Inhomogeneous Cosmologies Research Articles

Inhomogeneous cosmology and backreaction: Current status and future prospects

Astronomical observations reveal hierarchical structures in the universe, from galaxies, groups of galaxies, clusters and superclusters, to filaments and voids. On the largest scales, it seems that some kind of statistical homogeneity can be observed. As a result, modern cosmological models are based on spatially homogeneous and isotropic solutions of the Einstein equations, and the evolution of the universe is approximated by the Friedmann equations. In parallel to standard homogeneous cosmology, the field of inhomogeneous cosmology and backreaction is being developed. This field investigates whether small scale inhomogeneities via nonlinear effects can backreact and alter the properties of the universe on its largest scales, leading to a non-Friedmannian evolution. This paper presents the current status of inhomogeneous cosmology and backreaction. It also discusses future prospects of the field of inhomogeneous cosmology, which is based on a survey of 50 academics working in the field of inhomogeneous cosmology.

Lecture Notes on Gauge Theories and Inhomogeneous Cosmologies

<p>These lecture notes were meant to compile the main ideas on gauge field theory and modern cosmology, showing the recent contributions of the author in these areas, including classical thermodynamics, supergravity and quantum gravity. Although the author has conducted the work without wishing to exhaust the issues in question, this synthesis provides some essential formal approaches for further studies, with relevant and updated references, as well as indications of classical readings. Special attention was given to the application of Lyra's geometry, because of its growing importance in quantum cosmology, and to the so-called <<paleogravity>>, a model of supergravity developed by the author with the purpose to offer a classical representation for supergravity that could be made compatible with the quantum theory of spacetime also developed by himself. Also in general cosmology, the emphasis was on inhomogeneous models due to the debate they open on the validity of the standard model. In this sense, the author presented his perturbative formalism of weak gravitational lensing by defining an inhomogeneous cosmological refractive index within a Lemaître-Tolman universe. Lastly, some remarks on Stephani cosmology were organized in order to expose the most significant features of this approach. During the reading, it shall be possible to observe some title marks indicating items of interest, as well as small side texts with outgivings of renowned authors.</p><p><strong>Key words</strong><span>: gauge field theory, cosmology, Lyra's geometry, quantum gravity, supergravity.</span></p><p><span>======================================================================================================</span> </p><p>A presente coletânea de notas de aula foi organizada como compilação das principais idéias sobre teoria de campo de calibre e cosmologia moderna, mostrando as recentes contribuições do autor nessas áreas, incluindo termodinâmica clássica aplicada, supergravidade e gravitação quântica. Embora o trabalho tenha sido conduzido sem pretensões a esgotar os temas em apreço, a síntese produzida trata formalmente alguns aspectos essenciais para estudos posteriores, com referências relevantes e atualizadas, bem como indicações de leituras clássicas. Especial atenção foi dada à aplicação da geometria de Lyra, por sua crescente importância na cosmologia quântica, e à chamada << paleogravidade >>, um modelo de supergravidade desenvolvido pelo autor com o propósito de oferecer uma representação clássica da supergravidade, o qual seria compatível com a teoria quântica do espaço-tempo também desenvolvida pelo autor. Em cosmologia geral, foi dada ênfase em modelos não-homogêneos devido ao amplo debate que eles abrem sobre a validade do modelo padrão. Nesse sentido, o autor apresentou seu formalismo perturbativo de lentes gravitacionais fracas, definindo um índice de refração cosmológico inomogêneo dentro de um universo de Lemaître-Tolman. Por fim, algumas observações sobre a cosmologia de Stephani foram organizadas para expor as características mais significativas dessa abordagem. Durante a leitura, será possível observar algumas marcas indicativas de itens de interesse, bem como pequenos textos marginais com comentários de autores renomados.</p><p><strong>Palavras-chave</strong>: teoria de gauge, cosmologia, geometria de Lyra, gravidade quântica, supergravidade.</p><div><span style="font-size: 14px;"><br /></span></div>

