Particle horizon

About: Particle horizon is a research topic. Over the lifetime, 2096 publications have been published within this topic receiving 69137 citations.

Scalar fields in the late Universe: The mechanical approach

Abstract: In this paper, we consider the Universe at the late stage of its evolution and deep inside the cell of uniformity. At these scales, we consider the Universe to be filled with dust-like matter in the form of discretely distributed galaxies, a minimal scalar field, the cosmological constant and radiation as matter sources. We investigate such a Universe in the mechanical approach. This means that the peculiar velocities of the inhomogeneities (in the form of galaxies) as well as fluctuations of other perfect fluids are non-relativistic. Under such conditions, we investigate the theory of scalar perturbations. We show that, at the background level, the considered scalar field has a constant equation of state (EoS) parameter $w=-1/3$. The fluctuations of the energy density and pressure of this field are defined by the interaction between scalar field background and the gravitational potential of the system. These fluctuations are concentrated around the galaxies screening their gravitational potentials. The expressions for the gravitational potentials are found for all topologies of the Universe: flat, open and closed. The averaged (over the whole Universe) values of these potentials are equal to zero which also results in zero averaged values of the energy density fluctuations of the scalar field. We also determine the form of the potential for the considered scalar field.

Solution of the horizon and flatness problems by multiple inflations

Abstract: A simple treatment is developed to describe a universe that has undergone successive generic inflations of the type described by us in a previous work. By comparing general expressions for the horizon and flatness problems it is shown that allowing ''failed'' inflationary bubbles within the visible universe results in values of ..cap omega.. that may depart logarithmically from unity. An exact relationship between a scale and which period of inflation determined its density perturbation amplitude is derived. Restricting attention to the case of double inflation, it is shown that many visible scales today fell within the event horizons of both de Sitter phases. It is assumed that the later inflation will set the density perturbation amplitude of such scales when they reenter the post-inflation horizon. The relative values of the amplitudes are examined and it is shown that the amplitudes created by earlier inflations will tend to be higher than that of later epochs. The relative values of the number of e-folds of the two inflations give rise to four combinations that solve the horizon problem. Treating all values as equally likely, a double-valued spectrum is obtained in only one, and not a preferred one, of those combinations.

On the Necessity of Phantom Fields for Solving the Horizon Problem in Scalar Cosmologies

Abstract: We discuss the particle horizon problem in the framework of spatially homogeneous and isotropic scalar cosmologies. To this purpose we consider a Friedmann–Lemaitre–Robertson–Walker (FLRW) spacetime with possibly non-zero spatial sectional curvature (and arbitrary dimension), and assume that the content of the universe is a family of perfect fluids, plus a scalar field that can be a quintessence or a phantom (depending on the sign of the kinetic part in its action functional). We show that the occurrence of a particle horizon is unavoidable if the field is a quintessence, the spatial curvature is non-positive and the usual energy conditions are fulfilled by the perfect fluids. As a partial converse, we present three solvable models where a phantom is present in addition to a perfect fluid, and no particle horizon appears.

Combinatorics, observables, and String Theory: part II

Abstract: We investigate the string configuration that, in the framework of the theoretical scenario introduced in [1], corresponds to the most entropic configuration in the phase space of all the configurations of the universe. This describes a universe with four space-time dimensions, and the physical content is phenomenologically compatible with the experimental observations and measurements. Everything is determined in terms of the age of the universe, with no room for freely-adjustable parameters. We discuss how one obtains the known spectrum of particles and interactions, with massive neutrinos, no Higgs boson, and supersymmetry broken at the Planck scale. Besides the computation of masses and couplings, CKM matrix elements, cosmological constant, expansion parameters of the universe etc..., all resulting, within the degree of the approximation we used, in agreement with the experimental observations, we also discuss how this scenario passes the tests provided by cosmology and the constraints imposed by the physics of the primordial universe.

Time Variable Cosmological Constants from Cosmological Horizons

Abstract: In this paper, motivated from the fact that a de Sitter cosmological boundary corresponds to a positive cosmological constant, we consider time variable cosmological constants, dubbed {\it horizon cosmological constants} The horizon cosmological constants correspond to Hubble horizon, future event horizon and particle horizon are discussed respectively When the Hubble horizon is taken as a cosmological length scale, the effective equation of state of horizon cosmological constant is quintessence-like The values of model parameter $c$ will determine the current status of our universe When particle horizon is taken as the cosmological length scale, non viable cosmological model can be obtained for the requirement of $\Omega_{\Lambda}<1/3$ which conflicts with current comic observations When the future event horizon is taken as the role of cosmological length scale, the forms of effective equation of state of horizon cosmological constants are the the same as the holographic ones But, their evolutions are different because of the effective interaction with cold dark matter