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There's an interesting conference taking place this month at the NORDITA institute for theoretical physics in Stockholm. It brings together, from a number of different areas, researchers working on various models of what was happening around the start of expansion--replacing the "singularity" (the classical theory's failure) with some more physical model.

The conference is called *Perspectives of Fundamental Cosmology*

The topics of the hour talks include cosmology from the perspective of

Causal Dynamical Triangulations

Horava Gravity

Noncommutative (i.e. Spectral) Geometry

Loop and Spinfoam

Geometrodynamics

String

I count 51 things on the schedule, mostly 1-hour talks, but including a half-dozen discussion sessions on various topics. Part of why I find the conference interesting is that it brings together prominent experts from different fields with the aim of getting them talking with each other (Loll, Ambjorn, Lizzi, Brandenberger, Mersini, Kiefer, Sakellariadou, Nelson, Burgess, Sotiriou, Mavromatos, Thiemann, Schücker, Steinhacker, Hossenfelder...) The more I consider the lineup the more I am impressed by how incredibly diverse it is.

All these people are giving talks. Lewandowski is not on the list but his junior co-author Andrea Dapor is presenting their research.

Here is a highly incomplete sampling of abstracts of hour talks:

Claus Kiefer

I start by giving a general introduction to the motivation for and the problems of a theory of quantum gravity. I then briefly describe the main approaches - quantum general relativity (including loop quantum gravity) and string theory - and some of their applications. I then give a general introduction to quantum cosmology, in which I mainly deal with geometrodynamics, but make also remarks on loop quantum cosmology.

Will Nelson

Inflation is extremely successful at explaining the features of our late-time universe, however it is well known that it is incomplete. In particular the initial singularity persists and there is a period where quantum gravity effects must become important. In this talk I will discuss these quantum-gravity issues and describe how they can be tackled within Loop Quantum Cosmology. I will describe how we can make contact between LQC and QFT in curved space-times, what the 'true' trans-Planckian issue is within this context and what this approach predicts for observations.

Andrea Dapor

We present a new perspective on QFT on quantum cosmological space-times. Naively, the semiclassical limit of a quantum space-time can be taken by averaging the gravitational operators on a semiclassical state of geometry. The result is an effective classical metric (which in general does not obey Einstein equation). We show that, once quantum matter is taken into account, a new possibility arises: the dynamics of matter on quantum space-time can be equivalently described by standard QFT on a classical space-time, whose geometry is encoded in a so-called "dressed metric", a classical metric which is different than the naive semiclassical one. Such matter-dependent dressed metric can be thought of as the metric "seen" by the matter field.

We explore this interpretation and its consequences in various instances, such as scalar field on quantum cosmological space-times, addressing the question of observable effects on the propagation of matter. Indeed, since the dressed metric depends on the matter, Lorentz-violating phenomena are expected (such as deformation of dispersion relations). We prove that for massless field on FRW [1] or Bianchi I [2] quantum geometries, no violation takes place at test-field level. We present a way to take into account the matter back-reaction (in which case a deformation in the dispersion relation is present). Finally, we address the case of a massive field on FRW quantum geometry [3], and show that an isotropy-breaking is present already at the test-field level, since the dressed metric depends on the direction of propagation of the mode of the field.

References:

[1] A. Ashtekar, W. Kaminski, J. Lewandowski - "Quantum field theory on a cosmological, quantum space-time", Phys. Rev. D 79, 064030 (2009)

[2] A. Dapor, J. Lewandowski, Y. Tavakoli - "Lorentz Symmetry in QFT on Quantum Bianchi I Space-Time", Phys. Rev. D 86, 064013 (2012)

[3] A. Dapor, J. Lewandowski - "Isotropy-Breaking in Quantum Cosmology", in preparation [ http://arxiv.org/abs/1211.0161 ]

Francesca Vidotto

In the recent years the major developments in Loop Quantum Gravity have arisen in the covariant framework (spinfoam) where the dynamics is coded by transition amplitudes. These provide an evolution for "quanta of spacetime" in terms of a local product of interaction vertices, reconstructing a Lorentzian spacetime. The first part of the talk will be devoted to a general presentation of these results. The second part of the talk will show how these results find a good testing framework in cosmology and how, viceversa, spinfoam could provide a fertile framework for the description of the early-universe physics.

A sample discussion section led by Claus Kiefer:

My idea here is to give a short introduction myself (at most half an hour), followed by short presentations of participants and a general discussion. As for the topics of the short presentations, I suggest: "The no-boundary condition", "The tunnelling condition", "Boundary conditions in loop quantum cosmology", as well as other suggestions by participants.

The conference is leisurely paced--spread out over the better part of 4 weeks.

http://agenda.albanova.se/conferenceTimeTable.py?confId=2998

http://agenda.albanova.se/conferenceDisplay.py?confId=2998

Credit for organizing and arranging the program goes to Bojowald, Giesel, Hossenfelder, and Sakellariadou.

