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Publications (10 of 31) Show all publications
Dialektopoulos, K. F., Koivisto, T. S. & Capozziello, S. (2019). Noether symmetries in symmetric teleparallel cosmology. European Physical Journal C, 79(7), Article ID 606.
Open this publication in new window or tab >>Noether symmetries in symmetric teleparallel cosmology
2019 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 79, no 7, article id 606Article in journal (Refereed) Published
Abstract [en]

We consider a general theory of all possible quadratic, first-order derivative terms of the non-metricity tensor in the framework of Symmetric Teleparallel Geometry. We apply the Noether Symmetry Approach to classify those models that are invariant under point transformations in a cosmological background and we use the symmetries of these models to reduce the dynamics of the system in order to find analytical solutions.

Place, publisher, year, edition, pages
Springer, 2019
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-255751 (URN)10.1140/epjc/s10052-019-7106-8 (DOI)000476543200005 ()2-s2.0-85069538479 (Scopus ID)
Note

QC 20190813

Available from: 2019-08-13 Created: 2019-08-13 Last updated: 2019-08-13Bibliographically approved
Iosifidis, D. & Koivisto, T. S. (2019). Scale Transformations in Metric-Affine Geometry. UNIVERSE, 5(3), Article ID 82.
Open this publication in new window or tab >>Scale Transformations in Metric-Affine Geometry
2019 (English)In: UNIVERSE, ISSN 2218-1997, Vol. 5, no 3, article id 82Article in journal (Refereed) Published
Abstract [en]

This article presents an exhaustive classification of metric-affine theories according to their scale symmetries. First it is clarified that there are three relevant definitions of a scale transformation. These correspond to a projective transformation of the connection, a rescaling of the orthonormal frame, and a combination of the two. The most general second order quadratic metric-affine action, including the parity-violating terms, is constructed in each of the three cases. The results can be straightforwardly generalised by including higher derivatives, and implemented in the general metric-affine, teleparallel, and symmetric teleparallel geometries.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
scale invariance, Palatini formalism, metric affine gauge theory of gravity
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-251221 (URN)10.3390/universe5030082 (DOI)000464054300001 ()2-s2.0-85064415076 (Scopus ID)
Note

QC 20190524

Available from: 2019-05-24 Created: 2019-05-24 Last updated: 2019-05-24Bibliographically approved
Koivisto, T. S., Hohmann, M. & Zlosnik, T. (2019). The General Linear Cartan Khronon. UNIVERSE, 5(7), Article ID 168.
Open this publication in new window or tab >>The General Linear Cartan Khronon
2019 (English)In: UNIVERSE, ISSN 2218-1997, Vol. 5, no 7, article id 168Article in journal (Refereed) Published
Abstract [en]

A Cartan geometry of the General Linear symmetry is formulated by dividing out the displacements from the group. The resulting action is quadratic in curvature, polynomial in all the (minimal) variables, and describes an observer space that-in the symmetry-broken phase-reproduces the predictions of General Relativity in the presence of dark matter.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
conformal and metric-affine gauge theories of gravitation, Cartan geometry
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-257581 (URN)10.3390/universe5070168 (DOI)000478769900017 ()2-s2.0-85070388518 (Scopus ID)
Note

QC 20190920

Available from: 2019-09-20 Created: 2019-09-20 Last updated: 2019-10-04Bibliographically approved
Beltran Jimenez, J., Heisenberg, L. & Koivisto, T. S. (2019). The Geometrical Trinity of Gravity. UNIVERSE, 5(7), Article ID 173.
Open this publication in new window or tab >>The Geometrical Trinity of Gravity
2019 (English)In: UNIVERSE, ISSN 2218-1997, Vol. 5, no 7, article id 173Article in journal (Refereed) Published
Abstract [en]

The geometrical nature of gravity emerges from the universality dictated by the equivalence principle. In the usual formulation of General Relativity, the geometrisation of the gravitational interaction is performed in terms of the spacetime curvature, which is now the standard interpretation of gravity. However, this is not the only possibility. In these notes, we discuss two alternative, though equivalent, formulations of General Relativity in flat spacetimes, in which gravity is fully ascribed either to torsion or to non-metricity, thus putting forward the existence of three seemingly unrelated representations of the same underlying theory. Based on these three alternative formulations of General Relativity, we then discuss some extensions.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
general relativity, modified gravity, torsion, non-metricity
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-257582 (URN)10.3390/universe5070173 (DOI)000478769900006 ()2-s2.0-85070407323 (Scopus ID)
Note

