In EiBI gravity, spherical collapse yields lower linear thresholds, higher turnaround and virial overdensities, and modestly smaller turnaround radii than in ΛCDM, with effects increasing with the coupling κ̂_BI.
New insights on the matter-gravity coupling paradigm
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abstract
The coupling between matter and gravity in General Relativity is given by a proportionality relation between the stress tensor and the geometry. This is an oriented assumption driven by the fact that both the stress tensor and the Einstein tensor are divergenceless. However, General Relativity is in essence a nonlinear theory, so there is no obvious reason why the coupling to matter should be linear. On another hand, modified theories of gravity usually affect the vacuum dynamics, yet keep the coupling to matter linear. In this Letter we address the implications of consistent nonlinear gravity/matter coupling. The Eddington inspired Born-Infeld theory recently introduced by Banados and Ferreira provides an enlightening realization of such coupling modifications. We find that this theory coupled to a perfect fluid reduces to General Relativity coupled to a nonlinearly modified perfect fluid, leading to an ambiguity between modified coupling and modified equation of state. We discuss observational consequences of this degeneracy and argue that such a completion of General Relativity is viable from both an experimental and theoretical point of view through energy conditions, consistency, and singularity-avoidance perspectives. We use these results to discuss the impact of changing the coupling paradigm.
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Nonlinear dark-sector interaction models with a half-saturation sparseness scale are observationally preferred over their linear counterparts at >95% confidence for two of three cases.
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Studying spherical collapse and its implications in the Eddington-inspired Born-Infeld gravity theory
In EiBI gravity, spherical collapse yields lower linear thresholds, higher turnaround and virial overdensities, and modestly smaller turnaround radii than in ΛCDM, with effects increasing with the coupling κ̂_BI.
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Saturation Mechanisms in the Interacting Dark Sector
Nonlinear dark-sector interaction models with a half-saturation sparseness scale are observationally preferred over their linear counterparts at >95% confidence for two of three cases.