# Challenges for Early Universe Cosmology

Conference Date:
Tuesday, July 12, 2011 (All day) to Saturday, July 16, 2011 (All day)
Scientific Areas:
Cosmology

The goal of this conference is to encourage new thinking on important unresloved questions in early universe cosmology.

Anthony Aguirre, University of California, Santa Cruz

Andreas Albrecht, University of California, Davis

Nima Arkani-Hamed, Institute for Advanced Study

Tom Banks, Rutgers, University of California, Santa Cruz

Itzhak Bars, University of Southern California

Jose Juan Blanco-Pillado, Tufts University

Raphael Bousso, University of California, Berkeley

Latham Boyle, Perimeter Institute

Adam Brown, Princeton University

Sean Carroll, California Institute of Technology

Willy Fischler, University of Texas at Austin

Benjamin Freivogel, Massachusetts Institute of Technology

Jaume Garriga, University of Barcelona

Alan Guth, Massachusetts Institute of Technology

Daniel Harlow, Stanford University

James Hartle, University of California, Santa Barbara

Matt Johnson, Perimeter Institute

Justin Khoury, University of Pennsylvania

Matt Kleban, New York University

Louis Leblond, Perimeter Institute

Jean-Luc Lehners, Albert Einstein Institute

Paul McFadden, University of Amsterdam

Laura Mersini-Houghton, University of North Carolina

Rob Myers, Perimeter Institute

Radford Neal, University of Toronto

Don Page, University of Alberta

Hiranya Peiris, University College London

Sir Roger Penrose, University of Oxford

Michael Salem, Stanford University

Leonardo Senatore, Stanford University

Sarah Shandera, Perimeter Institute

Ben Shlaer, Tufts University

Douglas Stanford, Stanford University

Paul Steinhardt, Princeton University

Leonard Susskind, Stanford University

Gonzalo Torroba, Stanford University

Neil Turok, Perimeter Institute

Vitaly Vachurin, Stanford University

BingKan Xue, Princeton University

Anthony Aguirre, University of California, Santa Cruz

What Happens When Entropy Decreases

Closed systems never evolve to lower entropy states -- except when they do, which is if one waits a time that is exponential in the entropy change.  Thus macroscopic decreases in entropy are 'never' observed.  Yet in cosmology there are eternal systems in which downward entropy fluctuations of any magnitude eventually happen.  What is the nature of such fluctuations?  I will argue that these can be understood in a simple and general way that sheds light on our understanding of various interesting cosmological processes such as bubble nucleation, black/white hole formation, eternal stochastic inflation, Boltzmann brain formation, and other processes.

Andreas Albrecht, University of California, Davis

Infinity, Finiteness and Inflationary Cosmology

I analyze the various roles of infinity in current thinking about cosmology. Topics include initial conditions, attractor behavior, inflation and the precision and meaning of quantum measurements. I review the de Sitter equilibrium cosmology as an example of a finite cosmology, and present some new predictions that permit observable tests.

Nima Arkani-Hamed, Institute for Advanced Study

TBA

Tom Banks, Rutgers/University of California, Santa Cruz

Holographic Cosmology Inflation and Entropy

I provide a mathematical model of holographic cosmology whose coarse grained description is that of a homogeneous isotropic, flat universe, which makes a transitions from an FRW to an eternal de Sitter regime. Based on this model, I suggest some heuristic ideas which explain the low initial entropy of the universe and may provide a description of an inflationary era with small fluctuations.

Itzhak Bars, University of Southern California

Antigravity Predicted Between Crunch and Bang in a Cyclic Universe

Einstein's theory of General Relativity and its couplings to matter in 3+1 dimensions can be slightly enlarged with the requirement of a local scale (conformal) symmetry and the corresponding gauge degrees of freedom. This form of the theory is a prediction from 2T-gravity in 4+2 dimensions. It has no dimensionful constants, not even the gravitational constant, and requires all scalar fields to be conformally coupled to gravity and to the rest of matter. The theory can be gauge fixed to the usual gravity theory in the Einstein frame, thus generating the gravitational constant. Other physically equivalent forms of gauge fixing lead to the complete set of exact analytic solutions of the usual Friedmann equations, including radiation, curvature, anisotropy and a special potential for a scalar field coupled minimally to gravity. These analytic cosmological solutions, which are geodesically complete at singularities, reveal many surprising properties that are not noticeable with approximate cosmological solutions. Some aspects of the exact solutions will be reviewed in this lecture. In particular, it is predicted that the universe is cyclic and furthermore is has a period of antigravity between every big crunch and the following big bang.

