Particle horizon

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

Cosmological tests for determining real spatial and time dimensionalities of our Universe

Abstract: In the framework of the homogeneous and isotropic cosmological model with arbitrary (noninteger) space and time dimensionalities, we derive the three classical cosmological tests: visual bolometric magnitude, angular distance and the number of sources versus redshift. We also obtain the deceleration parameter and the age and modern radius of the Universe.

Wavepacket of the Universe and its spreading

Abstract: Wavepackets in quantum mechanics spread and the Universe in cosmology expands. We discuss a formalism where the two effects can be unified. The basic assumption is that the Universe is determined by a unitarily evolving wavepacket defined on space-time. Space-time is static but the Universe is dynamic. Spreading analogous to expansion known from observational cosmology is obtained if one regards time evolution as a discrete process with probabilities of jumps determined by a variational principle employing Kolmogorov-Nagumo-R\'enyi averages. The choice of the R\'enyi calculus implies that the form of the Universe involves an implicit fractal structure. The formalism automatically leads to two types of "time" parameters: $\tau$, with dimension of $x^0$, and dimensionless $\varepsilon=\ln \epsilon_\tau$, related to the form of diffeomorphism that defines the dynamics. There is no preferred time foliation, but effectively the dynamics leads to asymptotic concentration of the Universe on spacelike surfaces that propagate in space-time. The analysis is performed explicitly in $1+1$ dimensions, but the unitary evolution operator is brought to a form that makes generalizations to other dimensions and other fields quite natural.

Generalized Second Law of Thermodynamics in Emergent Universe

Abstract: In this letter, we have considered the FRW model of the emergent universe, which was presented in our previous work (Debnath, in Class. Quantum Gravity 25:205019, 2008). We have chosen one of the form of scale factor in such a way that the emergent scenario is possible in the universe. We have also considered the universe as a thermodynamical system with the horizon surface as a boundary of the system. The entropy and the radius of the event horizon have been calculated in the emergent scenario. When the emergent scenario occurs, we have shown that the generalized second law of thermodynamics is always satisfied for open, flat and closed models of the universe.

Measuring the baryon content of the universe: BBN versus CMB

Abstract: The relic abundance of baryons — the only form of stable matter whose existence we arecertain of — is a crucial parameter for many cosmological processes, as well as material evidence thatthere is new physics beyond the Standard Model. We discuss recent determinations of the cosmologicalbaryon density from analysis of the abundances of light elements synthesised at the end of “the firstthree minutes”, and from the observed temperature anisotropies imprinted on small angular-scales inthe cosmic microwave background when the universe was ∼ 10 5 yr old. 0.1 Introduction Begining as an uniformly distributed plasma in the radiation-dominated era, baryonsare now distributed in a hierarchy of structures, from galaxies to superclusters, whichhave formed through gravitational collapse and constitute the familiar visible universe.Although it is recognised that baryons are a dynamically unimportant component, out-weighed by the dark matter that actually holds such structures together, they constitutethe only form of stable matter which we know about and can study directly. Neverthelesstheir cosmological origin is a mystery, even harder to fathom than that of the much moredominant dark matter. This is not always appreciated, particularly by those who thinkof baryons as familiar ‘ordinary’ stuff, as opposed to the ‘exotic’ particles that particletheorists dream up such as supersymmetric neutralinos. However it follows from elemen-tary kinetic theory [1] that the relic abundance of massive particles (and anti-particles)which were in thermal equilibrium in the early universe is proportional to the inverse oftheir (velocity-averaged)self-annihilation cross-section: Ω

Cosmological tests of gravity Kazuya Koyama

Abstract: TBA The discovery of the accelerated expansion of the Universe has come relatively late in our study of the cosmos, but in showing that gravity can act repulsively, it has opened up many new questions about the nature of gravity and what the Universe might contain. Is the acceleration being driven by dark energy? Or is general relativity (GR) itself in error, requiring a modification at large scales to account for the late acceleration? Structure formation in our Universe can be different even if the geometry of the homogeneous and isotropic universe is the same in these two classes of models, offering a possibility to distinguish between them observationally. Non-linear structure formation is complicated by the fifth force that commonly appears in modified gravity models and new techniques are required to analyse it. We will discuss novel methods to test GR on cosmological scales, building on the recent developments of N-body simulations for modified gravity.