Two-point correlation functions in inhomogeneous and anisotropic cosmologies

Two-point correlation functions are ubiquitous tools of modern cosmology, appearing in disparate topics ranging from cosmological inflation to late-time astrophysics. When the background spacetime is maximally symmetric, invariance arguments can be used to fix the functional dependence of this function as the invariant distance between any two points. In this paper we introduce a novel formalism which fixes this functional dependence directly from the isometries of the background metric, thus allowing one to quickly assess the overall features of Gaussian correlators without resorting to the full machinery of perturbation theory. As an application we construct the CMB temperature correlation function in one inhomogeneous (namely, an off-center LTB model) and two spatially flat and anisotropic (Bianchi) universes, and derive their covariance matrices in the limit of almost Friedmannian symmetry. We show how the method can be extended to arbitrary N-point correlation functions and illustrate its use by constructing three-point correlation functions in some simple geometries.

The background Friedmannian Hubble constant in relativistic inhomogeneous cosmology and the age of the Universe

Context. In relativistic inhomogeneous cosmology, structure formation couples to average cosmological expansion. A conservative approach to modelling this assumes an Einstein-de Sitter model (EdS) at early times and extrapolates this forward in cosmological time as a “background model” against which average properties of today’s Universe can be measured. Aims. This modelling requires adopting an early-epoch-normalised background Hubble constant Hbg1. Methods. Here, we show that the ΛCDM model can be used as an observational proxy to estimate Hbg1 rather than choose it arbitrarily. We assume (i) an EdS model at early times; (ii) a zero dark energy parameter; (iii) bi-domain scalar averaging-division of the spatial sections into over- and underdense regions; and (iv) virialisation (stable clustering) of collapsed regions. Results. We find Hbg1= 37.7 ± 0.4 km s-1/ Mpc (random error only) based on a Planck ΛCDM observational proxy. Conclusions. Moreover, since the scalar-averaged expansion rate is expected to exceed the (extrapolated) background expansion rate, the expected age of the Universe should be much younger than 2/(3Hbg1) = 17.3 Gyr. The maximum stellar age of Galactic bulge microlensed low-mass stars (most likely: 14.7 Gyr; 68% confidence: 14.0–15.0 Gyr) suggests an age of about a Gyr older than the (no-backreaction) ΛCDM estimate.

Inhomogeneous anisotropic cosmology

In homogeneous and isotropic Friedmann-Robertson-Walker cosmology, the topology of the universe determines its ultimate fate. If the Weak Energy Condition is satisfied, open and flat universes must expand forever, while closed cosmologies can recollapse to a Big Crunch. A similar statement holds for homogeneous but anisotropic (Bianchi) universes. Here, we prove that arbitrarily inhomogeneous and anisotropic cosmologies with ``flat'' (including toroidal) and ``open'' (including compact hyperbolic) spatial topology that are initially expanding must continue to expand forever at least in some region at a rate bounded from below by a positive number, despite the presence of arbitrarily large density fluctuations and/or the formation of black holes. Because the set of 3-manifold topologies is countable, a single integer determines the ultimate fate of the universe, and, in a specific sense, most 3-manifolds are ``flat'' or ``open''. Our result has important implications for inflation: if there is a positive cosmological constant (or suitable inflationary potential) and initial conditions for the inflaton, cosmologies with ``flat'' or ``open'' topology must expand forever in some region at least as fast as de Sitter space, and are therefore very likely to begin inflationary expansion eventually, regardless of the scale of the inflationary energy or the spectrum and amplitude of initial inhomogeneities and gravitational waves. Our result is also significant for numerical general relativity, which often makes use of periodic (toroidal) boundary conditions.

Remarks on inhomogeneous anisotropic cosmology

Recently a new no-global-recollapse argument was given for some inhomogeneous and anisotropic cosmologies that utilizes surface deformation by the mean curvature flow. In this paper we discuss important properties of the mean curvature flow of spacelike surfaces in Lorentzian manifolds. We show that singularities may form during cosmic evolution and the theorems forbidding the global recollapse lose their validity. The time evolution of the spatial scalar curvature that may kinematically prevent the recollapse is determined in normal coordinates, which shows the impact of inhomogeneities explicitly. Our analysis indicates a caveat in numerical solutions that give rise to inflation.

