A response to Rubin & Heitlauf: "Is the expansion of the universe accelerating? All signs still point to yes" (1912.04257v2)

Abstract: We have shown (Colin et al., 2019) that the acceleration of the Hubble expansion rate inferred from Type Ia supernovae (SNe Ia) is, at $3.9\sigma$ significance, a dipole approximately aligned with the CMB dipole, while its monopole component, which can be interpreted as due to a Cosmological Constant or dark energy, is consistent with zero at $1.4\sigma$. This has been challenged by Rubin & Heitlauf (2019) who assert that the dipole arises because we made an incorrect assumption about the SNe Ia light-curve parameters (viz. took them to be sample- and redshift independent), and did not allow for the motion of the Solar system (w.r.t. the 'CMB frame' in which the CMB dipole supposedly vanishes). In fact what has an even larger impact on our finding is that we reversed the inconsistent "corrections" made for the peculiar velocities of the SNe Ia host galaxies w.r.t the CMB frame, which in fact serve to bias the lever arm of the Hubble diagram towards higher inferred values of the monopole. We demonstrate that even if all such corrections are made consistently and both sample- and redshift-dependence is allowed for in the standardisation of supernova light curves, the evidence for isotropic acceleration rises to just $2.8\,\sigma$. Thus the criticism of Rubin & Heitlauf serves only to highlight that "corrections" must be made to the SNe Ia data assuming the standard $\Lambda$CDM model, in order to recover it from the data.

• The paper critiques the treatment of light-curve parameters in SNe Ia data, challenging standard isotropic acceleration assumptions.
• It scrutinizes redshift frame choices by emphasizing that peculiar velocities are intrinsic signals rather than mere uncertainties.
• Results show that traditional velocity corrections can weaken the monopole acceleration signal, urging model-independent verification.

Overview of "A response to Rubin {content} Heitlauf: 'Is the expansion of the universe accelerating? All signs still point to yes'"

This paper by Colin et al. presents a critical analysis of the method used to infer the acceleration of the Universe's expansion based on Type Ia supernovae (SNe Ia) data. The authors respond specifically to the critique posed by Rubin and Heitlauf regarding the treatment of light-curve parameters and peculiar velocity corrections in the context of cosmic acceleration analysis. The findings challenge the isotropic acceleration interpretation central to the standard $\Lambda$CDM cosmological model and suggest significant anisotropic components are present in the data.

Key Points of the Critique

The authors address several primary criticisms levied by Rubin and Heitlauf:

1. Assumptions on Light-Curve Parameters: Rubin and Heitlauf propose that light-curve shapes and colors, parameterized within the SALT2 model, exhibit redshift dependence contrary to the Colin et al.'s assumption of independence. While Rubin and Heitlauf argue that the Bayesian Information Criterion supports additional parameters, Colin et al. contend that such extensions are not justified without more substantial evidence.
2. Choice of Redshift Frame: The authors discuss the implications of using heliocentric vs. CMB-frame redshifts. Historically, heliocentric redshifts have been standard; however, transitioning to the CMB-frame is claimed to align with isotropic universe assumptions. Colin et al. maintain that peculiar velocities should be seen as intrinsic signals rather than uncertainties to be corrected.
3. Data Inclusivity and Anisotropy: Colin et al. contend that the criticism regarding a supposed lack of representative data inclusivity is unfounded because the relevant datasets were not accessible or lacked detailed individual supernovae analyses, unlike those available from the JLA catalog.
4. Model Pathology: Finally, Colin et al. refute the argument that their model for dipole anisotropy is flawed by demonstrating that constraints are naturally emergent in physical data scenarios.

Numerical Results and Implications

The analysis finds that when peculiar velocity corrections are applied traditionally, as in the JLA catalog, the monopole component indicative of isotropic acceleration is significantly contingent upon assumptions about the data. Using only heliocentric redshifts, without transformation to the CMB frame, yields a much weaker statistical signal of acceleration, aligning with the argument that supernova data inherently includes nondirectional influences often sidelined by current models.

The authors also connect the significance of the isotropic acceleration claims to the adjustments made for motion within presumed cosmological frameworks, thus implying the need for more thorough model-independent verification methods.

Theoretical and Practical Implications

The paper highlights critical considerations regarding astrophysical datasets' handling and the models used to interpret cosmic acceleration. The potential misunderstanding and misapplication of peculiar velocity corrections in a cosmological context drive home the importance of referencing framework choices' explicit assumptions. Practically, the research suggests that the standard methodologies require revisiting with these concerns in mind to authenticate claims about cosmic acceleration and potentially redefine understandings related to dark energy.

Future Research Directions

Given the paper's conclusions, future explorations might focus on refining redshift correction methods that do not a priori assume isotropy or the validity of $\Lambda$CDM parameters. Enhanced simulations and rigorous statistical testing can aid in developing models that better respect observed inhomogeneities and align results with empirical, isotropic observations.

Overall, this work prompts a cautious re-evaluation of established cosmological beliefs and underscores the ongoing conversation around cosmic expansion's true nature. The findings reinforce the necessity for better data handling techniques and a sharper lens on fundamental cosmological model premises.