The KBC void and Hubble tension contradict $Λ$CDM on a Gpc scale $-$ Milgromian dynamics as a possible solution (2009.11292v2)

Abstract: The KBC void is a local underdensity with the observed relative density contrast $\delta \equiv 1 - \rho/\rho_{0} = 0.46 \pm 0.06$ between 40 and 300 Mpc around the Local Group. If mass is conserved in the Universe, such a void could explain the $5.3\sigma$ Hubble tension. However, the MXXL simulation shows that the KBC void causes $6.04\sigma$ tension with standard cosmology ($\Lambda$CDM). Combined with the Hubble tension, $\Lambda$CDM is ruled out at $7.09\sigma$ confidence. Consequently, the density and velocity distribution on Gpc scales suggest a long-range modification to gravity. In this context, we consider a cosmological MOND model supplemented with $11 \, \rm{eV}/c^{2}$ sterile neutrinos. We explain why this $\nu$HDM model has a nearly standard expansion history, primordial abundances of light elements, and cosmic microwave background (CMB) anisotropies. In MOND, structure growth is self-regulated by external fields from surrounding structures. We constrain our model parameters with the KBC void density profile, the local Hubble and deceleration parameters derived jointly from supernovae at redshifts $0.023 - 0.15$, time delays in strong lensing systems, and the Local Group velocity relative to the CMB. Our best-fitting model simultaneously explains these observables at the $1.14\%$ confidence level (${2.53 \sigma}$ tension) if the void is embedded in a time-independent external field of ${0.055 \, a_{_0}}$. Thus, we show for the first time that the KBC void can naturally resolve the Hubble tension in Milgromian dynamics. Given the many successful a priori MOND predictions on galaxy scales that are difficult to reconcile with $\Lambda$CDM, Milgromian dynamics supplemented by $11 \, \rm{eV}/c^{2}$ sterile neutrinos may provide a more holistic explanation for astronomical observations across all scales.

• The paper identifies a combined 7.09σ tension between predictions of the standard Lambda-CDM cosmological model and observations of the KBC void and the Hubble constant.
• Analysis shows the KBC void alone creates a 6.04σ tension with Lambda-CDM predictions and contributes to the existing 5.3σ Hubble tension.
• Milgromian dynamics is presented as a possible solution, with simulations indicating it can explain the KBC void's structure and better fit local Hubble constant measurements than Lambda-CDM.

An Analysis of Tensions in Cosmological Models with Respect to the KBC Void and Hubble Constant

The paper "The KBC Void and Hubble Tension Contradict ΛCDM on a Gpc Scale—Milgromian Dynamics as a Possible Solution" by M. Haslbauer et al. addresses a critical and longstanding problem in cosmology regarding the discrepancies between observed cosmological features and predictions from the ΛCDM model. Specifically, it focuses on the conflict between the observed KBC void and the Hubble tension when contrasted with the standard Lambda-Cold Dark Matter (ΛCDM) model predictions.

Key Findings and Analysis

1. KBC Void and Its Implications:
   • The KBC void presents a significant underdensity observed locally in the Universe, characterized by a relative density contrast of δ = 0.46 ± 0.06. This discrepancy provides a severe challenge to the ΛCDM model, predicting lower density fluctuations on this scale.
   • The authors utilized the MXXL simulation to analyze cosmic variance within standard cosmology, finding the KBC void results in a 6.04σ tension, significantly challenging the ΛCDM model's validity.
2. Hubble Tension:
   • The Hubble tension refers to the disagreement between locally measured Hubble constant values and those derived from the Cosmic Microwave Background (CMB) via the ΛCDM model. This tension has reached 5.3σ, suggesting systemic issues in the standard model of cosmology.
   • When considering the Hubble tension together with the KBC void, the combined discrepancy with ΛCDM predictions is at a robust 7.09σ, necessitating a reevaluation of the underlying cosmological assumptions.
3. Milgromian Dynamics (MOND) as an Alternative:
   • The authors explore Milgromian Dynamics and its application on cosmological scales to provide a solution to these tensions. MOND, originally developed to address galactic rotation curves without invoking dark matter, offers a framework that may naturally account for larger density fluctuations due to its modified gravitational theory.
   • Within this framework, the authors propose the νHDM model, which incorporates sterile neutrinos to accommodate galaxy cluster observations and aims to preserve the successes of MOND on galactic scales.
4. Simulation Results:
   • Through semi-analytic simulations, Haslbauer et al. found that MOND significantly accelerates structure formation compared to ΛCDM. The structure on a 300 Mpc scale grows more efficiently, resulting in a void consistent with observations.
   • The paper demonstrates that MOND, with appropriate parameterizations, can produce large-scale structures consistent with the KBC void and fit the local Hubble constant measurements at a tension level as low as 2.53σ when compared to observations.
5. Potential Impact and Future Directions:
   • If MOND or a related model is viable, it would necessitate a paradigmatic shift from the ΛCDM framework, incorporating modified gravity theories at large scales.
   • The paper suggests the need for further investigations into large-scale structure simulations within a MOND paradigm, testing theoretical assumptions further and refining the understanding of MOND's implications on cosmological scales.

Conclusion

The findings of this paper highlight the profound implications of the KBC void and Hubble tension on our understanding of cosmology. The introduction of Milgromian dynamics offers promising avenues that transcend the limitations of the ΛCDM model when explaining observed cosmological phenomena. Enhanced theoretical and computational efforts in the MOND framework are imperative to fully address these discrepancies, ensuring the integration of cosmological observations across all scales. Both the cosmological community's experimental efforts and theoretical advancements are crucial to further exploring and validating or falsifying MOND and related models in the future.