Cosmological backreaction in higher-derivative gravity expansions

We calculate a general effective stress-energy tensor induced by cosmological inhomogeneity in effective theories of gravity where the action is Taylor-expandable in the Riemann tensor and covariant derivatives of the Riemann tensor. This is of interest as an effective fluid that might provide an alternative to the cosmological constant, but it also applies to gravitational waves. We use an adaptation of Green and Wald's weak-averaging framework, which averages over perturbations in the field equation where the perturbation length scales are small compared to the averaging scale. In this adaptation, the length scale of the effective theory, 1/M, is also taken to be small compared with the averaging scale. This ensures that the perturbation length scales remain in fixed proportion to the length scale of the effective theory as the cosmological averaging scale is taken to be large. We find that backreaction from higher-derivative terms in the effective action can continue to be important in the late universe, given a source of sufficiently high-frequency metric perturbations. This backreaction might also provide a window on exotic particle physics in the far ultraviolet.

Problem of time in slightly inhomogeneous cosmology

The problem of time (PoT) is a multi-faceted conceptual incompatibility between various areas of Theoretical Physics. While usually stated as between GR and QM, in fact 8/9ths of it is already present at the classical level. Thus we adopt a ‘top-down’ classical and then quantum approach. I consider a local resolution to the PoT that is Machian, which was previously realized for relational triangle and minisuperspace models. This resolution has three levels: classical, semiclassical and combined semiclassical-histories-records. This article’s specific model is a slightly inhomogeneous cosmology considered for now at the classical level. This is motivated by how the inhomogeneous fluctuations that underlie structure formation—galaxies and CMB hotspots—might have been seeded by quantum cosmological fluctuations, as magnified by some inflationary mechanism. In particular, I consider the perturbations about case of this involving up to second order, which has a number of parallels with the Halliwell–Hawking model but has a number of conceptual differences and useful upgrades. The article’s main features are that the elimination part of the model’s thin sandwich is straightforward, but the modewise split of the constraints fail to be first-class constraints. Thus the elimination part only arises as an intermediate geometry between superspace and Riem. The reduced geometries have surprising singularities influenced by the matter content of the Universe, though the N-body problem anticipates these with its collinear singularities. I also give a ‘basis set’ of Kuchař beables for this model arena.

A simple, heuristic derivation of our ‘no backreaction’ results

We provide a simple discussion of our results on the backreaction effects of density inhomogeneities in cosmology without mentioning one-parameter families or weak limits. Emphasis is placed on the manner in which ‘averaging’ is done and the fact that one is solving Einstein’s equation. The key assumptions and results that we rigorously derived within our original mathematical framework are thereby explained in a heuristic way.

An Inhomogeneous Space–Time Patching Model Based on a Nonlocal and Nonlinear Schrödinger Equation

We consider an integrable, nonlocal and nonlinear, Schr\"odinger equation (NNSE) as a model for building space-time patchings in inhomogeneous loop quantum cosmology (LQC). We briefly review exact solutions of the NNSE, specially those obtained through "geometric equivalence" methods. Furthemore, we argue that the integrability of the NNSE could be linked to consistency conditions derived from LQC, under the assumption that the patchwork dynamics behaves as an integrable many-body system.

