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- Steffen Gielen

Phd: University of Cambridge 2011

Areas of Research:

Phone: x6321

I aim to connect various non-perturbative approaches to quantum gravity, which often involve some form of discrete structures, to more conventional physical theories based on general relativity. At the moment I am focussing on the derivation of classical cosmology from fundamental quantum gravity, various models of quantum cosmology, and the role of Lorentz invariance in canonical gravity.

The most natural place to look for testable predictions from quantum gravity is in cosmology. A major first step towards making the connection to cosmology is to be able to describe quantum gravity states that describe macroscopic, homogeneous or approximately homogeneous universes like the one around us, and to be able to do calculations within quantum gravity to derive the effective dynamics of such states. With Daniele Oriti and Lorenzo Sindoni, I have recently managed to complete these steps in the group field theory approach to quantum gravity. We show that certain states whose structure is analogous to condensate states that appear in Bose-Einstein condensation describe macroscopic spatially homogeneous geometries, and that in a semiclassical regime and in the isotropic case, the dynamics they satisfy is given precisely by the Friedmann equation for pure GR. Extensions of the formalism to matter fields and inhomogeneities are currently investigated.

Another recent interest of mine has been a geometric study of `spontaneous' symmetry breaking in gravity, using Cartan geometry. With Derek Wise, I have given a reformulation of Ashtekar variables where local Lorentz symmetry is broken to a rotational subgroup through local observers that specify a local notion of `time'. We then take this picture further by defining general relativity on `observer space' which include all such local observers simultaneously. In general relativity this space is just a direct product of spacetime with the space of unit timelike vectors at each point, but if proposals such as relative locality point in the right direction it could be a more general space. Observer space provides a very general geometric framework for quantum gravity phenomenology, such as in the study of Lorentz-violating theories, or in Finsler geometry.

- 2012 - 2013 Postdoctoral Fellow (Riemann Fellow), Riemann Center for Geometry and Physics, Hannover, Germany
- Jan.2012 - present Interdisciplinary Postdoctoral Researcher at Perimeter Institute
- 2010 - 2011 Postdoctoral Fellow, Albert Einstein Institute, Potsdam, Germany
- 2007 - 2010 PhD student, Relativity and Gravitation group, DAMTP, Cambridge

- 2011 Vaclav Votruba Prize, Doppler Institute, Prague, Czech Republic
- 2007 Mayhew Prize, DAMTP, University of Cambridge

- Cosmology from Group Field Theory Formalism for Quantum Gravity, Steffen Gielen, Daniele Oriti, Lorenzo Sindoni, Phys. Rev. Lett. 111 (2013) 031301, arxiv:gr-qc/1303.3576
- Steffen Gielen, Comment on "Causality-violating Higgs singlets at the LHC", Phys. Rev. D 88, 068701 (2013), arXiv: 1302.1711
- Contracted Lorentz Invariance for Gravity with a Preferred Foliation, Steffen Gielen, Phys. Rev. D 88 (2013) 064026, arxiv:gr-qc/1301.1692
- Lifting General Relativity to Observer Space, Steffen Gielen, Derek K. Wise, J. Math. Phys. 54 (2013) 052501, arxiv:gr-qc/1210.0019
- Linking Covariant and Canonical General Relativity via Local Observers, Steffen Gielen, Derek K. Wise, Gen. Rel. Grav. 44, 3103-3109 (2012), arxiv:gr-qc/1206.0658
- Group field cosmology: a cosmological field theory of quantum geometry, Gianluca Calcagni, Steffen Gielen, Daniele Oriti, Class. Quantum Grav. 29 (2012) 105005, arxiv:gr-qc/1201.4151
- Gary W. Gibbons, Steffen Gielen, C. N. Pope, Neil Turok, Measures on Mixing Angles, Phys.Rev.D79:013009,2009, arXiv: 0810.4813
- Gary W. Gibbons, Steffen Gielen, C. N. Pope, Neil Turok, Naturalness of CP Violation in the Standard Model, Phys.Rev.Lett.102:121802,2009, arXiv: 0810.4368
- Homogeneous cosmologies as group field theory condensates, Steffen Gielen, Daniele Oriti, Lorenzo Sindoni, arXiv: 1311.1238
- Spontaneous breaking of Lorentz symmetry for canonical gravity, proceedings of "Relativity and Gravitation - 100 years after Einstein in Prague", arXiv: 1210.1787

- Space as a quantum gravity condensate, Workshop "Quantum Gravity and Fundamental Cosmology", Albert Einstein Institute, Potsdam
- Cosmology with quantum gravity condensates, Astroparticle Seminar, SISSA, Trieste
- Cosmology with group field theory condensates, Quantum Gravity Seminar, Albert Einstein Institute, Potsdam
- Cosmology with quantum gravity condensates, Quantum Gravity Seminar, Radboud University Nijmegen
- Space as a group field theory condensate, Relativity Lunch, Imperial College London
- Cosmology with group field theory condensates, Group Seminar, Institute for Quantum Gravity, Erlangen
- Homogeneous cosmologies in discrete quantum gravity, 2nd Mediterranean Conference on Classical and Quantum Gravity, Veli Losinj
- Observer dependent spacetimes, Research Training Group Models of Gravity Colloquium, Hannover
- General relativity on observer space, Group Seminar, Institute for Quantum Gravity, Erlangen
- Spontaneous breaking of Lorentz symmetry for canonical gravity, at ``Relativity and Gravitation - 100 years after Einstein in Prague``, Prague
- Cartan geometry and spontaneous breaking of Lorentz symmetry in gravity, Riemann Center for Geometry and Physics, Hannover
- Spontaneous breaking of Lorentz symmetry for canonical gravity, German Physics Society (DPG) Spring Meeting Goettingen
- PIRSA:13070039, Quantum Cosmology - 1, 2013-07-22, Loops 13

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