Research Repository

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Inhomogeneous universe from group field theory condensate (2019)
Journal Article
Gielen, S. (2019). Inhomogeneous universe from group field theory condensate. Journal of Cosmology and Astroparticle Physics, 2019(2), doi:10.1088/1475-7516/2019/02/013

One of the fundamental challenges for quantum cosmology is to explain the emergence of our macroscopic Universe from physics at the Planck scale. In the group field theory (GFT) approach to quantum gravity, such a macroscopic universe results from th... Read More

Group field theory and its cosmology in a matter reference frame (2018)
Journal Article
Gielen, S. (2018). Group field theory and its cosmology in a matter reference frame. Universe, 4(10), (103). doi:10.3390/universe4100103. ISSN 2218-1997

While the equations of general relativity take the same form in any coordinate system, choosing a suitable set of coordinates is essential in any practical application. This poses a challenge in background-independent quantum gravity, where coordinat... Read More

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). doi:10.1088/1361-6382/aadbd1. ISSN 0264-9381

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

Cosmological evolution as squeezing: a toy model for group field cosmology (2018)
Journal Article
Adjei, E., Gielen, S., & Wieland, W. (2018). Cosmological evolution as squeezing: a toy model for group field cosmology. Classical and Quantum Gravity, 35(10), doi:10.1088/1361-6382/aaba11. ISSN 0264-9381

We present a simple model of quantum cosmology based on the group field theory (GFT) approach to quantum gravity. The model is formulated on a subspace of the GFT Fock space for the quanta of geometry, with a fixed volume per quantum. In this Hilbert... Read More

Quantum propagation across cosmological singularities (2017)
Journal Article
Gielen, S., & Turok, N. (2017). Quantum propagation across cosmological singularities. Physical Review D, 95(10), doi:10.1103/physrevd.95.103510. ISSN 2470-0010

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

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), doi:10.1088/0264-9381/33/22/224002. ISSN 0264-9381

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

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

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

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), doi:10.1103/physrevd.91.043526. ISSN 1550-7998

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

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), doi: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

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

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