The tension between early and late Universe probes of the Hubble constant has motivated various new FLRW cosmologies. Here, we reanalyse the Hubble tension with a recent age of the Universe constraint. This allows us to restrict attention to matter and a dark energy sector that we treat without assuming a specific model. Assuming analyticity of the Hubble parameter H(z), and a generic low redshift modification to flat ΛCDM, we find that low redshift data (z 2.5) and well-motivated priors only permit a dark energy sector close to the cosmological constant Λ. This restriction rules out late Universe modifications within FLRW. We show that early Universe physics that alters the sound horizon can yield an upper limit of H 0 ∼ 71 ± 1 km s −1 Mpc −1 . Since various local determinations may be converging to H 0 ∼ 73 km s −1 Mpc −1 , a breakdown of the FLRW framework is a plausible resolution. We outline how future data, in particular strongly lensed quasar data, could also provide further confirmations of such a resolution.

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 ΛCDM cosmology, we observe a correlation between higher H 0 and the CMB dipole direction, confirming our previous results for strongly lensed quasars [Classical Quantum Gravity 38, 184001 (2021)]. Concretely, we record a ∼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 ≲ 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 .

We investigate the cosmological viability of the generalized proca theory. We first implement the background and linear perturbation equations of motion in the Boltzmann code and then study the constraints on the parameters of the generalized proca theory after running MCMC against the cosmological data set. With Planck + HST data, we obtain the constraint h=0.7334−0.0269+0.0246, which indicates that the tension between early universe and late time universe within this theory is removed. By adding other late-time data sets (BAO, RSD, etc.) we show that the tension is reduced, as the 2σ allowed region for h in Proca, h=0.7041−0.0087+0.0094, overlaps with the 2σ region of the HST data.

The Cosmological Principle (CP) -- the notion that the Universe is spatially isotropic and homogeneous on large scales -- underlies a century of progress in cosmology. It is conventionally formulated through the Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) cosmologies as the spacetime metric, and culminates in the successful and highly predictive $\Lambda$-Cold-Dark-Matter ($\Lambda$CDM) model. Yet, tensions have emerged within the $\Lambda$CDM model, most notably a statistically significant discrepancy in the value of the Hubble constant, $H_0$. Since the notion of cosmic expansion determined by a single parameter is intimately tied to the CP, implications of the $H_0$ tension may extend beyond $\Lambda$CDM to the CP itself. This review surveys current observational hints for deviations from the expectations of the CP, highlighting synergies and disagreements that warrant further study. Setting aside the debate about individual large structures, potential deviations from the CP include variations of cosmological parameters on the sky, discrepancies in the cosmic dipoles, and mysterious alignments in quasar polarizations and galaxy spins. While it is possible that a host of observational systematics are impacting results, it is equally plausible that precision cosmology may have outgrown the FLRW paradigm, an extremely pragmatic but non-fundamental symmetry assumption.