Jose Juan Blanco-Pillado, Tufts University

Transdimensional Tunneling in the Multiverse

I will introduce a simple 6d model of flux compactification that shows a remarkable rich landscape of vacua with different number of large and compact dimensions. I will then describe the instantons interpolating between these different vacua as well as some the implications of a transdimensional multiverse of this form.

Raphael Bousso, University of California, Berkeley

The Measure Problem: Successes and Challenges

Adam Brown, Princeton University

Populating the Whole Landscape

Sean Carroll, California Institute for Technology

The Arrow of Time in an Eternal Universe

If we imagine that the universe is truly eternal, special challenges arise for attempts to solve cosmological fine-tuning problems, especially the low entropy of the early universe.  If the space of states is finite, the universe should spend most of its time near equilibrium.  If the space of states is infinite, it becomes difficult to understand why our universe was in a particular low-entropy state.

I will discuss approaches to addressing this problem in a model-independent fashion.

Willy Fischler, University of Texas at Austin

Holographic Cosmology Part 1

We will describe how a quantum mechanical description of a flat FRW with equation of state pressure =energy density, emerges.

Ben Freivogel, MIT

TBA

Jaume Garriga, University of Barcelona

TBA

Alan Guth, MIT

TBA

James Hartle, University of California, Santa Barbara

Eternal Inflation in the Light of Quantum Cosmology

If the universe is a quantum mechanical system it has a quantum state.This  state supplies a probabilistic measure for alternative histories of the universe. During eternal inflation these histories typically develop large inhomogeneities that lead to a mosaic structure on superhorizon scales consisting of homogeneous patches separated by inflating regions. As observers we do not see this structure directly. Rather our observations are confined to a small, nearly homogeneous region within our past light cone. This talk will describe how the probabilities for these observations can be calculated from the probabilities supplied by the quantum state without introducing a further ad hoc measure.

Matt Johnson, Perimeter Institute

Bringing Terminal Vacua Back to Life With Classical Transitions

In this talk, I will describe how the collision of Minkowski or crunching bubbles can re-start inflation in a portion of the bubble interior. Consistent with various singularity theorems, such collisions can only seed a lasting inflationary phase with energy density lower than that of the parent vacuum.

Justin Khoury, University of Pennsylvania

Scale Invariance from Spontaneous Breaking of Conformal Symmetry

I will discuss a novel framework of the very early universe which addresses the traditional horizon and flatness problems of big bang cosmology and predicts a scale invariant spectrum of perturbations. Unlike inflation, this scenario requires no exponential superluminal expansion of space-time. Instead, the early universe is described by a conformal field theory minimally coupled to gravity. The conformal fields develop a time-dependent expectation value which breaks the flat space so(4,2) conformal symmetry down to so(4,1), the symmetries of de Sitter, giving perturbations a scale invariant spectrum. The solution is an attractor, at least in the case of a single time-dependent field. Meanwhile, the metric background remains approximately flat but slowly contracts, which makes the universe increasingly flat, homogeneous and isotropic. The essential features of the scenario depend only on the symmetry breaking pattern and not on the details of the underlying lagrangian.

Matt Kleban, New York University

TBA

Jean-Luc Lehners, Albert Einstein Institute

Does the Cyclic Universe Avoid the Measure Problem?

In its best understood version, the Steinhardt-Turok cyclic universe contains two crucial ingredients: an unstable field trajectory during the ekpyrotic phase, and the subsequent brane collision corresponding to the crunch/bang transition. These two features act as strong selection principles and determine the broad physical properties of the universe emerging from the bang. As such, they significantly alleviate (and perhaps resolve) the measure problem that is inherent to all cosmological models that produce universes with a range of physical properties.

