Traversable wormholes in $f(T,\tau )$ gravity: a complete classification of the non-exotic sector

Abstract

We study static and spherically symmetric traversable wormholes in $f(T,\tau )$ gravity, where the torsion scalar $T$ is coupled to the trace $\tau$ of the matter energy--momentum tensor. We consider the linear model $f(T,\tau )=T+\beta \tau$ with an anisotropic fluid and the mean-pressure matter Lagrangian $\Lm=\Pmean=(p_r+2p_t)/3$. The field equations are obtained for the Morris--Thorne geometry without fixing the redshift or shape function at the outset. For a constant redshift function, the energy-condition problem takes a simple form. On the branch $\beta >8\pi$ and for $b(r)>0$, the energy density together with the null, weak, and strong energy conditions is satisfied throughout the spacetime if and only if $rb(r)$ is non-increasing. The same condition also implies asymptotic flatness, $b(r)<r$ outside the throat, and $b^{\prime }(r_0)\le -1$. The allowed geometries can therefore be written as $b(r)=r_0^{2}h(r)/r$, where $h(r_0)=1$ and $h(r)$ is positive and non-increasing. For the representative family $b(r)=r_0(r_0/r{)}^n$, the null, weak, and strong energy conditions hold for $n\ge 1$, while the dominant energy condition requires $n\ge 3(\beta -2\pi )/(\beta -6\pi )$. We also separate the physical matter from the effective source and show how the trace coupling allows the physical matter to remain non-exotic although the effective source violates the null energy condition. Finally, we examine the marginal case $b(r)=r_0^2/r$ with a non-constant redshift function. A decreasing redshift function can improve the tangential null energy condition at the throat, but this improvement cannot be maintained throughout an asymptotically flat exterior. These results show that the matter--torsion coupling can support a broad class of traversable wormholes without requiring exotic physical matter.

11 pages, 4 figures