Journal ArticleDOI
Isotropic and semi-isotropic observations in cosmology
Roy Maartens,David R. Matravers +1 more
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In this article, the authors investigated the geometric consequences for dust universes of isotropies within the set of idealized galactic observations, including area distance, number counts, proper motions and image distortion.Abstract:
We investigate the geometric consequences for dust universes of isotropies within the set of idealized galactic observations---area distance, number counts, proper motions and image distortion. Anisotropy in the Hubble parameter is shown to indicate non-zero proper motions. If the proper motions and distortion are assumed to be zero (semi-isotropic observations), then the vorticity vanishes, the Hubble and deceleration parameters are isotropic, and Einstein's equations strongly constrain the area distance and the number counts. These constraints allow limited anisotropy, and in principle they provide indirect tests for the presence of proper motions and distortion. If, further, we assume isotropy of the area distance and number counts (isotropic observations), then we prove that Einstein's equations force the spacetime geometry to be isotropic. With the Copernican principle, this leads to an observational proof of the standard FLRW model.read more
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Beyond ΛCDM: Problems, solutions, and the road ahead
Philip Bull,Yashar Akrami,Julian Adamek,Tessa Baker,Emilio Bellini,Jose Beltrán Jiménez,Eloisa Bentivegna,Stefano Camera,Sebastien Clesse,Jonathan H. Davis,Enea Di Dio,Jonas Enander,Alan Heavens,Lavinia Heisenberg,Bin Hu,Claudio Llinares,Roy Maartens,Edvard Mörtsell,Seshadri Nadathur,Johannes Noller,Roman Pasechnik,Marcel S. Pawlowski,Thiago S. Pereira,Miguel Quartin,A. Ricciardone,Signe Riemer-Sørensen,Massimiliano Rinaldi,Jeremy Sakstein,Ippocratis D. Saltas,Vincenzo Salzano,Ignacy Sawicki,Adam R. Solomon,Adam R. Solomon,Adam R. Solomon,Douglas Spolyar,Glenn D. Starkman,Daniele A. Steer,Ismael Tereno,L. Verde,Francisco Villaescusa-Navarro,Mikael von Strauss,Hans A. Winther +41 more
TL;DR: There is a persistent interest in extending cosmology beyond the standard model, ΛCDM, motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm as mentioned in this paper.
Journal ArticleDOI
Beyond $\Lambda$CDM: Problems, solutions, and the road ahead
Philip Bull,Yashar Akrami,Julian Adamek,Tessa Baker,Emilio Bellini,Jose Beltrán Jiménez,Eloisa Bentivegna,Stefano Camera,Sebastien Clesse,Jonathan H. Davis,Enea Di Dio,Jonas Enander,Alan Heavens,Lavinia Heisenberg,Bin Hu,Claudio Llinares,Roy Maartens,Edvard Mörtsell,Seshadri Nadathur,Johannes Noller,Roman Pasechnik,Marcel S. Pawlowski,Thiago S. Pereira,Miguel Quartin,Angelo Ricciardone,Signe Riemer-Sørensen,Massimiliano Rinaldi,Jeremy Sakstein,Ippocratis D. Saltas,Vincenzo Salzano,Ignacy Sawicki,Adam R. Solomon,Douglas Spolyar,Glenn D. Starkman,Daniele A. Steer,Ismael Tereno,Licia Verde,Francisco Villaescusa-Navarro,Mikael von Strauss,Hans A. Winther +39 more
TL;DR: There is a persistent interest in extending cosmology beyond the standard model, $\Lambda$CDM as discussed by the authors, motivated by a range of apparently serious theoretical issues, involving such questions as the cosmological constant problem, the particle nature of dark matter, the validity of general relativity on large scales, the existence of anomalies in the CMB and on small scales, and the predictivity and testability of the inflationary paradigm.
Journal ArticleDOI
Accelerated expansion from structure formation
TL;DR: In this article, the authors discuss the physics of backreaction-driven accelerated expansion and show that large-scale smoothness does not imply that the effect of inhomogeneity and anisotropy on the expansion rate is small.
Journal ArticleDOI
Light-cone averaging in cosmology: formalism and applications
Maurizio Gasperini,Maurizio Gasperini,Giovanni Marozzi,Fabien Nugier,Gabriele Veneziano,Gabriele Veneziano +5 more
TL;DR: In this paper, a general gauge invariant formalism for defining cosmological averages that are relevant for observations based on light-like signals is presented, which involve either null hypersurfaces corresponding to a family of past light-cones or compact surfaces given by their intersection with timelike hypersurface.
References
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Journal ArticleDOI
A class of homogeneous cosmological models
TL;DR: In this article, the authors studied the field equations under the assumption that the source of the gravitational field is a perfect fluid, and there exists a group of motions simply transitive on three-surfaces orthogonal to the fluid flow vector.
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Isotropic Solutions of the Einstein‐Liouville Equations
TL;DR: In this article, the authors examined the gravitational field generated by a gas whose one-particle distribution function obeys the Liouville equation under the following assumptions: First, the distribution is locally isotropic in momentum space with respect to some world-velocity field; second, if the particles have rest mass zero, the gas is irrotational.
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Global existence of solutions of the spherically symmetric Vlasov-Einstein system with small initial data
Gerhard Rein,Alan D. Rendall +1 more
TL;DR: In this paper, it was shown that global asymptotically flat singularity-free solutions of the spherically symmetric Vlasov-Einstein system exist for all initial data which are sufficiently small in an appropriate sense.
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The fluid-ray tetrad formulation of Einstein's field equations
TL;DR: In this article, the authors describe the fluid-ray tetrad, which is based on the four-velocity u of the cosmological fluid and the null vector k which lies along the generators of the null geodesics in the past light cones C-(p0) centred on the worldline C.