The conference is called *Perspectives of Fundamental Cosmology*

The topics of the hour talks include cosmology from the perspective of

Causal Dynamical Triangulations

Horava Gravity

Noncommutative (i.e. Spectral) Geometry

Loop and Spinfoam

Geometrodynamics

String

I count 51 things on the schedule, mostly 1-hour talks, but including a half-dozen discussion sessions on various topics. Part of why I find the conference interesting is that it brings together prominent experts from different fields with the aim of getting them talking with each other (Loll, Ambjorn, Lizzi, Brandenberger, Mersini, Kiefer, Sakellariadou, Nelson, Burgess, Sotiriou, Mavromatos, Thiemann, Schücker, Steinhacker, Hossenfelder...) The more I consider the lineup the more I am impressed by how incredibly diverse it is.

All these people are giving talks. Lewandowski is not on the list but his junior co-author Andrea Dapor is presenting their research.

Here is a highly incomplete sampling of abstracts of hour talks:

Claus Kiefer

**Quantum Gravity and Quantum Cosmology: A General Introduction**I start by giving a general introduction to the motivation for and the problems of a theory of quantum gravity. I then briefly describe the main approaches - quantum general relativity (including loop quantum gravity) and string theory - and some of their applications. I then give a general introduction to quantum cosmology, in which I mainly deal with geometrodynamics, but make also remarks on loop quantum cosmology.

Will Nelson

**Extending inflation to the quantum gravity era**Inflation is extremely successful at explaining the features of our late-time universe, however it is well known that it is incomplete. In particular the initial singularity persists and there is a period where quantum gravity effects must become important. In this talk I will discuss these quantum-gravity issues and describe how they can be tackled within Loop Quantum Cosmology. I will describe how we can make contact between LQC and QFT in curved space-times, what the 'true' trans-Planckian issue is within this context and what this approach predicts for observations.

Andrea Dapor

**Isotropy-breaking in Quantum Cosmology**We present a new perspective on QFT on quantum cosmological space-times. Naively, the semiclassical limit of a quantum space-time can be taken by averaging the gravitational operators on a semiclassical state of geometry. The result is an effective classical metric (which in general does not obey Einstein equation). We show that, once quantum matter is taken into account, a new possibility arises: the dynamics of matter on quantum space-time can be equivalently described by standard QFT on a classical space-time, whose geometry is encoded in a so-called "dressed metric", a classical metric which is different than the naive semiclassical one. Such matter-dependent dressed metric can be thought of as the metric "seen" by the matter field.

We explore this interpretation and its consequences in various instances, such as scalar field on quantum cosmological space-times, addressing the question of observable effects on the propagation of matter. Indeed, since the dressed metric depends on the matter, Lorentz-violating phenomena are expected (such as deformation of dispersion relations). We prove that for massless field on FRW [1] or Bianchi I [2] quantum geometries, no violation takes place at test-field level. We present a way to take into account the matter back-reaction (in which case a deformation in the dispersion relation is present). Finally, we address the case of a massive field on FRW quantum geometry [3], and show that an isotropy-breaking is present already at the test-field level, since the dressed metric depends on the direction of propagation of the mode of the field.

References:

[1] A. Ashtekar, W. Kaminski, J. Lewandowski - "Quantum field theory on a cosmological, quantum space-time", Phys. Rev. D 79, 064030 (2009)

[2] A. Dapor, J. Lewandowski, Y. Tavakoli - "Lorentz Symmetry in QFT on Quantum Bianchi I Space-Time", Phys. Rev. D 86, 064013 (2012)

[3] A. Dapor, J. Lewandowski - "Isotropy-Breaking in Quantum Cosmology", in preparation [ http://arxiv.org/abs/1211.0161 ]

Francesca Vidotto

**Spinfoam and Cosmology**In the recent years the major developments in Loop Quantum Gravity have arisen in the covariant framework (spinfoam) where the dynamics is coded by transition amplitudes. These provide an evolution for "quanta of spacetime" in terms of a local product of interaction vertices, reconstructing a Lorentzian spacetime. The first part of the talk will be devoted to a general presentation of these results. The second part of the talk will show how these results find a good testing framework in cosmology and how, viceversa, spinfoam could provide a fertile framework for the description of the early-universe physics.

A sample discussion section led by Claus Kiefer:

**Boundary Conditions in Quantum Cosmology**My idea here is to give a short introduction myself (at most half an hour), followed by short presentations of participants and a general discussion. As for the topics of the short presentations, I suggest: "The no-boundary condition", "The tunnelling condition", "Boundary conditions in loop quantum cosmology", as well as other suggestions by participants.

The conference is leisurely paced--spread out over the better part of 4 weeks.

http://agenda.albanova.se/conferenceTimeTable.py?confId=2998

http://agenda.albanova.se/conferenceDisplay.py?confId=2998

Credit for organizing and arranging the program goes to Bojowald, Giesel, Hossenfelder, and Sakellariadou.

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