Particle horizon

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

How strong is the evidence for accelerated expansion

Abstract: We test the present expansion of the universe using supernova type Ia data without making any assumptions about the matter and energy content of the universe or about the parametrization of the deceleration parameter. We assume the cosmological principle to apply in a strict sense. The result strongly depends on the data set, the light curve fitting method and the calibration of the absolute magnitude used for the test, indicating strong systematic errors. Nevertheless, in a spatially flat universe there is at least 5σ evidence for acceleration which drops to 1.8σ in an open universe.

The particle versus the future event horizon in an interacting holographic dark energy model

Abstract: Choosing the future event horizon as the horizon of the flat Friedmann–Lemaitre–Robertson–Walker universe, we show that the interacting holographic dark energy model is able to explain the phantom divide line crossing. We show that if we take the particle event horizon as the horizon of the universe, besides describing the ω = −1 crossing (based on astrophysical data), we are able to determine appropriately the ratio of dark matter to dark energy density at the transition time. In this approach, after the first transition from the quintessence to the phantom phase, there is another transition from the phantom to the quintessence phase which avoids the big rip singularity.

Recycling the universe using scalar fields

Abstract: We examine the behavior of a closed oscillating universe filled with a homogeneous scalar field and find that, contrary to naive expectations, such a universe expands to larger volumes during successive expansion epochs. This intriguing behavior introduces an arrow of time in a system which is time reversible. The increase in the maximum size of the universe is closely related to the work done on or by the scalar field during one complete oscillatory cycle which, in turn, is related to the asymmetry in the scalar field equation of state during expansion and collapse. Our analysis shows that scalar fields with polynomial potentials $V(\ensuremath{\varphi})=\ensuremath{\lambda}{\ensuremath{\varphi}}^{q},$ $qg1,$ lead to a growing oscillation amplitude for the universe: the increase in amplitude between successive oscillations is more significant for smaller values of q. Such behavior allows for the effective recycling of the universe. A recycled universe can be quite old and can resolve the flatness problem. These results have strong bearing on cosmological models in which the role of dark matter is played by a scalar field. They are also relevant for chaotic inflationary models of the early universe since they demonstrate that, even if the universe fails to inflate the first time around, it will eventually do so during future oscillatory cycles. Thus, the space of initial conditions favorable for chaotic inflation increases significantly.

Non-adiabatic-like accelerated expansion of the late universe in entropic cosmology

Abstract: In ``entropic cosmology,'' instead of a cosmological constant $\ensuremath{\Lambda}$, an extra driving term is added to the Friedmann equation and the acceleration equation, taking into account the entropy and the temperature on the horizon of the universe. By means of the modified Friedmann and acceleration equations, we examine a non-adiabatic-like accelerated expansion of the universe in entropic cosmology. In this study, we consider a homogeneous, isotropic, and spatially flat universe, focusing on the single-fluid- (single-component-) dominated universe at late times. To examine the properties of the late universe, we solve the modified Friedmann and acceleration equations, neglecting high-order corrections for the early universe. We derive the continuity (conservation) equation from the first law of thermodynamics, assuming nonadiabatic expansion caused by the entropy and temperature on the horizon. Using the continuity equation, we formulate the generalized Friedmann and acceleration equations, and propose a simple model. Through the luminosity distance, it is demonstrated that the simple model agrees well with both the observed accelerated expansion of the Universe and a fine-tuned standard $\ensuremath{\Lambda}\mathrm{CDM}$ (lambda cold dark matter) model. However, we find that the increase of the entropy for the simple model is likely uniform, while the increase of the entropy for the standard $\ensuremath{\Lambda}\mathrm{CDM}$ model tends to become gradually slower, especially after the present time. In other words, the simple model predicts that the present time is not a special time, unlike for the prediction of the standard $\ensuremath{\Lambda}\mathrm{CDM}$ model.

Negative mass in general relativity

Abstract: Mechanics is considered in a universe containing negative mass. Demanding (i) conservation of momentum, (ii) principle of equivalence, (iii) no runaway motions, (iv) no Schwarzschild black holes, and (v) the inertial and active gravitational masses of a body shall have the same sign, we find thatall mass must be negative. Some properties of such a universe are investigated. We show that a neutral spherical body of arbitrarily small size is possible, and observers external to it can communicate with each other by light rays without horizon problems. There are no cosmological models with a power-law big bang, and there is an abundance of nonsingular models. Like electric charges would attract each other, and unlike ones would repel. This could produce stars and galaxies held together by charge and not gravity. The investigation does not suggest any reason why mass in the real universe should be positive.