arXiv subjects

Miguel Zumalacarregui

Publications and source records attributed to Miguel Zumalacarregui.

23 records · Page 2Linked to original sources

arXiv2013

DBI Galileons in the Einstein Frame: Local Gravity and Cosmology

It is shown that a disformally coupled theory in which the gravitational sector has the Einstein-Hilbert form is equivalent to a quartic DBI Galileon Lagrangian, possessing non-linear higher derivative interactions, and hence allowing for the Vainshtein effect. This Einstein Frame description considerably simplifies the dynamical equations and highlights the role of the different terms. The study of highly dense, non-relativistic environments within this description unravels the existence of a disformal screening mechanism, while the study of static vacuum configurations reveals the existence of a Vainshtein radius, at which the asymptotic solution breaks down. Disformal couplings to matter also allow the construction of Dark Energy models, which behave differently than conformally coupled ones and introduce new effects on the growth of Large Scale Structure over cosmological scales, on which the scalar force is not screened. We consider a simple Disformally Coupled Dark Matter model in detail, in which standard model particles follow geodesics of the gravitational metric and only Dark Matter is affected by the disformal scalar field. This particular model is not compatible with observations in the linearly perturbed regime. Nonetheless, disformally coupled theories offer enough freedom to construct realistic cosmological scenarios, which can be distinguished from the standard model through characteristic signatures.

astro-ph.CO↗

arXiv2012

Screening Modifications of Gravity through Disformally Coupled Fields

It is shown that extensions to General Relativity, which introduce a strongly coupled scalar field, can be viable if the interaction has a non-conformal form. Such disformal coupling depends upon the gradients of the scalar field. Thus, if the field is locally static and smooth, the coupling becomes invisible in the solar system: this is the disformal screening mechanism. A cosmological model is considered where the disformal coupling triggers the onset of accelerated expansion after a scaling matter era, giving a good fit to a wide range of observational data. Moreover, the interaction leaves signatures in the formation of large-scale structure that can be used to probe such couplings.

astro-ph.CO↗

arXiv2012

Tension in the Void: Cosmic Rulers Strain Inhomogeneous Cosmologies

New constraints on inhomogeneous Lemaître-Tolman-Bondi (LTB) models alternative to Dark Energy are presented, focusing on adiabatic profiles with space-independent Big Bang and baryon fraction. The Baryon Acoustic Oscillation (BAO) scale at early times is computed in terms of the asymptotic value and then projected to different redshifts by following the geodesics of the background metric. Additionally, a model-independent method to constraint the local expansion rate using a prior on supernovae luminosity is presented. Cosmologies described by an adiabatic GBH matter profile with Ω_{out}=1 and Ω_{out}\leq 1 are investigated using a Markov Chain Monte Carlo analysis including the latest BAO data from the WiggleZ collaboration and the local expansion rate from the Hubble Space Telescope, together with Union-II type Ia supernovae data and the position and height of the Cosmic Microwave Background acoustic peaks. The addition of BAO data at higher redshifts increases considerably their constraining power and represents a new drawback for this type of models, yielding a value of the local density parameter Ω_{in}>0.2 which is 3σapart from the value Ω_{in}<0.15 found using supernovae. The situation does not improve if the asymptotic flatness assumption is dropped, and a Bayesian analysis shows that constrained GBH models are ruled out at high confidence. We emphasize that these are purely geometric probes, that only recently have become sufficiently constraining to independently rule out the whole class of adiabatic LTB models.

astro-ph.CO↗

arXiv2012

Modified Entropic Gravity and Cosmology

It has been recently proposed that gravity might be an entropic force. Although a well defined fundamental description for such a mechanism is still lacking, it is still possible to address the viability of phenomenological models of entropic-inspired modified gravities. I will summarize some recent work directed to using cosmology as a tool to constraint scenarios in which the modifications are aimed to explain the physics behind dark energy and inflation. A phenomenological modification is able to explain cosmic acceleration at the background level and fit observations, but simple inflation models with higher curvature corrections are in conflict with late time matter domination.

astro-ph.CO↗

arXiv2011

Constraining Entropic Cosmology

It has been recently proposed that the interpretation of gravity as an emergent, entropic force might have nontrivial implications to cosmology. Here two approaches are investigated: in one, the Friedman equation receives entropic contributions from the usually neglected surface terms, and in another, the extra terms are derived from quantum corrections to the entropy formula. UV terms may drive inflation, avoiding a recently derived no-go theorem, though in some cases leading to a graceful exit problem. IR terms can generate dark energy, alleviating the cosmological constant problem. The quantum corrections are bounded by their implications to the BBN, and the surface terms are constrained in addition by their effect upon the behavior of matter. Likelihood analyses are performed to constrain the modifications by the SNeIa, BAO and CMB data. It is found that a monomial correction to the area-entropy formula results in late acceleration in very good agreement with observations, which then turn out to be compatible with positive curvature. The evolution of perturbations is deduced by assuming the Jebsen-Birkhoff theorem. Distinct signatures can then be identified in the large scale structure formation. Furthermore, it is shown that the visible universe satisfies the Bekenstein bound.

astro-ph.CO↗