Inhomogeneous cosmology with quasi-vacuum effective equation of state on Lyra manifold

<p>A class of inhomogeneous Lemaître-Tolman cosmological models is obtained in the context of Lyra’s geometry. Cosmological models in Lyra’s geometry are studied under the condition of the minimal coupling of matter with the displacement vector field and the varying Λ term. Exact solutions to the model equations are obtained subject to the quasi-vacuum effective equation of state. As a result, the displacement field as well as the cosmological term can be expressed in terms of the energy density of matter. The rate of expansion and the deceleration parameter of the model are also studied</p>

Pressure in Lemaître–Tolman–Bondi solutions and cosmologies

Lemaître–Tolman–Bondi solutions have traditionally been confined to systems with no pressure in which the gravity is due to massive dust, but the solutions are little changed in form if, as in cosmology, the pressure is uniform in space at each comoving time. This allows the equations of cosmology to be deduced in a manner that more closely resembles classical mechanics. It also gives some inhomogeneous solutions with growing condensations and black holes. We give criteria by which the sizes of different closed models of the Universe can be compared and discuss conditions for self-closure of inhomogeneous cosmologies with a -term.

Magnetized cosmological perturbations in the post-recombination era

We study inhomogeneous magnetised cosmologies through the post-recombination era in the framework of Newtonian gravity and the ideal-magnetohydrodynamic limit. The nonlinear kinematic and dynamic equations are derived and linearised around the Newtonian counterpart of the Einstein-de Sitter universe. This allows for a direct comparison with the earlier relativistic treatments of the issue. Focusing on the evolution of linear density perturbations, we provide new analytic solutions which include the effects of the magnetic pressure as well as those of the field's tension. We confirm that the pressure of field inhibits the growth of density distortions and can induce a purely magnetic Jeans length. On scales larger than the aforementioned characteristic length the inhomogeneities grow, though slower than in non-magnetised universes. Wavelengths smaller than the magnetic Jeans length typically oscillate with decreasing amplitude. We also identify a narrow range of scales, just below the Jeans length, where the perturbations exhibit a slower power-law decay. In all cases, the effect of the field is proportional to its strength and increases as we move to progressively smaller lengths.

Generalized Second Law of Thermodynamics in Parabolic LTB Inhomogeneous Cosmology**Supported financially by Research Institute for Astronomy & Astrophysics of Maragha (RIAAM), Iran

We study thermodynamics of the parabolic Lemaitre–Tolman–Bondi (LTB) cosmology supported by a perfect fluid source. This model is the natural generalization of the flat Friedmann–Robertson–Walker (FRW) universe, and describes an inhomogeneous universe with spherical symmetry. After reviewing some basic equations in the parabolic LTB cosmology, we obtain a relation for the deceleration parameter in this model. We also obtain a condition for which the universe undergoes an accelerating phase at the present time. We use the first law of thermodynamics on the apparent horizon together with the Einstein field equations to get a relation for the apparent horizon entropy in LTB cosmology. We find out that in LTB model of cosmology, the apparent horizon's entropy could be feeded by a term, which incorporates the effects of the inhomogeneity. We consider this result and get a relation for the total entropy evolution, which is used to examine the generalized second law of thermodynamics for an accelerating universe. We also verify the validity of the second law and the generalized second law of thermodynamics for a universe filled with some kinds of matters bounded by the event horizon in the framework of the parabolic LTB model.

Origin of structure in the universe: quantum cosmology reconsidered

Based on a more careful canonical analysis, we motivate a reduced quantization—in the sense of superspace quantization—of slightly inhomogeneous cosmology in place of the Dirac quantization in the existing literature, and provide it in the vacuum case. This is attained through consideration of configuration space geometries at various levels of reduction. Some of these have the good fortunate of being flat. Geometrically natural coordinates for these are interpreted in terms of the original redundant formulation’s well-known mode expansion coefficients.

Identifying cosmological perturbations in group field theory condensates

AbstractOne 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 idea has been successfully applied within the setting of group field theory (GFT), a quantum field theory of ‘atoms of space’ which can form such a condensate. We further clarify the interpretation of this mean-field approximation, and show how it can be used to obtain a semiclassical description of the GFT, in which the mean field encodes a classical statistical distribution of geometric data. In this sense, GFT condensates are quantum homogeneous geometries that also contain statistical information about cosmological inhomogeneities. We show in the isotropic case how this information can be extracted from geometric GFT observables and mapped to quantities of observational interest. Basic uncertainty relations of (non-commutative) Fourier transforms imply that this statistical description can only be compatible with the observed near-homogeneity of the Universe if the typical length scale associated to the distribution is much larger than the fundamental ‘Planck’ scale. As an example of effective cosmological equations derived from the GFT dynamics, we then use a simple approximation in one class of GFT models to derive the ‘improved dynamics’ prescription of holonomy corrections in loop quantum cosmology.

