The distinctions between $\Lambda$CDM and $f(T)$ gravity according Noether symmetry

TL;DR

This paper uses Noether symmetry to analyze different $f(T)$ gravity models, finding that the optimal form closely resembles $ ext{Lambda}$CDM but with notable differences, especially in conserved quantities and symmetries.

Contribution

It introduces a systematic Noether symmetry approach to identify the most suitable $f(T)$ models that mimic $ ext{Lambda}$CDM and highlights key differences in symmetries and conserved quantities.

Findings

The optimal $f(T)$ form is $ ext{alpha} T + eta T^{-1}$.

Power-law and exponential forms are less effective.

Significant differences in symmetries between $ ext{Lambda}$CDM and $f(T)$ gravity.

Abstract

Noether's theory offers us a useful tool to research the conserved quantities and symmetries of the modified gravity theories, among which the $f(T)$ theory, a generally modified teleparallel gravity, has been proposed to account for the dark energy phenomena. By the Noether symmetry approach, we investigate the power-law, exponential and polynomial forms of $f(T)$ theories. All forms of $f(T)$ concerned in this work possess the time translational symmetry, which is related with energy condition or Hamilton constraint. In addition, we find out that the performances of the power-law and exponential forms are not pleasing. It is rational adding a linear term $T$ to $T^n$ as the most efficient amendment to resemble the teleparallel gravity or General Relativity on small scales, ie., the scale of the solar system. The corresponding Noether symmetry indicates that only time translational symmetry remains. Through numerically calculations and observational data-sets constraining, the optimal form $\alpha T + \beta T^{-1}$ is obtained, whose cosmological solution resembles the standard $\Lambda$CDM best with lightly reduced cosmic age which can be alleviated by introducing another $T^m$ term. More important is that we find the significant differences between $\Lambda$CDM and $f(T)$ gravity. The $\Lambda$CDM model has also two additional symmetries and corresponding positive conserved quantities, except the two negative conserved quantities.