arXiv subjects

Vincent Deledicque

Publications and source records attributed to Vincent Deledicque.

6 recordsLinked to original sources

arXiv2024

Interpretation of SNIa and BAO cosmic probes results using a two-regions model of the universe

This article revisits the interpretation of cosmic probes such as SNIa and BAO under a two-regions model of the universe. Standard cosmological analyses assume homogeneity, yet observations are predominantly conducted in overdense regions where matter is clustered, potentially biasing conclusions about cosmic expansion. We refine an existing two-regions framework that accounts for both overdense and underdense areas, demonstrating that the apparent acceleration of the universe, typically attributed to dark energy, can emerge as a consequence of observational bias rather than a fundamental cosmological constant. By applying this model to SNIa and BAO measurements, we show that inhomogeneities can affect redshift and distance estimates in a way that mimics cosmic acceleration. This interpretation provides a natural resolution to the coincidence problem and offers an alternative explanation for the observed tensions in Hubble constant measurements. Additionally, it may shed light on the unexpectedly early formation of galaxies observed by JWST. These findings challenge the necessity of dark energy and suggest that the observed acceleration may be an artefact of the spatial distribution of matter rather than a true universal phenomenon.

astro-ph.CO↗

arXiv2023

Development of a model to investigate the effect of the bias in SNIa measurements related to the inhomogeneity of space

It has been suggested recently that the appparent accelerated expansion of the universe could be explained by a bias in the SNIa measurements. Such events indeed occur mainly in overdense regions, where matter is located, and whose dynamics can perhaps not been considered as representative of the one of the universe. In this article, we develop a model to investigate in more detail the effect of this bias. This model depends on one single parameter, related to the void fraction of space, and leads to simple analytical relations. We in particular determine the average metric tensor in overdense regions, and deduce that the scale factor and the rate at which time progresses in such regions differ significantly from the corresponding values expected on the average space. We then quantitatively deduce how redshift and luminosity distance measurements are affected by the bias, taking into account the perturbation of the metric tensor. Using a value for the void fraction corresponding to the order of magnitude found in the literature, we show that the model is able to predict a distance modulus versus redshift relation being in excellent aggreement with the one corresponding to a universe characterized by $\Omega_{m,0} = 0.3$ and $\Omega_{\Lambda,0} = 0.7$.

astro-ph.CO↗

arXiv2022

Dark energy and the fitting problem

At a global scale, the universe is generally fitted by an idealized manifold described by the FLRW metric. This is in particular the case when probing the universe to determine its dynamics. The process that fits the idealized manifold to the real universe is however not uniquely defined. This process may depend on the cosmic probe that has been used for the measurements, and could hence lead to different observed temporal evolutions of the scale factor. A correct interpretation of the observed accelerated expansion of the universe requires therefore first a thorough understanding of the fitting process that has been implicitly applied. In this article we establish the fitting processes for the SNIa, BAO and CMB cosmic probes, and deduce the related averaged Einstein equations. We demonstrate that the way these fittings have been applied in practice lead to an apparent dark energy effect. We also highlight the conceptual differences in the fitting processes between the SNIa and BAO probes on the one hand, and the CMB probe on the other hand. Considering those differences, we then show how the so-called Hubble tension can be explained.

astro-ph.CO↗

arXiv2021

Dark energy explained by a bias in the measurements

Typical cosmological models are based on the postulate that space is homogeneous. Space however contains overdense regions in which matter is concentrating, leaving underdense regions of almost void. The evolution of the scale factor of the universe has been established from measurements on SNIa. Since such events occur in regions were matter is present, we may expect that most of the SNIa are located in overdense regions. This means that the evolution of the scale factor has been established in a biased manner, by considering only information coming from overdense regions, excluding the one from the underdense regions. We develop a simple model to analyze the effect of this bias, and show that it leads to the appearance of a new tensor in the Einstein equation of general relativity, which can account for the apparent acceleration of the expansion of the universe. We further show that this tensor tends to be proportional to the FLRW metric tensor, and that the constant of proportionality quantitatively corresponds to the measured cosmological constant with a remarkable accuracy. We finally explain why these properties remain valid for other techniques used in determining the dynamics of the universe, such as the baryon acoustic oscillations.

astro-ph.CO↗

arXiv2020

Dark energy explained by an inadequate fitting of the FLRW metric

Approximating a real manifold by an idealized one requires to calibrate the parameters characterizing the idealized manifold in function of the real one. This calibration is a purely conventional process and can generally be done in several ways, leading to different fittings. In practice, however, all possible fittings cannot be considered as representative of the real manifold. Approximating the real metric of the universe by the FLRW metric would be adequate only if both corresponding structures, defined by the space-time interval, are equivalent on large scales. This requirement puts some constraints on what would be a representative FLRW metric. We show that the way how measurements on SNIa are interpreted to determine the evolution of the scale factor implicitly define the calibration process, and that this one is compatible with the aforementioned constraints. On a theoretical point of view, this indicates that the as fitted FLRW metric would indeed be representative of the real one. On a practical point of view, however, we show that a bias in the measurements could invalidate this conclusion. The bias comes from the fact that SNIa are not randomly distributed over space, but are probably mostly located in regions were matter is largely present, i.e., in overdense regions. We explain how this bias could account for the apparent accelerated expansion of the universe, without needing to introduce the dark energy assumption. We show in particular that this bias leads to an inadequate fitting of the FLRW metric, resulting in the appearance of a new term in the evolution equation of the related scale factor, being equivalent to the cosmological constant.

gr-qc↗

arXiv2019

A simplified general relativistic model to analyze the structure of spiral galaxies

The dark matter hypothesis, which is not called into question here, explains why typical rotation curves of spiral galaxies do not follow a Keplerian profile. It is however not sufficient in itself to explain why the whole matter distribution in spiral galaxies is such that the rotation curve generally presents a flat profile in the disk region. To understand this property, a model considering general relativistic effects is developed. It is stressed that the aim is not to explain the flat rotation curve of spiral galaxies without dark matter. More specifically, the analytical stationary solution of an axisymmetric rotating pressureless fluid for the linearized equations of the theory of general relativity is determined. It is demonstrated that this solution leads to some constraints on the rotation curve, by looking to its limit behavior when neglecting general relativistic effects. In particular, the positiveness of the density imposes the rotation curve to be flat in the regions where the density and general relativistic effects are small, such as in the disk region. General relativistic effects hence remain negligible in the disk region, but their consideration proved to be necessary to establish the aforementioned constraint. Such constraint cannot be derived from a Newtonian approach. The model is finally applied on two specific cases to demonstrate its ability to predict the rotation curve from a typical density profile along the galactic plane. These examples suggest that for some galaxies, general relativistic effects can be significant close to the bulge region and should be taken into account to have a proper understanding of their rotation curve.

astro-ph.GA↗