Modeling effective FRW cosmologies with perfect fluids from states of the hybrid quantum Gowdy model

We employ recently developed approximation methods in the hybrid quantization of the Gowdy ${T}^{3}$ model with linear polarization and a massless scalar field to obtain physically interesting solutions of this inhomogeneous cosmology. More specifically, we propose some particular approximate solutions of the quantum Gowdy model constructed in such a way that, for the Hamiltonian constraint, they effectively behave as those corresponding to a flat homogeneous and isotropic universe filled with a perfect fluid, even though these quantum states are far from being homogeneous and isotropic. We analyze how one can get different perfect fluid effective behaviors, including the cases of dust, radiation, and a cosmological constant.

Inhomogeneous viscous fluids for inflation

In this paper, we investigate inhomogeneous viscous fluid cosmology for inflation. Several toy models are presented in the attempt to analyze how inflation can be realized according with cosmological data by making use of an inhomogeneous EoS parameter for the fluid and/or by introducing a viscosity to have a graceful exit from inflation. The results will be compared with the ones of scalar field representation and discussed. We will pay attention on the possibility to recover the reheating and therefore the Friedmann universe.

Cosmological perturbations: Vorticity, isocurvature and magnetic fields

In this paper, I review some recent, interlinked, work undertaken using cosmological perturbation theory — a powerful technique for modeling inhomogeneities in the universe. The common theme which underpins these pieces of work is the presence of nonadiabatic pressure, or entropy, perturbations. After a brief introduction covering the standard techniques of describing inhomogeneities in both Newtonian and relativistic cosmology, I discuss the generation of vorticity. As in classical fluid mechanics, vorticity is not present in linearized perturbation theory (unless included as an initial condition). Allowing for entropy perturbations, and working to second order in perturbation theory, I show that vorticity is generated, even in the absence of vector perturbations, by purely scalar perturbations, the source term being quadratic in the gradients of first order energy density and isocurvature, or nonadiabatic pressure perturbations. This generalizes Crocco's theorem to a cosmological setting. I then introduce isocurvature perturbations in different models, focusing on the entropy perturbation in standard, concordance cosmology, and in inflationary models involving two scalar fields. As the final topic, I investigate magnetic fields, which are a potential observational consequence of vorticity in the early universe. I briefly review some recent work on including magnetic fields in perturbation theory in a consistent way. I show, using solely analytical techniques, that magnetic fields can be generated by higher order perturbations, albeit too small to provide the entire primordial seed field, in agreement with some numerical studies. I close this paper with a summary and some potential extensions of this work.

Problem of time: facets and Machian strategy

The problem of time is that "time" in each of ordinary quantum theory and general relativity are mutually incompatible notions. This causes difficulties in trying to put these two theories together to form a theory of quantum gravity. The problem of time has eight facets in canonical approaches. I clarify that all but one of these facets already occur at the classical level, and reconceptualize and re-name some of these facets as follows. The frozen formalism problem becomes temporal relationalism, the thin sandwich problem becomes configurational relationalism, via the notion of best matching. The problem of observables becomes the problem of beables, and the functional evolution problem becomes the constraint closure problem. I also outline how each of the global and multiple-choice problems of time have their own plurality of facets. This article additionally contains a local resolution to the problem of time at the conceptual level and which is actually realizable for the relational triangle and minisuperspace models. This resolution is, moreover, Machian, and has three levels: classical, semiclassical, and a combined semiclassical-histories-timeless records scheme. I end by delineating the current frontiers of this program toward resolution of the problem of time in the cases of full general relativity and of slightly inhomogeneous cosmology.