Paul McFadden, University of Amsterdam

Inflationary Cosmology: A Holographic Perspective

We present a holographic framework for inflationary universes, in particular those that are either asymptotically de Sitter or asymptotically power-law.  This framework reveals how cosmological observables, including the primordial power spectrum and non-Gaussianities, are encoded in the correlation functions of a three-dimensional non-gravitational quantum field theory.  Introducing a simple yet general class of holographic models, we obtain distinctive observational predictions that are compatible with current observational data and may be either confirmed or excluded by Planck.

Laura Mersini-Houghton, University of North Carolina

Selection of the Initial Conditions from the Landscape Multiverse

I will briefly review the predictions of the theory of the Selection of the Initial Conditions of the Universe from the Landscape Multiverse and focus on recent and upcoming evidence. In this theory, the wavefunction of the universe propagating on the landscape is localized via Anderson localization. Decoherence of the wavefunction is triggered by the backreaction of massive superhorizon fluctuations. Thus the selection of the initial conditions is determined by the quantum dynamics of gravitational (vacuum energy) vs. matter degrees of freedom. Dynamics selects only high energy universes as 'survivors' while low energy universe become 'terminal'.

I will describe how the nonlocal quantum entanglement associated with decoherence provides a second source of perturbations and gives rise to a series of derived predictions.  Three of the signatures of the theory predicted in 2006 (the giant void; a suppressed \sigma_8; and, the dark flow) were tested soon afterwards. The fourth prediction will be tested by LHC in a year.

Radford Neal, University of Toronto

Probability and Anthropic Reasoning in Small, Large, and Infinite Universes

I will argue that anthropic reasoning is unnecessary or misleading when the universe/multiverse is small enough that another observer with exactly your memories is unlikely to exist.  Instead, one can  evaluate theories or make predictions in the standard Bayesian way,   based on the conditional probability of something unknown given all  that you do know.  Things are not so clear when the universe is large enough that all competing theories predict that an observer with your exact memories exists with probability close to one.  I will discuss issues that arise in such large or infinite universes, such as "Boltzmann brains", and will argue that pending better understanding of these issues one should be hesitant to draw conclusions different from those that would apply to a small iverse.

Alberto Nicolis, Columbia University

Galilean Genesis

We propose a novel cosmological scenario, in which standard inflation is replaced by an expanding phase with a drastic violation of the Null Energy Condition (NEC): \dot H >> H^2. The model is based on the recently introduced Galileon theories, which allow NEC violating solutions without instabilities. The unperturbed solution describes a Universe that is asymptotically Minkowski in the past, expands with increasing energy density until it exits the regime of validity of the effective field theory and reheats. This solution is a dynamical attractor and the Universe is driven to it, even if it is initially contracting. Adiabatic perturbations turn out to be cosmologically irrelevant. The model, however, suggests a new way to produce a scale invariant spectrum of isocurvature perturbations, which can be later

converted to adiabatic: the Galileon is forced by symmetry to couple to the other fields as a dilaton; the effective metric it yields on the NEC violating solution is that of de Sitter space, so that all light scalars will automatically acquire a nearly scale-invariant spectrum of perturbations.

Don Page, University of Alberta

Entropy in the Universe

A positive cosmological constant allows arbitrarily many different quantum states, but apparently only if there can be big bangs and/or big crunches.  Without any big bang or big crunch, the entropy may be limited by the Gibbons-Hawking entropy of pure deSitter, and the matter entropy might even more limited by a value roughly the three-fourths power of the Gibbons-Hawking entropy.  A classical analogue of an upper limit on the entropy is the finite canonical measure for nonsingular cosmologies.

