Another Observational Problem With ΛCDM

The problems with the ΛCDM model, which is the "Standard Model of Cosmology," aren't limited to galaxy scale problems that have been widely discussed. Increasingly, there are problems with it at the time depth and scale reserved to cosmology.

The latest problem is that the model assumes that at a large enough scale the universe is isotropic and homogeneous (i.e. that it is basically the same in all directions). But compelling observational evidence contradicts this assumption. 

The evidence for dark energy also isn't as strong as commonly believed in the first place. As the body text of a new preprint explains, contrary to a late 1990s study of 93 SNe Ia supernova that showed clear evidence of an accelerated expansion of the universe, a "principled statistical analysis of a bigger 740 star 2014 Joint Lightcurve Analysis catalogue data set done in a 2016 "demonstrated that the evidence for acceleration is rather marginal i.e. < 3σ[.]" 

Furthermore, it observes that: "The observed Universe is not quite isotropic. It is also manifestly inhomogeneous."

This doesn't necessarily mean that the concept of dark energy is fundamentally wrong, or that dark energy phenomena aren't real. But if these fundamental assumptions are flawed and the evidence is fairly equivocal to start with, then it follows that our estimates of the magnitude of dark energy phenomena are far less reliable and precise than currently widely believed.

In the late 1990's, observations of 93 Type Ia supernovae were analysed in the framework of the FLRW cosmology assuming these to be `standard(isable) candles'. It was thus inferred that the Hubble expansion rate is accelerating as if driven by a positive Cosmological Constant Λ. This is still the only direct evidence for the `dark energy' that is the dominant component of the standard ΛCDM cosmological model. Other data such as BAO, CMB anisotropies, stellar ages, the rate of structure growth, etc are all `concordant' with this model but do not provide independent evidence for accelerated expansion. 

Analysis of a larger sample of 740 SNe Ia shows that these are not quite standard candles, and highlights the "corrections" applied to analyse the data in the FLRW framework. The latter holds in the reference frame in which the CMB is isotropic, whereas observations are made in our heliocentric frame in which the CMB has a large dipole anisotropy. This is assumed to be of kinematic origin i.e. due to our non-Hubble motion driven by local inhomogeneity in the matter distribution. 

The ΛCDM model predicts how this peculiar velocity should fall off as the averaging scale is raised and the universe becomes sensibly homogeneous. However observations of the local `bulk flow' are inconsistent with this expectation and convergence to the CMB frame is not seen. Moreover the kinematic interpretation implies a corresponding dipole in the sky distribution of high redshift quasars, which is rejected by observations at 4.9σ. The acceleration of the Hubble expansion rate is also anisotropic at 3.9σ and aligned with the bulk flow. Thus dark energy may be an artefact of analysing data assuming that we are idealised observers in an FLRW universe, when in fact the real universe is inhomogeneous and anisotropic out to distances large enough to impact on cosmological analyses.

Roya Mohayaee, Mohamed Rameez, Subir Sarkar, "Do supernovae indicate an accelerating universe?" arXiv:2106.03119 (June 6, 2021).