Hints of FLRW Breakdown from Supernovae (2106.02532v4)

Abstract: A 10\% difference in the scale for the Hubble parameter constitutes a clear problem for cosmology. Here, considering angular distribution of Type Ia supernovae (SN) within the Pantheon compilation and working within flat $\Lambda$CDM cosmology, we observe a correlation between higher $H_0$ and the CMB dipole direction, confirming our previous results for strongly-lensed quasars \cite{Krishnan:2021dyb}. Concretely, we record a $\sim 1$ km/s/Mpc variation in $H_0$ at antipodal points on the sky within the Pantheon sample, which is evident in the Low $z$ subsample ($z \lesssim 0.075$) and gets enhanced by higher redshift SN. Our work raises the possibility that we may be at the precision required to probe anisotropic Hubble expansions, while providing a concrete prediction for future inferences of $H_0$.

• The paper identifies up to a 1 km/s/Mpc variation in H0 across the sky, hinting at a potential breakdown of the FLRW model.
• It employs Type Ia supernovae data alongside quasar observations to explore deviations from cosmic isotropy.
• The findings suggest new physics or measurement refinements, encouraging further research using diverse astronomical datasets.

Hints of FLRW Breakdown from Supernovae: An Analysis

The paper "Hints of FLRW Breakdown from Supernovae" presents a compelling examination of potential anisotropy in the Hubble parameter ($H_0$), challenging the conventional Friedmann-Lemaître-Robertson-Walker (FLRW) model assumptions. The research leverages data from Type Ia supernovae (SN Ia) in the Pantheon compilation, alongside previous studies on strongly-lensed quasars, to probe this possible inconsistency.

Study Rationale and Key Observations

The investigation arises from the persistent Hubble tension—a ~10% discrepancy between $H_0$ values inferred from observations of the early Universe and those derived from the late Universe, notably using Cepheid-calibrated distance ladders. This paper hypothesizes that this tension may signal deviations from the FLRW paradigm wherein isotropy and homogeneity are foundational assumptions. By analyzing SN Ia's angular distribution, the authors detect a correlation between higher $H_0$ values and the Cosmic Microwave Background (CMB) dipole direction, echoing observations in prior quasar studies.

In terms of numerical results, the authors report a notable variation of approximately 1 km/s/Mpc in $H_0$ at antipodal points on the sky, identified within the Pantheon supernova sample. These variations are most apparent in the Low $z$ subsample (redshifts $z \lesssim 0.075$), and they become even more pronounced at higher redshifts. The findings suggest that existing datasets might already possess the precision needed to detect anisotropic Hubble expansions.

Interpretation of Results

The paper suggests three potential explanations for the observed anisotropies in $H_0$:

1. A genuine breakdown of the FLRW model at large redshifts, consistent with past discrepancies between CMB and large-scale structure dipoles.
2. Local structure along the line of sight influencing measurements.
3. A statistical coincidence, which the paper evaluates as having a probability of 12% for the quasar results and 6.5% for the Type Ia supernova results.

While the statistical significance of these findings may not independently suffice to revise cosmological models, the consistent pattern across different astronomical objects and data compilations points to the first hypothesis—a potential anisotropic Hubble expansion—as a serious contender.

Implications and Future Directions

The ramifications of an anisotropic $H_0$ challenge fundamental aspects of cosmology and suggest possible new physics or misconceived measurements of the CMB dipole. Practically, these findings urge refinements in how we model and interpret cosmological observations, especially those concerning large structures and cosmic expansion.

Future research should aim to confirm these anisotropies with more precise data and alternative methodologies that could account for or eliminate the potential local structures' influence. Additionally, expanding the observational set to include more diverse datasets (e.g., gamma-ray bursts or other supernova compilations) could validate the universality of these results.

In summary, while not conclusively refuting the FLRW assumptions, this paper presents a nuanced perspective on cosmological isotropy, underlining the need for ongoing scrutiny of the Hubble tension and its broader implications for understanding cosmic evolution.