vis black hole thermodynamics, the initial value problem in mean curvature flow and beyond-GR physics, and the issue of theoretical control of spacelike singularities as a few examples.

Following the New York meeting, the group had an extremely full and productive workshop at IAS. We would like to thank the Simons Foundation for the support of both meetings and the arrangements in between which all went very smoothly.

Beyond Einstein Cosmology (Gabadadze)

There were three talks presented by  the “Beyond Einstein Cosmology” group.  Claudia de Rham summarized recent developments on  various strongly coupled theories and their possible applications in  astrophysics  and cosmology. Notably the question of gravitational wave production in such theories goes hand in hand with the understanding of the time dependent Vainshtein mechanism that she investigated and showed to be  qualitatively similar but  quantitatively differed from the static Vainshtein mechanism. Claudia also  summarized the present and possible future observational bounds on graviton mass giving an exhaustive list of the constraints.

David Pirtskhalava’s talk  addressed two  questions of a tremendous magnitude: (1) whether one can come up with  meaningful theories that would, unlike inflation, be geodesically past complete,  and (2) whether such theories can be used to solve the old cosmological constant (CC) problem. In the direction (1) David reviewed  the null energy violating theories that seem to provide a natural framework  for geodesically past complete early universe and discussed perturbative stability of such theories in various contexts.  In the direction (2) David presented an existence proof  model in which the CC is be adjusted before the universe enters the FRW phase.

Greg Gabadadze talked about a  proposal to embed massive gravity and bigravity in a five dimensional braneworld theory to address the strong coupling issue of these theories. He presented an embedding that raises the strong scale by many orders of magnitude, and also proposed a theory in which the strong scale  could  be raised all the way up to the 5D Planck scale.  Greg outlined how this  theory can be used for geodesically complete cosmology in the early universe, as well as for the description of  the late time accelerated expansion of the universe.