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- Steffen Gielen

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University of Sheffield

Areas of Research:

Our current understanding of cosmology is that the Universe began at the Big Bang; the Big Bang itself is a singularity, where general relativity breaks down. This breakdown motivates the search for quantum gravity; the fact that our Universe seems to have started in a singularity gives observational relevance to this search: a theory that resolves the Big Bang singularity should make cosmological predictions.

The aim of my research is to develop new models for the early Universe, which connect quantum gravity to cosmology, demanding that these resolve the singularity problem. Doing this strengthens the theoretical foundations of modern cosmology, and can provide observational tests for quantum gravity.

I am currently working on two scenarios in which quantum effects replace the Big Bang singularity by a non-singular "bounce". The first scenario starts from group field theory, a non-perturbative approach to quantum gravity in which space and time themselves emerge from discrete quantum structures that form a "condensate". The second scenario uses semiclassical methods in quantum cosmology to describe the passage through the bounce as akin to quantum-mechanical tunnelling; one can directly calculate the transition of cosmological perturbations through the bounce.

Exploration of these complementary directions should shed light on what aspects of quantum gravity lead to a consistent theory of singularity resolution; this is particularly important in light of the growing interest in a variety of different bounce scenarios (as discussed in a successful workshop at Perimeter in June 2017).

- 2014 - 2016 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

- 2017 Royal Society University Research Fellowship
- 2013 Marie Curie Intra-European Fellowship, European Commission
- 2011 Vaclav Votruba Prize, Doppler Institute, Prague, Czech Republic
- 2007 Mayhew Prize, DAMTP, University of Cambridge

- Quantum propagation across cosmological singularities, Steffen Gielen and Neil Turok, Phys.Rev. D95 (2017) no.10, 103510, arXiv: 1612.02792
- 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

- Cosmology from group field theory condensates, plenary talk at Loops '17, University of Warsaw
- Perfect Bounce in quantum cosmology, McGill University Theoretical High Energy Seminar
- 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 group field theory condensates, Quantum Gravity Seminar, Albert Einstein Institute, Potsdam
- Cosmology with quantum gravity condensates, Astroparticle Seminar, SISSA, Trieste
- Cosmology with quantum gravity condensates, Quantum Gravity Seminar, Radboud University Nijmegen
- Cosmology with group field theory condensates, Group Seminar, Institute for Quantum Gravity, Erlangen
- 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:17060094, Welcome and Opening Remarks, 2017-06-26, Bounce Scenarios in Cosmology
- PIRSA:17060101, Discussion Session 2, 2017-06-26, Bounce Scenarios in Cosmology
- 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

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