On observational signatures of multi-fractional theory

TL;DR

This study investigates the multi-fractional $q$-derivatives theory's cosmological implications, finding it incompatible with certain observational data and unable to resolve existing tensions in standard cosmology.

Contribution

It provides a detailed analysis of the multi-fractional $q$-derivatives model's predictions for cosmic expansion and structure growth, confronting it with current observational data.

Findings

Multi-fractional model increases structure growth, conflicting with low-redshift data.

Hubble constant is reduced, conflicting with local measurements.

Parameters remain unconstrained due to degeneracy in observational data.

Abstract

We study the multi-fractional theory with $q$-derivatives, where the multi-fractional measure is considered to be in the time direction. The evolution of power spectra and also the expansion history of the universe are investigated in the $q$-derivatives theory. According to the matter power spectra diagrams, the structure growth would increase in the multi-fractional model, expressing incompatibility with low redshift measurements of large scale structures. Furthermore, concerning the diagrams of Hubble parameter evolution, there is a reduction in the value of Hubble constant which conflicts with local cosmological constraints. Thus, primary numerical investigations imply that $q$-derivatives theory has no potential to relieve observational tensions. We also explore the multi-fractional model with current observational data, principally Planck 2018, weak lensing, supernovae, baryon acoustic oscillations (BAO), and redshift-space distortions (RSD) measurements. Numerical analysis reveals that the degeneracy between multi-fractional parameters makes them remain unconstrained under observations. Furthermore, observational constraints on $H_0$ and $\sigma_8$, detect no significant departure from standard model of cosmology.