On generalized Lemaitre–Tolman–Bondi metric: Fractal matter at the end of matter–antimatter recombination

Many recent researches have investigated the deviations from the Friedmannian cosmological model, as well as their consequences on unexplained cosmological phenomena, such as dark matter and the acceleration of the Universe. On one hand, a first-order perturbative study of matter inhomogeneity returned a partial explanation of dark matter and dark energy, as relativistic effects due to the retarded potentials of far objects. On the other hand, the fractal cosmology, now approximated by a Lemaitre–Tolman–Bondi (LTB) metric, results in distortions of the luminosity distances of SNe Ia, explaining the acceleration as apparent. In this work, we extend the LTB metric to ancient times. The origin of the fractal distribution of matter is explained as the matter remnant after the matter–antimatter recombination epoch. We show that the evolution of such a inhomogeneity necessarily requires a dynamical generalization of LTB, and we propose a particular solution.

Exploring the evidence for a large local void with supernovae Ia data

In the present work we utilise the most recent publicly available SN Ia compilations and implement a well formulated cosmological model based on LTB metric in presence of cosmological constant Λ (ΛLTB) to test for signatures of large local inhomogeneities at z ≤ 0.15. Local underdensities in this redshift range have been previously found based on luminosity density data and galaxy number counts. Our main constraints on the possible local void using the Pantheon SN Ia dataset are: redshift size of $z_{\rm size}=0.068^{+0.021}_{-0.030}$; density contrast of $\delta \Omega _0/\Omega _0 = -10.5_{-7.4}^{+9.3}\%$ between 16th and 84th percentiles. Investigating the possibility to alleviate the ∼ $9\%$ disagreement between measurements of present expansion rate H0 coming from calibrated local SN Ia and high-z CMB data, we find large local void to be a very unlikely explanation alone, consistently with previous studies. However, the level of matter inhomogeneity at a scale of ∼100Mpc that is allowed by SN Ia data, although not expected from ΛCDM cosmic variance calculations, could be the origin of additonal systematic error in distance ladder measurements based on SN Ia. Fitting low-redshift Pantheon data with a cut 0.023 < z < 0.15 to the ΛLTB model and to the Taylor expanded luminosity distance formula we estimate that this systematic error amounts to $1.1\%$ towards the lower H0 value. A test for local anisotropy in Pantheon SN Ia data yields null evidence. Analysis of luminosity density data provides a constraint on contrast of large isotropic void $\delta \Omega _0/\Omega _0 = -51.9\%\pm 6.3\%$, which is in ∼ 4σ tension with SN Ia results. More data is necessary to better constrain the local matter density profile and understand the disagreement between SN Ia and luminosity density samples.

Non-stationarity of low flows and their relevance to river modelling during drought periods

Changes in groundwater storage lead to a reduction in groundwater contribution to river flow and present as non-stationarity, especially during low-flow conditions. Conventional river models typically ignore this non-stationarity, and, hence, their predictions of declines in low flows during drought periods are likely to be compromised. The present study assesses non-stationarity and highlights its implications for river modelling. A quantile regression analysis showed non-stationarity of low flows in the Namoi catchment (Australia), with statistically significant downward trends in the 10th percentile of log-transformed baseflow (10-LTB). This highlighted the usefulness of the 10-LTB metric to identify non-stationarity and, hence, alert modellers to the importance of adopting models that explicitly account for groundwater processes when modelling such river systems.

Inhomogeneous matter distribution and supernovae

This work investigates a simple inhomogeneous cosmological model within the Lema[Formula: see text]tre–Tolman–Bondi (LTB) metric. The mass-scale function of the LTB model is taken to be [Formula: see text] and would correspond to a fractal distribution for [Formula: see text]. The luminosity distance for this model is computed and then compared to supernovae (SNe) data. Unlike LTB models which have, in the most general case, two free functions, our model has only two free parameters as the flat Standard Model of cosmology. The best-fit obtained is a matter distribution with an exponent of [Formula: see text] revealing that SNe data do not favor those fractal models.

The impact of superstructures in the Cosmic Microwave Background

In 2008, Granett et al. claimed a direct detection of the integrated Sachs-Wolfe (iSW) effect, through the stacking of CMB patches at the positions of identified superstructures. Additionally, the high amplitude of their measured signal was reported to be at odds with predictions from the standard model of cosmology. However, a closer inspection of these results prompts multiple questions, more specifically about the amplitude and significance of the expected signal. We propose here an original theoretical prediction of the iSW effect produced by such superstructures. We use simulations based on GR and the LTB metric to reproduce cosmic structures and predict their exact theoretical iSW effect on the CMB. The amplitudes predicted with this method are consistent with the signal measured when properly accounting the contribution of the non-negligible (and fortuitous) primordial CMB fluctuations to the total signal. It also highlights the tricky nature of stacking measurements and their interpretation.

The impact of superstructures in the Cosmic Microwave Background

In 2008, Granett et al. claimed a direct detection of the integrated Sachs- Wolfe (iSW) effect by a stacking approach of patches of the CMB, at the positions of identified superstructures. However, the high amplitude of their measured signal seems to be at odds with predictions from the standard model of cosmology. However, multiple questions arise from these results and their expected value : I propose here an original theoretical prediction of the iSW effect produced by such superstructures. I use simulations based on GR and the LTB metric to reproduce cosmic structures and predict their exact full theoretical iSW effect. Expected amplitudes are consistent with the measured signal ; however the latter shows non-reproducible features that are hardly compatible with ΛCDM predictions.

MODIFIED AVERAGING PROCESSES IN COSMOLOGY AND THE STRUCTURED FRW MODEL

We study the volume averaging of inhomogeneous metrics within GR and discuss its shortcomings, such as gauge dependence, singular behavior as a result of caustics, and causality violations. To remedy these shortcomings, we suggest some modifications to this method. As a case study we focus on the inhomogeneous structured FRW model based on a flat LTB metric. The effect of averaging is then studied in terms of an effective backreaction fluid. It is shown that, contrary to the claims in the literature, the backreaction fluid behaves like a dark matter component, instead of dark energy, having a density of the order of 10-5 times the matter density, and, most importantly, it is gauge-dependent.