Inflation by itself does not explain the arrow of time and seems to require that not only a small region have suitable initial conditions.  A special quantum state, such as the Symmetric-Bounce one, with a suitable measure, such as volume averaging and Agnesi weighting, appears possible to explain the arrow of time and other observations, such as high order that would not be expected for Boltzmann brain observations, distant stars that would not be expected if we were fluctuations of empty de Sitter spacetime, and positive cosmological constant that appears not to dominate many other measures.

Hiranya Peiris, University College London

First Observational Tests of Eternal Inflation

The eternal inflation scenario predicts that our observable universe resides inside a single bubble embedded in a vast inflating multiverse. Collisions between bubble universes imprinted in the CMB sky provide a powerful observational test of this idea. I will describe a robust algorithm for non-Gaussian source detection in massive datasets, and present its application to the search for bubble collision signatures in CMB data from WMAP.

Sir Roger Penrose, University of Oxford

Conformal Cyclic Cosmology: Some Striking New Observational Support

Michael Salem, Stanford University

Observable signatures of anisotropic bubble nucleation

Our universe may have formed via bubble nucleation in an eternally-inflating background.  Furthermore, the background may have a compact dimension---the modulus of which tunnels out of a metastable minimum during bubble nucleation---which subsequently grows to become one of our three large spatial dimensions.  We discuss some potential observational signatures of this scenario.

Leonardo Senatore, Stanford University

On Slow Roll Eternal Inflation

Sarah Shandera, Perimeter Institute

Infrared Challenges for Inflation

I will review some recent work on infrared issues for scalar fields in exact and quasi de Sitter space. Renewed interest in this topic has been driven by the observational potential for a more accurate determination of statistics of the primordial curvature perturbations, especially non-Gaussianity. Interestingly, the resulting questions are not only relevant for mapping inflationary models to observation but also link directly to more fundamental questions about the initial state, eternal inflation, and the long time dynamics of interacting quantum fields in curved space. Infrared questions provide a precisely calculable way to put pressure on inflation as a rigorous and consistent framework, ready to confront future observations.

Ben Shlaer, Tufts University

A critique of Coleman - De Luccia

In many respects, de Sitter space behaves like a system at finite temperature in finite volume.  I will extend this to include the lack of first-order phase transitions. This rules out exponential decay in the de Sitter landscape, which changes the global structure in a significant way.

Paul J. Steinhardt, Princeton University

Meeting the Challenges

This talk will be a heuristic discussion of the challenges for the big bang inflationary picture and possible approaches for addressing them.

Leonard Susskind, Stanford University

TBA

Gonzalo Torroba, Stanford University

A Simple Harmonic Universe

We explore simple but novel solutions of general relativity which, classically, approximate cosmologies cycling through an infinite set of bounces."   These solutions require curvature K=+1, and are supported by a negative cosmological term and matter with -1 < w < -1/3.  They can be studied within the regime of validity of general relativity.  We argue that quantum mechanically, particle production leads eventually to a departure from the regime of validity of semiclassical general relativity, likely yielding a singular crunch.

Vitaly Vanchurin, Stanford University

Physics Beyond the Standard Theory of Inflation and Dark Energy

Inflationary cosmology not only provided a simple solution to various cosmological problems, but also made predictions later confirmed by observations. Despite of its success, a straightforward extrapolation of the theory to higher energy scales led to new problems and seems to require new physics. In this talk I review the new problems, discuss their possible resolutions and speculate on possible predictions of the new physics.

BingKan Xue, Princeton University

Curvature and Anisotropy Near a Nonsingular Bounce

Problematic growths of curvature and anisotropy are found in nonsingular bouncing cosmologies that include both an ekpyrotic phase and a bouncing phase. Classically, initial curvature and anisotropy that are suppressed during the ekpyrotic phase will grow back exponentially during the nonsingular bouncing phase. Besides, curvature perturbations and anisotropy are generated by quantum fluctuations during the ekpyrotic phase. In the bouncing phase, an adiabatic curvature perturbation grows to dominate and gives rise to a blue spectrum that spoils the scale-invariance. Meanwhile, a scalar shear perturbation grows nonlinear and creates an overwhelming anisotropy that disrupts the nonsingular bounce altogether.