https://doi.org/10.1140/epjc/s10052-020-7961-3
Regular Article - Theoretical Physics
Scalar perturbations in f(T) gravity using the 
covariant approach
1
Astronomy and Astrophysics Research and Development Department, Ethiopian Space Science and Technology Institute, Addis Ababa, Ethiopia
2
Department of Physics, College of Natural and Computational Science, Wolkite University, Wolkite, Ethiopia
3
Department of Physics, College of Science and Technology, University of Rwanda, Kigali, Rwanda
4
Center for Space Research, North-West University, Vanderbijlpark, South Africa
5
Cosmology and Gravity Group, Mathematics and Applied Mathematics, University of Cape Town, Rondebosch, 7701, South Africa
6
Institute of Theoretical Astrophysics, University of Oslo, Oslo, Norway
* e-mail: shambel.sahlu@wku.edu.et
Received:
16
January
2020
Accepted:
22
April
2020
Published online:
14
May
2020
The cosmological scalar perturbations of standard matter are investigated in the context of extended teleparallel f(T) gravity theories using the 
covariant formalism. After a review of the background gravitational field equations of f(T) gravity and the introduction of the covariant perturbation variables, the usual scalar and harmonic decomposition have been performed, and the analysis of the growth of the density contrasts in the quasi-static approximation for two non-interacting fluids scenarios, namely torsion-dust and torsion-radiation mixtures is presented for the generic f(T) gravity theory. Special applications to two classes of f(T) gravity toy models, namely 
and 
, have then been made within the observationally viable regions of their respective parameter spaces, and the growth of the matter density contrast for both torsion-dust and torsion-radiation epochs of the Universe has been examined. The exact solutions of the dust perturbations, with growing amplitudes in cosmic time, are obtained for some limiting cases of n. Similarly, the long- and short-wavelength modes in the torsion-radiation case are treated, with the amplitudes either oscillating or monotonically growing with time. Overall, it is noted that f(T) models contain a richer set of observationally viable structure growth scenarios that can be tested against up-and-coming observational data and can accommodate currently known features of the large-scale structure power spectrum in the general relativistic and 
limits.
© The Author(s), 2020
