Does Hubble Tension Signal a Breakdown in FLRW Cosmology? (2105.09790v3)

Abstract: The tension between early and late Universe probes of the Hubble constant has motivated various new FLRW cosmologies. Here, we reanalyse the Hubble tension with a recent age of the Universe constraint. This allows us to restrict attention to matter and a dark energy sector that we treat without assuming a specific model. Assuming analyticity of the Hubble parameter $H(z)$, and a generic low redshift modification to flat $\Lambda$CDM, we find that low redshift data ($z \lesssim 2.5$) and well-motivated priors only permit a dark energy sector close to the cosmological constant $\Lambda$. This restriction rules out late Universe modifications within FLRW. We show that early Universe physics that alters the sound horizon can yield an upper limit of $H_0 \sim 71 \pm 1$ km/s/Mpc. Since various local determinations may be converging to $H_0 \sim 73$ km/s/Mpc, a breakdown of the FLRW framework is a plausible resolution. We outline how future data, in particular strongly lensed quasar data, could also provide further confirmations of such a resolution.

Does Hubble Tension Signal a Breakdown in FLRW Cosmology?

The paper "Does Hubble Tension Signal a Breakdown in FLRW Cosmology?" provides a comprehensive analysis of the discrepancies between different methods of determining the Hubble constant $H_0$, a key parameter in cosmology that denotes the rate of expansion of the universe. This tension, notably between early and late Universe measurements, challenges the standard Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological model, especially the flat Lambda Cold Dark Matter ($\Lambda$CDM) paradigm.

Summary of Key Findings

The authors re-evaluate the Hubble tension in light of recent constraints on the age of the Universe. They particularly focus on two influential datasets: measurements from the early universe such as the Cosmic Microwave Background (CMB) and late-time observations like supernovae and baryon acoustic oscillations (BAO).

1. Framework and Approach: By assuming the analyticity of the Hubble parameter $H(z)$ and considering low-redshift modifications to flat $\Lambda$CDM, the paper focuses on a sector consisting of matter and dark energy without adhering to a specific cosmological model. This analysis leverages Taylor expansions to quantitatively understand the Universe's dynamics over small and large redshifts, constrained by observation-based priors.
2. Constraints and Results: The paper finds that modifications at low redshifts ($z \lesssim 2.5$) imply a dark energy component resembling the cosmological constant $\Lambda$. Their analysis places an upper limit on $H_0$ of approximately 71 ± 1 km/s/Mpc, while local determinations often suggest $H_0$ values around 73 km/s/Mpc, indicating a possible inconsistency within the FLRW framework.
3. Implications of Hubble Tension: Given the upper limit on $H_0$, the authors suggest that resolving this tension may necessitate new physics from the early Universe that affects the sound horizon at recombination or might hint at a breakdown in the FLRW cosmology. They discuss the potential of early Universe modifications, such as altering sound horizon physics, as a possible resolution, although such solutions remain speculative pending further observational evidence.
4. Future Developments: The paper highlights various avenues for future research. Anisotropic observations of strong gravitational lensing (such as H0LiCOW lens systems) could provide insights into non-FLRW cosmologies. Additionally, upcoming data sets from missions like LSST could further test these cosmological principles and contribute to resolving the Hubble tension.

Conclusion and Speculation

The authors provide a nuanced analysis that cautions against a simple resolution of the Hubble tension within the well-trodden paths of FLRW cosmology adjustments. Instead, they foreshadow a need for novel theoretical frameworks that could encompass the subtleties suggested by varying observations. This work underscores the necessity for additional empirical data, particularly from cosmological probes capable of testing isotropy and variance in cosmic acceleration across different cosmic latitudes and observational scales. The pursuit of such data remains a critical focal point for ongoing and future investigations into the underlying structure and dynamics of our Universe.