Homogeneous Cosmological Background Research Articles

Consistent cosmological structure formation on all scales in relativistic extensions of MOND

General relativity manifests very similar equations in different regimes, notably in large scale cosmological perturbation theory, non-linear cosmological structure formation, and in weak field galactic dynamics. The same is not necessarily true in alternative gravity theories, in particular those that possess MONDian behaviour (“relativistic extensions” of MOND). In these theories different regimes are typically studied quite separately, sometimes even with the freedom in the theories chosen differently in different regimes. If we wish to properly and fully test complete cosmologies containing MOND against the ΛCDM paradigm then we need to understand cosmological structure formation on all scales, and do so in a coherent and consistent manner. We propose a method for doing so and apply it to generalised Einstein-Aether theories as a case study. We derive the equations that govern cosmological structure formation on all scales in these theories and show that the same free function (which may contain both Newtonian and MONDian branches) appears in the cosmological background, linear perturbations, and non-linear cosmological structure formation. We show that MONDian behaviour on galactic scales does not necessarily result in MONDian behaviour on cosmological scales, and for MONDian behaviour to arise cosmologically, there will be no modification to the Friedmann equations governing the evolution of the homogeneous cosmological background. We comment on how existing N-body simulations relate to complete and consistent generalised Einstein-Aether cosmologies. The equations derived in this work allow consistent cosmological N-body simulations to be run in these theories whether or not MONDian behaviour manifests on cosmological scales.

Dynamic stability of [formula omitted]-varying non-zero torsion cosmology

This paper aims to study the stability analysis of Friedmann-like spacetime in the presence of torsion through dynamical system techniques. For this purpose, we consider equations of motion which describe the evolution in an isotropic and homogeneous cosmological background with non-vanishing torsion and convert them into the first order autonomous system of differential equations. In order to identify system’s equilibrium points, we implement an ansatz for the torsion term ϕ(t)=λH. Furthermore, within this framework, the cosmological scenario of the varying vacuum is being considered where dark matter interacts with vacuum. We take into account various forms of interaction term to study the effects of linear and non-linear interaction and examine the stability in each case. It is found that these cosmological solutions can describe different evolutionary phases of the universe and behave as stable attractor for the specific values of the parameters. For each case of interaction model, the phase space portraits exhibit the attractive behavior of the universe.

Phase space description of nonlocal teleparallel gravity

We study cosmological solutions in nonlocal teleparallel gravity or f(T) theory, where T is the torsion scalar in teleparallel gravity. This is a natural extension of the usual teleparallel gravity with nonlocal terms. In this work the phase space portrait proposed to describe the dynamics of an arbitrary flat, homogeneous cosmological background with a number of matter contents, both in early- and late-time epochs. The aim was to convert the system of the equations of the motion to a first order autonomous dynamical system and to find fixed points and attractors using numerical codes. For this purpose, firstly we derive effective forms of cosmological field equations describing the whole cosmic evolution history in a homogeneous and isotropic cosmological background and construct the autonomous system of the first order dynamical equations. In addition, we investigate the local stability in the dynamical systems called “the stable/unstable manifold” by introducing a specific form of the interaction between matter, dark energy, radiation and a scalar field. Furthermore, we explore the exact solutions of the cosmological equations in the case of a de Sitter spacetime. In particular, we examine the role of an auxiliary function called “gauge” eta in the formation of such cosmological solutions and show whether the de Sitter solutions can exist or not. Moreover, we study the stability issue of the de Sitter solutions both in vacuum and non-vacuum spacetimes. It is demonstrated that, for nonlocal f(T) gravity, the stable de Sitter solutions can be produced even in vacuum spacetime.

Cosmological law of universal gravitation

Without exceeding the limits of the conventional ΛCDM paradigm, we argue for Yukawa law of interparticle interaction as the law of gravitation in the real expanding inhomogeneous universe. It covers the whole space and comes up to take place of Newtonian gravity, which is restricted exclusively to sub-horizon distances. The large-scale screening of gravitational interaction between every two nonrelativistic massive particles is ensured by the homogeneous cosmological background (specifically, by the nonzero average rest mass density of nonrelativistic matter). We take advantage of the uniform matter distribution case (i.e. the homogeneous universe limit) to demonstrate superiority of Yukawa gravity. Attention is also devoted to the concrete particular case of inhomogeneity.

