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Gravity with more or less gauging (2018)
Journal Article
Gielen, S., de León Ardón, R., & Percacci, R. (2018). Gravity with more or less gauging. Classical and Quantum Gravity, 35(19), 195009. https://doi.org/10.1088/1361-6382/aadbd1

General relativity is usually formulated as a theory with gauge invariance under the diffeomorphism group, but there is a 'dilaton' formulation where it is in addition invariant under Weyl transformations, and a 'unimodular' formulation where it is o... Read More about Gravity with more or less gauging.

Quantum propagation across cosmological singularities (2017)
Journal Article
Gielen, S., & Turok, N. (2017). Quantum propagation across cosmological singularities. Physical Review D, 95(10), Article 103510. https://doi.org/10.1103/physrevd.95.103510

The initial singularity is the most troubling feature of the standard cosmology, which quantum effects are hoped to resolve. In this paper, we study quantum cosmology with conformal (Weyl) invariant matter. We show that it is natural to extend the sc... Read More about Quantum propagation across cosmological singularities.

Emergence of a low spin phase in group field theory condensates (2016)
Journal Article
Gielen, S. (2016). Emergence of a low spin phase in group field theory condensates. Classical and Quantum Gravity, 33(22), Article 224002. https://doi.org/10.1088/0264-9381/33/22/224002

Recent results have shown how quantum cosmology models can be derived from the effective dynamics of condensate states in group field theory (GFT), where ‘cosmology is the hydrodynamics of quantum gravity’: the classical Friedmann dynamics for homoge... Read More about Emergence of a low spin phase in group field theory condensates.

Quantum cosmology from group field theory condensates: a review (2016)
Journal Article
Gielen, S., & Sindoni, L. (2016). Quantum cosmology from group field theory condensates: a review. Symmetry, Integrability and Geometry: Methods and Applications, 12, doi:10.3842/sigma.2016.082

We give, in some detail, a critical overview over recent work towards deriving a cosmological phenomenology from the fundamental quantum dynamics of group field theory (GFT), based on the picture of a macroscopic universe as a “condensate” of a large... Read More about Quantum cosmology from group field theory condensates: a review.

Perfect quantum cosmological bounce (2016)
Journal Article
Gielen, S., & Turok, N. (2016). Perfect quantum cosmological bounce. Physical Review Letters, 117(2), Article 021301. https://doi.org/10.1103/physrevlett.117.021301

We study quantum cosmology with conformal matter comprising a perfect radiation fluid and a number of conformally coupled scalar fields. Focusing initially on the collective coordinates (minisuperspace) associated with homogeneous, isotropic backgrou... Read More about Perfect quantum cosmological bounce.

Identifying cosmological perturbations in group field theory condensates (2015)
Journal Article
Gielen, S. (2015). Identifying cosmological perturbations in group field theory condensates. Journal of High Energy Physics, 2015(8), doi:10.1007/jhep08(2015)010

One proposal for deriving effective cosmological models from theories of quantum gravity is to view the former as a mean-field (hydrodynamic) description of the latter, which describes a universe formed by a ‘condensate’ of quanta of geometry. This i... Read More about Identifying cosmological perturbations in group field theory condensates.

Perturbing a quantum gravity condensate (2015)
Journal Article
Gielen, S. (2015). Perturbing a quantum gravity condensate. Physical Review D - Particles, Fields, Gravitation and Cosmology, 91(4), Article 043526. https://doi.org/10.1103/physrevd.91.043526

In a recent proposal using the group field theory approach, a spatially homogeneous (generally anisotropic) universe is described as a quantum gravity condensate of “atoms of space”, which allows the derivation of an effective cosmological Friedmann... Read More about Perturbing a quantum gravity condensate.

Quantum cosmology from quantum gravity condensates: cosmological variables and lattice-refined dynamics (2014)
Journal Article
Gielen, S., & Oriti, D. (2014). Quantum cosmology from quantum gravity condensates: cosmological variables and lattice-refined dynamics. New Journal of Physics, 16(12), Article 123004. https://doi.org/10.1088/1367-2630/16/12/123004

In the context of group field theory condensate cosmology, we clarify the extraction of cosmological variables from the microscopic quantum gravity degrees of freedom. We show that an important implication of the second quantized formalism is the dep... Read More about Quantum cosmology from quantum gravity condensates: cosmological variables and lattice-refined dynamics.

Quantum cosmology of (loop) quantum gravity condensates: an example (2014)
Journal Article
Gielen, S. (2014). Quantum cosmology of (loop) quantum gravity condensates: an example. Classical and Quantum Gravity, 31(15), doi:10.1088/0264-9381/31/15/155009. ISSN 0264-9381

Spatially homogeneous universes can be described in (loop) quantum gravity as condensates of elementary excitations of space. Their treatment is easiest in the second-quantized group field theory formalism, which allows the adaptation of techniques f... Read More about Quantum cosmology of (loop) quantum gravity condensates: an example.

Homogeneous cosmologies as group field theory condensates (2014)
Journal Article
Gielen, S., Oriti, D., & Sindoni, L. (2014). Homogeneous cosmologies as group field theory condensates. Journal of High Energy Physics, 2014(6), doi:10.1007/jhep06(2014)013

We give a general procedure, in the group field theory (GFT) formalism for quantum gravity, for constructing states that describe macroscopic, spatially homogeneous universes. These states are close to coherent (condensate) states used in the descrip... Read More about Homogeneous cosmologies as group field theory condensates.