QC 20190920

Available from: 2019-09-20 Created: 2019-09-20 Last updated: 2019-09-20Bibliographically approved
Koivisto, T. S. & Tsimperis, G. (2019). The Spectrum of Teleparallel Gravity. UNIVERSE, 5(3), Article ID 80.
Open this publication in new window or tab >>The Spectrum of Teleparallel Gravity
2019 (English)In: UNIVERSE, ISSN 2218-1997, Vol. 5, no 3, article id 80Article in journal (Refereed) Published
Abstract [en]

The observer's frame is the more elementary description of the gravitational field than the metric. The most general covariant, even-parity quadratic form for the frame field in arbitrary dimension generalises the New General Relativity by nine functions of the d'Alembertian operator. The degrees of freedom are clarified by a covariant derivation of the propagator. The consistent and viable models can incorporate an ultra-violet completion of the gravity theory, an additional polarisation of the gravitational wave, and the dynamics of a magnetic scalar potential.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
teleparallel theory of gravity, nonlocal theories of gravity
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-251222 (URN)10.3390/universe5030080 (DOI)000464055600003 ()2-s2.0-85064399933 (Scopus ID)
Note

QC 20190524

Available from: 2019-05-24 Created: 2019-05-24 Last updated: 2019-05-24Bibliographically approved
Koivisto, T. S. (2018). An integrable geometrical foundation of gravity. International Journal of Geometric Methods in Modern Physics (IJGMMP), 15, Article ID 1840006.
Open this publication in new window or tab >>An integrable geometrical foundation of gravity
2018 (English)In: International Journal of Geometric Methods in Modern Physics (IJGMMP), ISSN 0219-8878, Vol. 15, article id 1840006Article in journal (Refereed) Published
Abstract [en]

In a talk at the conference Geometrical Foundations of Gravity at Tartu, 2017, it was suggested that the affine spacetime connection could be associated with purely fictitious forces. This leads to gravitation in a flat and smooth geometry. Fermions are found to nevertheless couple with the metrical connection and a phase gauge field. The theory is reviewed in this proceeding in a Palatini, and in a metric-affine gauge formulation.

Place, publisher, year, edition, pages
World Scientific Publishing Co. Pte Ltd, 2018
Keywords
Gauge theories of gravitation, Palatini variation, geometrical foundations
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-240784 (URN)10.1142/S0219887818400066 (DOI)000453056800007 ()2-s2.0-85053039556 (Scopus ID)
Note

QC 20190110

Available from: 2019-01-10 Created: 2019-01-10 Last updated: 2019-01-10Bibliographically approved
Beltran Jimenez, J., Heisenberg, L. & Koivisto, T. S. (2018). Coincident general relativity. Physical Review D: covering particles, fields, gravitation, and cosmology, 98(4), Article ID 044048.
Open this publication in new window or tab >>Coincident general relativity
2018 (English)In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 98, no 4, article id 044048Article in journal (Refereed) Published
Abstract [en]

The metric-affine variational principle is applied to generate teleparallel and symmetric teleparallel theories of gravity. From the latter we discover an exceptional class which is consistent with a vanishing affine connection. Based on this remarkable property, this work proposes a simpler geometrical formulation of general relativity that is oblivious to the affine spacetime structure, thus fundamentally depriving gravity of any inertial character. The resulting theory is described by the Hilbert action purged from the boundary term and is more robustly underpinned by the spin-2 field theory, where an extra symmetry is now manifest, possibly related to the double-copy structure of the gravity amplitudes. This construction also provides a novel starting point for modified gravity theories, and the paper presents new and simple generalizations where analytical self-accelerating cosmological solutions arise naturally in the early-and late-time Universe.

