Analysis of Pantheon+ supernova data suggests evidence of sign-changing pressure of the cosmological fluid

TL;DR

This work extends the spread-LDF, a model-independent luminosity-distance fitting method, to the Pantheon+ SN Ia dataset (augmented by GRBs) to reconstruct the expansion history under FLRW without committing to a specific matter-energy model. By deriving $\dot a(z)$, $\ddot a(z)$, and $H(z)$ from $d_L(z)$ and then applying a semi-model-independent GR framework, the authors quantify $\rho(z)$, $p(z)$, and the effective EOS $w(z)$, finding that a non-matter component is required and that $w(z)$ is roughly linear with $z$ up to $1.5$. The CPL parametrization provides a better fit to the reconstructed $H(z)$ than $\Lambda$CDM, yet the analysis reveals a sign change in pressure around $z \approx 1$, implying $w_{DE}(z)$ crosses the phantom divide and inviting interpretations in terms of generalized dark energy or possible modifications to fundamental assumptions. Overall, the paper demonstrates that the expansion history inferred from high-quality SN Ia data is consistent with acceleration and a dark-energy–like component, while highlighting potential new physics or methodological refinements if the sign-change result is confirmed by future data.

Abstract

In this work, we revisit/reinterpret/extend the model-independent analysis method (which we now call spread - luminosity distance fitting, spread-LDF) from our previous work. We apply it to the updated supernova type Ia catalogue, Pantheon+ and recent GRB compilations. The procedure allows us, using only FLRW assumption, to construct good approximations for expansion history of the universe, re-confirming its acceleration to be a robust feature. When we also assume General Relativity ("GR"), we can demonstrate, without any matter/energy model in mind, the need for (possibly nonconstant) dark energy ("GDE"). We find hints for positive pressure of GDE at z>1 with implications on either the complexity of dark energy, or the validity of one of the cosmological principle, interpretation of SN Ia data, or GR.

Figures (23)

• Figure 1: Luminosity distance $d_L(z)$ versus redshift for Type Ia supernovae in the Pantheon+ compilation.
• Figure 2: Luminosity distance $d_L(z)$ as a function of redshift for the full observational dataset. Black points represent Type Ia supernovae from the Pantheon+ compilation; red points represent the rescaled GRB dataset. The GRBs extend the redshift range beyond the supernova domain.
• Figure 3: Pantheon+ data in terms of luminosity distance and standard redshift, the $N=2$ to 10 fits for the first family, simple polynomials and ${\rm \Lambda}$CDM (red) model, and the $1$-$\sigma$ confidence-level of the best-fitting member ($N=3$) of the family F1. The insets show (i) a magnified view of the right end of the data, and (ii) a bar chart of the BIC values for the F1 fits ($N=2$ to $10$).
• Figure 4: BIC values in Table \ref{['families']}, illustrated as a 3D bar chart for different function families and redshift variables. The main plot (right) provides a close-up view by truncating the tallest bars for better visibility of the lower values. An inset (left) shows the full-scale chart including all bar heights for completeness.
• Figure 5: Best-fitting curves from each functional family applied to the Pantheon+ dataset, using the standard redshift variable $y_0=z$. Each curve represents the optimal model (based on BIC) within its respective function class. This comparison illustrates how different model families approximate the observed luminosity distance–redshift relation.