On the homogeneity of SnIa absolute magnitude in the Pantheon+ sample

Abstract

We have analysed the Pantheon+ sample using a new likelihood model that replaces the single SnIa absolute magnitude parameter M used in the standard likelihood model of Brout et. al. [1] with two absolute magnitude parameters M<, M> and a transition distance dcrit that determines the distance at which M changes from M< to M>. The use of this likelihood dramatically changes the quality of fit to the Pantheon+ sample for a ΛCDM background by ∆χ 2 = −19.6 (∆AIC = −15.5 for two additional parameters). The tension between the M< and M> best fit values is at a level more than 3σ with a best fit dcrit very close to 20M pc. The origin of this improvement of fit and M< − M> tension is that the new likelihood model, successfully models two signals hidden in the data: 1. The well known systematic effect called volumetric redshift scatter bias which is due to asymmetric peculiar velocity variations at redshifts z < 0.01 induced by unequal projected volumes at lower and higher distances compared to a given distance and 2. A mild signal for a change of intrinsic SnIa luminosity at about 20M pc. This interpretation of the results is confirmed by truncating the z < 0.01 Hubble diagram data from the Pantheon+ data where the above systematic is dominant and showing that the M< − M> tension decreases from above 3σ to a little less than 2σ. It is also confirmed by performing a Monte Carlo simulation to compare the merged SnIa absolute luminosities Mi of SnIa+Cepheid hosts, obtained from the SH0ES data, with the anticipated luminosities in the context of a homogeneous single absolute magnitude M . This simulation shows that the maximum significance of the SnIa luminosity transition (Σ ≡ |M>−M<|

A Bias-free Cosmological Analysis with Quasars Alleviating H 0 Tension

“…Indeed, for the case of both parameters free to vary, the results with and without evolution lie on the opposite side of the Planck (no evolution) and SNe Ia values for H 0 (for fixed evolution). We also would like to stress that the H 0 trend toward the value of 70 km s −1 Mpc −1 may also stem from the fact that SNe Ia are uncalibrated (namely, calibrated arbitrarily for an absolute magnitude M = − 19.35 and thus consequently at H 0 = 70 km s −1 Mpc −1 , see also Perivolaropoulos & Skara 2023); thus, the trend may not stem from the underlying physics, but from this calibration choice (A. Riess, private communication).…”

Suggestions of decreasing dark energy from supernova and BAO data

“…To ensure that these local effects are not having a significant effect on the results for our model parameters, we typically employ a z < 0.01 cut apart from data with Cepheid calibrators. We have compared this with an alternative treatment of low z data considered in [55] which uses all data but introduces an independent absolute magnitude parameter for SNe Ia with z smaller than some z crit . This gives similar results for the main parameters of interest, as we describe below.…”

“…A summary of the marginalized parameter values and their uncertainties, assuming the z < 0.01 cut, can be found in table 1. As we described above, as a check on the sensitivity to local effects we have also followed [55] in fitting distinct parameters M 1 , M 2 for the absolute magnitude of SNe Ia with z smaller than or larger than 0.005 respectively. We find very comparable results…”

Effects of a Late Gravitational Transition on Gravitational Waves and Anticipated Constraints

“…A smoking-gun prediction of this class of models is a sudden change in the SnIa absolute magnitude at a distance d c < 60 Mpc. Recent analyses have indicated hints for hidden signals in the SH0ES [11] and Pantheon+ SnIa data that are consistent with such a transition [40]. A physical mechanism for the realization of such a transition includes an explicit symmetry breaking in the context of the symmetron-screening mechanism (the asymmetron [45]) as well as a first-order phase transition in the context of a scalar-tensor theory [46].…”

“…The problem of this class of models is that the significant level of E(z) deformation needed is not consistent with a wide range of other observations constraining the form of E(z) like BAO and SnIa data [34][35][36][37]. • Ultra-late time models [36,[38][39][40][41][42] that investigate the possible presence of either an unaccounted-for systematic effect and/or a change in the fundamental physics taking place during the last 150 Myrs (redshift z < 0.01) when the calibration of standard candles like SnIa is performed [11]. The problems of this class of models includes fine tuning since the change in physical laws should not only occur at very late times but also should be consistent with other local observations [43].…”