A simultaneous solution to the Hubble tension and observed bulk flow within 250 ${h^{-1}}$ Mpc (2311.17988v1)

Abstract: The $\Lambda$ cold dark matter ($\Lambda$CDM) standard cosmological model is in severe tension with several cosmological observations. Foremost is the Hubble tension, which exceeds $5\sigma$ confidence. Galaxy number counts show the Keenan-Barger-Cowie (KBC) supervoid, a significant underdensity out to 300~Mpc that cannot be reconciled with $\Lambda$CDM cosmology. Haslbauer et al. previously showed that a high local Hubble constant arises naturally due to gravitationally driven outflows from the observed KBC supervoid. The main prediction of this model is that peculiar velocities are typically much larger than expected in the $\Lambda$CDM framework. This agrees with the recent discovery by Watkins et al. that galaxies in the CosmicFlows-4 catalogue have significantly faster bulk flows than expected in the $\Lambda$CDM model on scales of $100-250 \, h^{-1}$~Mpc. The rising bulk flow curve is unexpected in standard cosmology, causing $4.8\sigma$ tension at $200 \, h^{-1}$~Mpc. In this work, we determine what the semi-analytic void model of Haslbauer et al. predicts for the bulk flows on these scales. We find qualitative agreement with the observations, especially if our vantage point is chosen to match the observed bulk flow on a scale of $50 \, h^{-1}$~Mpc. This represents a highly non-trivial success of a previously published model that was not constrained by bulk flow measurements, but which was shown to solve the Hubble tension and explain the KBC void consistently with the peculiar velocity of the Local Group. Our results suggest that several cosmological tensions can be simultaneously resolved if structure grows more efficiently than in the $\Lambda$CDM paradigm on scales of tens to hundreds of Mpc.

A Simultaneous Solution to the Hubble Tension and Observed Bulk Flow Within 250 h^{-1} Mpc

The standard ΛCDM cosmological model faces significant challenges in accounting for various cosmological observations, particularly the Hubble tension and the observed bulk flow. The discrepancy, known as the Hubble tension, stems from the divergence between the locally measured Hubble constant (H₀) and that inferred from the cosmic microwave background (CMB) measurements, which exceeds 5σ confidence levels. Another issue arises from the existence of the Keenan-Barger-Cowie (KBC) supervoid, an underdensity out to 300 Mpc that standard ΛCDM cosmology struggles to reconcile.

The authors propose a compelling solution to these tensions through a model that incorporates gravitationally driven outflows resulting from the KBC supervoid. This framework predicts that galaxies exhibit larger peculiar velocities compared to ΛCDM expectations, aligning with recent findings from Watkins et al., which report unexpectedly high bulk flows in the CosmicFlows-4 catalogue. Specifically, the bulk flow anomalously rises on scales between 100-250 h⁻¹ Mpc, leading to a significant 4.8σ tension with the ΛCDM model at 200 h⁻¹ Mpc.

This paper explores the semi-analytic void model developed by Haslbauer et al. to predict the scale-dependent bulk flows and tests it against observational data. The model forecasts high peculiar velocities that are consistent with Watkins et al.'s observations under specific conditions: particularly, if the observer's vantage point reflects the measured bulk flow at a scale of 50 h⁻¹ Mpc. The success of the model in predicting these observed peculiar velocities, despite not being directly constrained by bulk flow measurements, presents a robust case for considering alternative dynamical models.

The implications of this research are substantial, suggesting that accelerated structure growth on cosmological scales could resolve multiple tensions within the ΛCDM framework. It supports alternative models like Milgromian dynamics, which posit more efficient structure formation on scales of tens to hundreds of Mpc. Moreover, it opens avenues for future developments in cosmology that incorporate revised initial conditions or non-standard dynamics to account for these observed deviations.

Overall, this paper contributes to the ongoing discourse on the viability of the ΛCDM model by presenting observationally consistent evidence for a dynamic cosmic structure model. It emphasizes the need for further empirical investigation and theoretical refinement, possibly using large-scale numerical simulations within alternative frameworks like Milgromian dynamics, to better grasp the intricacies of our universe's expansion and structure formation.