Place, publisher, year, edition, pages
American Physical Society, 2018
National Category
Geometry
Identifiers
urn:nbn:se:kth:diva-234593 (URN)10.1103/PhysRevD.98.044048 (DOI)000442886000010 ()2-s2.0-85052663064 (Scopus ID)
Note

QC 20180914

Available from: 2018-09-14 Created: 2018-09-14 Last updated: 2018-09-14Bibliographically approved
Golovnev, A. & Koivisto, T. S. (2018). Cosmological perturbations in modified teleparallel gravity models. Journal of Cosmology and Astroparticle Physics (11), Article ID 012.
Open this publication in new window or tab >>Cosmological perturbations in modified teleparallel gravity models
2018 (English)In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 11, article id 012Article in journal (Refereed) Published
Abstract [en]

Cosmological perturbations are considered in f (T) and in scalar-torsion f (phi) T teleparallel models of gravity. Full sets of linear perturbation equations are accurately derived and analysed at the relevant limits. Interesting features of generalisations to other teleparallel models, spatially curved backgrounds, and rotated tetrads are pointed out.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2018
Keywords
cosmological perturbation theory, modified gravity, dark energy theory
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-239806 (URN)10.1088/1475-7516/2018/11/012 (DOI)000450240200001 ()2-s2.0-85057624846 (Scopus ID)
Note

QC 20190107

Available from: 2019-01-07 Created: 2019-01-07 Last updated: 2019-01-07Bibliographically approved
Harko, T., Koivisto, T. S., Lobo, F. S. N., Olmo, G. J. & Rubiera-Garcia, D. (2018). Coupling matter in modified Q gravity. Physical Review D: covering particles, fields, gravitation, and cosmology, 98(8), Article ID 084043.
Open this publication in new window or tab >>Coupling matter in modified Q gravity
Show others...
2018 (English)In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 98, no 8, article id 084043Article in journal (Refereed) Published
Abstract [en]

We present a novel theory of gravity by considering an extension of symmetric teleparallel gravity. This is done by introducing, in the framework of the metric-affine formalism, a new class of theories where the nonmetricity Q is nonminimally coupled to the matter Lagrangian. More specifically, we consider a Lagrangian of the form L similar to f(1)(Q) + f(2)(Q)L-M, where f(1) and f(2) are generic functions of Q, and L-M is the matter Lagrangian. This nonminimal coupling entails the nonconservation of the energy-momentum tensor, and consequently the appearance of an extra force. The formulation of the gravity sector in terms of the Q instead of the curvature may result in subtle improvements of the theory. In the context of nonminimal matter couplings, we are therefore motivated to explore whether the new geometrical formulation in terms of the Q, when implemented also in the matter sector, would allow more universally consistent and viable realizations of the nonminimal coupling. Furthermore, we consider several cosmological applications by presenting the evolution equations and imposing specific functional forms of the functions f(1)(Q) and f(2)(Q), such as power-law and exponential dependencies of the nonminimal couplings. Cosmological solutions are considered in two general classes of models, and found to feature accelerating expansion at late times.

Place, publisher, year, edition, pages
American Physical Society, 2018
National Category
Mathematics
Identifiers
urn:nbn:se:kth:diva-239091 (URN)10.1103/PhysRevD.98.084043 (DOI)000448458600003 ()2-s2.0-85056200414 (Scopus ID)
Note

QC 20181121

Available from: 2018-11-21 Created: 2018-11-21 Last updated: 2019-08-20Bibliographically approved
Zlosnik, T., Urban, F., Marzola, L. & Koivisto, T. S. (2018). Spacetime and dark matter from spontaneous breaking of Lorentz symmetry. Classical and quantum gravity, 35(23), Article ID 235003.
Open this publication in new window or tab >>Spacetime and dark matter from spontaneous breaking of Lorentz symmetry
2018 (English)In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 35, no 23, article id 235003Article in journal (Refereed) Published
Abstract [en]

It is shown that a spontaneously-broken gauge theory of the Lorentz group contains Ashtekar's chiral formulation of general relativity accompanied by dust. From this perspective, gravity is described entirely by a connection omega valued in the Lie algebra of the complexified Lorentz group and a Lorentz-charged scalar field tau. The model is 'pre-geometric' in the sense that the spacetime metric may be constructed only in the symmetry-broken regime. We speculate on the extent to which this dust may account for dark matter and on the behaviour of the theory in regimes where the symmetry is not broken.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2018
Keywords
gravitation, dark matter, cosmology
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-239462 (URN)10.1088/1361-6382/aaea96 (DOI)000449540100001 ()2-s2.0-85056876999 (Scopus ID)
Note

QC 20181128

Available from: 2018-11-28 Created: 2018-11-28 Last updated: 2018-11-28Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-9403-8565

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