# Steffen Gielen

Imperial College London - Department of Physics

Areas of Research:

## Research Interests

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, and various models of quantum cosmology.

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.

These results have started a whole new programme for connecting effective quantum cosmology models with fundamental theories of quantum gravity, which I keep exploring with Daniele Oriti and others. For instance, I currently investigate extensions of the formalism to matter fields and inhomogeneities.

## Positions Held

• from Oct. 2014 Marie Curie Postdoctoral Fellow, Imperial College
• 2012 - 2013 Postdoctoral Fellow (Riemann Fellow), Riemann Center for Geometry and Physics, Hannover, Germany
• Jan.2012 - 2014 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

## Awards

• 2013 Marie Curie Intra-European Fellowship, European Union
• 2011 Vaclav Votruba Prize, Doppler Institute, Prague, Czech Republic
• 2007 Mayhew Prize, DAMTP, University of Cambridge

## Recent Publications

• Quantum cosmology of (loop) quantum gravity condensates: An example, Steffen Gielen, Class. Quant. Grav. 31, 155009 (2014), arXiv: 1404.2944
• Homogeneous cosmologies as group field theory condensates, Steffen Gielen, Daniele Oriti, Lorenzo Sindoni, JHEP 1406 013 (2014), arXiv: 1311.1238
• 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
• Quantum cosmology from quantum gravity condensates: cosmological variables and lattice-refined dynamics, Steffen Gielen, Daniele Oriti, arXiv: 1407.8167
• Spontaneous breaking of Lorentz symmetry for canonical gravity, proceedings of "Relativity and Gravitation -- 100 years after Einstein in Prague", Springer Proceedings in Physics 57 (2014) 497-503, arXiv: 1210.1787

## Seminars

• Space as a quantum gravity condensate, 2014 Atlantic General Relativity Conference, University of New Brunswick
• Conformal observer space, Cartan geometry and shape dynamics, International Shape Dynamics Workshop, University of New Brunswick
• 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 group field theory condensates, Group Seminar, Institute for Quantum Gravity, Erlangen
• Cosmology with quantum gravity condensates, Quantum Gravity Seminar, Radboud University Nijmegen
• Space as a group field theory condensate, Relativity Lunch, Imperial College London
• 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:14050135, Cosmological Dynamics of Space As a Quantum Gravity Condensate, 2014-05-22, Quantum Gravity Day 2014
• PIRSA:13070039, Quantum Cosmology - 1, 2013-07-22, Loops 13