arXiv subjects

Vasileios Kalaitzidis

Publications and source records attributed to Vasileios Kalaitzidis.

3 recordsLinked to original sources

arXiv2026

Comparing measures of the Hubble and BAO tensions in $Λ$CDM and possible solutions in $f(Q)$ gravity

We test whether $f(Q)$ symmetric teleparallel gravity theories can solve the Hubble tension consistently with DESI DR2 BAO. We consider three $f(Q)$ functional forms: logarithmic, exponential, and hyperbolic tangent. We extend these models by allowing a cosmological constant, and compare to phenomenological models with a flexible exponential, hyperbolic secant, and polynomial decay addition to the standard $Λ$CDM $H(z)$. We test these models against DESI DR2 BAO, CMB ($Planck$ 2018 + SPT-3G + ACT DR6), local $H_0$, and Cosmic Chronometer data. The logarithmic and hyperbolic tangent $f(Q)$ models do not provide an adequate solution, but the exponential model does. Furthermore, it slightly reduces the $(Ω_m, H_0 r_d)$ parameter space tension between CMB and BAO datasets to $2.56σ$, down from $2.65σ$ for $Λ$CDM. Although $Λ$CDM faces only $1.66σ$ tension in DESI data space, the $1σ$ higher tension in parameter space suggests a real anomaly. The models assisted by the cosmological constant perform slightly better still, at the cost of undermined theoretical motivation. They also perform poorly once local $H_0$ measurements are included. The phenomenological models fit all data reasonably well, yet the best-fitting models predict isotropically averaged BAO distances exceeding the DESI DR2 measurements at all redshifts. This highlights the difficulties of finding a theoretically motivated solution to the Hubble tension while remaining consistent with BAO data.

astro-ph.CO↗

arXiv2026

The local void model for the Hubble and BAO tensions

The inconsistency between the locally inferred Hubble constant and the value inferred from the cosmic microwave background assuming the $Λ$CDM cosmological model has persisted, turning into an important problem. An emergent underlying trend is that this Hubble tension is driven by data confined to the very low-redshift Universe (typically $z < 0.15$). Most intermediate-redshift measurements remain mutually consistent with $H_0^\mathrm{CMB}$, the $Λ$CDM expectation anchored by the CMB. This Perspective examines if a large local void can explain the Hubble tension and its appearance only at low $z$. For an observer residing within a large underdensity, such as the Milky Way inside the claimed KBC void, gravitationally induced outflows and redshift can inflate the locally inferred recession scale $cz'$ despite having $H_0 = H_0^\mathrm{CMB}$. We summarise evidence suggestive of a local underdensity from multi-wavelength galaxy number counts, discuss the dynamical requirements implied by the amplitude of inferred bulk flows, and connect the solution to the emerging low-redshift BAO distance anomaly ($α_{\mathrm{iso}} < 1$). Previously published semi-analytic void models anticipated the observed redshift dependence of BAO deviations and predict a rapid convergence to CMB-consistent expansion for $z \gtrsim 0.2$, aligning with reconstructions of $H_0(z)$ from BAO plus uncalibrated Type Ia supernovae. We conclude by looking to future tests, including improved mapping of the local density and velocity field, fits to galaxy distance catalogues at the field level, kinematic Sunyaev-Zel'dovich constraints on coherent outflows, fast radio bursts, and the long-term prospect of redshift drift measurements as a direct probe of time-varying non-cosmological redshift contributions.

astro-ph.CO↗

arXiv2025

Testing the local void hypothesis using baryon acoustic oscillation measurements over the last twenty years

A promising solution to the Hubble tension is a local void that is roughly 20% underdense out to 300 Mpc, as suggested by galaxy number counts in the near-infrared. Gravitationally driven outflows from this KBC void might inflate redshifts enough to solve the Hubble tension, a scenario explored in detail by Haslbauer et al. We obtain predictions for the baryon acoustic oscillation (BAO) observables in their best-fitting void models and in the homogeneous $Planck$ cosmology. We compare these models against our compilation of available BAO measurements from the past twenty years. We find that the quality and quantity of available measurements are best using the isotropically averaged distance $D_{\mathrm{V}}$. Taking its ratio with the expected value in the homogeneous model yields good agreement with unity at high redshift, but a discrepancy appears that systematically grows with decreasing redshift. Assuming independent uncertainties, the 42 considered $D_{\mathrm{V}}$ observations give a total $χ^2$ of 75.7 for the void-free model, while the void models give only $47.3 - 51.2$ depending on the density profile. This represents a reduction in overall tension from $3.3σ$ without a void to $1.1σ- 1.4σ$ in the void models. The $χ^2$ differences are smaller when considering measurements of the angular BAO scale or its redshift depth. The void-free model provides the worst fit in almost every case. Overall, our results suggest that recent evidence of BAO observables deviating from expectations in the homogeneous $Planck$ cosmology could indicate a local void, which was motivated by considerations unrelated to BAO data or the Hubble tension.

astro-ph.CO↗