Deceleration parameter

About: Deceleration parameter is a research topic. Over the lifetime, 1776 publications have been published within this topic receiving 89440 citations.

Is the expansion of the universe accelerating? All signs still point to yes a local dipole anisotropy cannot explain dark energy

Abstract: Type Ia supernovae (SNe Ia) provided the first strong evidence that the expansion of the universe is accelerating. With SN samples now more than ten times larger than those used for the original discovery and joined by other cosmological probes, this discovery is on even firmer ground. Two recent, related studies (Nielsen et al. 2016 and Colin et al. 2019, hereafter N16 and C19, respectively) have claimed to undermine the statistical significance of the SN Ia constraints. Rubin & Hayden (2016) (hereafter RH16) showed N16 made an incorrect assumption about the distributions of SN Ia light-curve parameters, while C19 also fails to remove the impact of the motion of the solar system from the SN redshifts, interpreting the resulting errors as evidence of a dipole in the deceleration parameter. Building on RH16, we outline the errors C19 makes in their treatment of the data and inference on cosmological parameters. Reproducing the C19 analysis with our proposed fixes, we find that the dipole parameters have little effect on the inferred cosmological parameters. We thus affirm the conclusion of RH16: the evidence for acceleration is secure.

Multipole decomposition of the general luminosity distance 'Hubble law' -- a new framework for observational cosmology

Abstract: We present the luminosity distance series expansion to third order in redshift for a general space-time with no assumption on the metric tensor or the field equations prescribing it. It turns out that the coefficients of this general 'Hubble law' can be expressed in terms of a finite number of physically interpretable multipole coefficients. The multipole terms can be combined into effective direction dependent parameters replacing the Hubble constant, deceleration parameter, curvature parameter, and 'jerk' parameter of the Friedmann-Lema\^itre-Robertson-Walker (FLRW) class of metrics. Due to the finite number of multipole coefficients, the exact anisotropic Hubble law is given by 9, 25, 61 degrees of freedom in the $\mathcal{O}(z)$, $\mathcal{O}(z^2)$, $\mathcal{O}(z^3)$ vicinity of the observer respectively, where $z\!:=\,$redshift. This makes possible model independent determination of dynamical degrees of freedom of the cosmic neighbourhood of the observer and direct testing of the FLRW ansatz. We argue that the derived multipole representation of the general Hubble law provides a new framework with broad applications in observational cosmology.

Reconstruction of modified gravity with ghost dark energy models

Abstract: In this work, we reconstruct the f(R) modified gravity for different ghost and generalized-ghost dark energy (DE) models in FRW flat universe, which describes the accelerated expansion of the universe. The equation of state and deceleration parameter of reconstructed f(R) gravity have been calculated. The equation of state and deceleration parameter of reconstructed f(R)-ghost/generalized-ghost DE, have been calculated. We show that the corresponding f(R) gravity of ghost/generalized-ghost DE model can behave like phantom or quintessence. Also the transition between deceleration to acceleration regime is indicated by deceleration parameter diagram for reconstructed f(R) generalized-ghost DE model.

Deceleration parameter in tilted Friedmann universes

Abstract: Large-scale peculiar motions are believed to reflect the local inhomogeneity and anisotropy of the Universe, triggered by the ongoing process of structure formation. As a result, realistic observers do not follow the smooth Hubble flow but have a peculiar ``tilt'' velocity relative to it. Our local group of galaxies, in particular, moves with respect to the universal expansion at a speed of roughly $600\text{ }\text{ }\mathrm{km}/\mathrm{sec}$. Relative motion effects are known to interfere with the observations and their interpretation. The strong dipolar anisotropy seen in the cosmic microwave background, for example, is not treated as a sign of real universal anisotropy, but as a mere artifact of our peculiar motion relative to the Hubble flow. With these in mind, we look into the implications of large-scale bulk motions for the kinematics of their associated observers, by adopting a tilted Friedmann model. Our aim is to examine whether the deceleration parameter measured in the rest frame of the bulk flow can differ from that of the actual Universe due to relative-motion effects alone. We find that there is a difference, which depends on the speed as well as the scale of the bulk motion. The faster and the smaller the drifting domain, the larger the difference. In principle, this allows relatively slow peculiar velocities to have a disproportionately strong effect on the value of the deceleration parameter measured by observers within bulk flows of, say, a few hundred megaparsecs. In fact, under certain circumstances, it is even possible to change the sign of the deceleration parameter. It goes without saying that all these effects vanish identically in the Hubble frame, which makes them an illusion and mere artifact of the observers' relative motion.

Interacting dark matter and holographic dark energy in an anisotropic universe

Abstract: In this paper we have studied the anisotropic and homogeneous Bianchi type-I universe filled with interacting Dark matter and Holographic dark energy Here we discussed two models, in first model the solutions of the field equations are obtained for constant value of deceleration parameter where as in the second model the solutions of the field equations are obtained for special form of deceleration parameter It is shown that for suitable choice of interaction between dark matter and holographic dark energy there is no coincidence problem (unlike ΛCDM) Also, in all the resulting models the anisotropy of expansion dies out very quickly and attains isotropy after some finite time The Statefinder diagnostic is applied to both the models in order to distinguish between our dark energy models with other existing dark energy models The physical and geometrical aspects of the models are also discussed