Magnetization of fluid phonons and large-scale curvature perturbations

The quasinormal mode of a gravitating and magnetized fluid in a spatially flat, isotropic and homogeneous cosmological background is derived in the presence of the fluid sources of anisotropic stress and of the entropic fluctuations of the plasma. The obtained gauge-invariant description involves a system of two coupled differential equations of which the physical content is analyzed in all the most relevant situations. The Cauchy problem of large-scale curvature perturbations during the radiation dominated stage of expansion can be neatly formulated, and its general solution is shown to depend on five initial data assigned when the relevant physical wavelengths are larger than the particle horizon. The consequences of this approach are explored.

O(d,d)-invariance in Inhomogeneous String Cosmologies with Perfect Fluid

In the first part of the present paper, we showthat O(d,d)-invariance usually known in a homogeneouscosmological background written in terms of proper timecan be extended to backgrounds depending on one or several coordinates [which may be anyspace-like or time-like coordinate(s)]. In all cases,the presence of a perfect fluid is taken into accountand the equivalent duality transformation in Einstein frame is explicitly given. In the second part,we present several concrete applications to somefour-dimensional metrics, including inhomogeneous ones,which illustrate the different duality transformations discussed in the first part. Note that most ofthe dual solutions given here do not seem to be known inthe literature.

Relativistic plasma in a homogeneous cosmological background.

The linearized theory of a relativistic plasma in a radiation background, proposed recently by Holcomb and Tajima, is extended for a more general background with the metric components being an arbitrary power function of time (\ensuremath{\sim}${\mathit{t}}^{\mathit{n}}$). It is noted that the electric field falls off faster and the redshift of the electromagnetic radiation frequency is also greater in a more rapidly expanding background universe. The dispersion relation for the transverse vibration of the electromagnetic field, however, appears to be independent of the parameter n. The wave equation for the electric field is solved for a de Sitter background also.

Multisoliton solutions to Einstein's equations.

We discuss a multisoliton solution to Einstein's equations in vacuum. The solution is interpreted as many gravitational solitons propagating and colliding on a homogeneous cosmological background. Following a previous letter, we characterize the solitons by their localizability and by their peculiar properties under collisions. Furthermore, we define an associated frame-dependent velocity field which illustrates the solitonic character of these gravitational solitons in the classical sense.

L'Univers à grande échelle Les grandes structures et leur formation

A few years ago, the elaboration of catalogues of galaxies based on decades of observations seemed to give the clue of the actual distribution of matter in the Universe. Moreover, the study of the growth by gravitational instability of initial inhomogeneities spread out on a globally homogeneous cosmological background gave some light on the formation of galaxies. However, technological developments allowing the obtention of large samples of recessional velocities of galaxies leading via the Hubble law to the 3rd dimension have modified the preceding vision. Galaxies seem now to cluster in large aggregates (clusters and superclusters) organized in an interconnected network with large voids, empty of bright galaxies. Besides, if galaxy formation occurs only in specially selected regions, this knowledge of the large scale structure of the Universe does not reflect the true distribution of matter ! One loophole comes probably from elementary particle physics. The effort to unify fundamental interactions appears to have profound cosmological implications. Perhaps, the matter of our world (baryons) is only a negligible part of the (dark) matter constituted of axions, or neutrinos, photinos... governing the dynamics of the whole Universe and these « cold » or « hot » « inos » thus play a main role in the formation of the observed structures. In this domain, things evolve very rapidly, and this paper tends to give a snapshot of the strange situation to which astrophysicians are now confronted. Stellar studies have precedingly shown the close link between stellar and atomic scales. If confirmed, the recent developments of Cosmology and Particle Physics seem to establish the ultimate connection between the Cosmos and the microworld.