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I am rephrasing an answer to another PSE question as an new question here.

Dark matter is invoked to explain the 'observed space-time geometry that does not match with predictions'.

According to the Einstein-Cartan version of gravity, there are two sources which "distort" local space-time:

  1. Mass (or more generally energy-momentum tensor), which induces space-time curvature.
  2. Spin (spin current from Dirac fermions), which induces space-time torsion.

As an alternative to missing/dark mass, can we attribute the "observed space-time geometry that does not match with predictions" to dark spin current?

As per Poplawski, the non-zero spin/torsion approach has another nice property that "averts the unphysical big-bang singularity, replacing it with a cusp-like bounce at a finite minimum scale factor, before which the Universe was contracting".


Added note:

As an alternative to dark matter, the MOND (modified Newtonian dynamics) hypothesis explains nicely the flat rotation curve of a spiral galaxy (Tully-Fisher law). But MOND failed miserably at inter-galactic scale and has nothing to say about cosmological scale dark matter, though there are some stilted versions of relativistic MOND extension. So "dark spin current" may come to the rescue.

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Yes, at least torsion can be used to explain the relic dark matter density from the big bang:

https://arxiv.org/abs/1611.03651

A quick read of this paper (a peer-reviewed version of which can be found in PRD): The minimal interaction of torsion coupled to fermions is non-renormalizable, but taking non-minimal coupling to fermions (whose effect turns out can be ignored anyway) and the simplist possible coupling to scalars (Higgs, a free parameter), one can match the Planck data pretty easily.

They consider two scenarios; in the first, the torsion acquires mass solely from the Higgs field. In the second, the torsion acquires mass from both the Higgs field, and from a higher scale symmetry breaking.

They found that for the first case, it is not possible to consider all of the relic DM to be due to torsion. However, in the second case, the parameter spaces allows torsion to be the sole source of relic DM.

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  • $\begingroup$ Interesting. Besides the cosmological scale DM, can your theory explain the flat rotation curve of a spiral galaxy (Tully-Fisher law)? $\endgroup$ – MadMax Oct 26 '18 at 15:49
  • $\begingroup$ @MadMax: It's not my theory, it's been published in Physical Review D by Belyaev, Shapiro, and Thomas, and no, it does not include any predictions of flat rotation curves (to the best of my knowledge). Of course, those can be explained by other non-dark matter means (see Cooperstock-Tieu galaxy models). $\endgroup$ – levitopher Oct 28 '18 at 3:41
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Interesting, why not? All the properties that dark matter has been assigned, are more suitable for empty (spinning) space than anything else. These are being cold, transparent, not interacting in any other way except gravitational.

However the difference between dark spin and dark matter, in my opinion would be detectable. In case of spin, the effects (e.g. lansing and uniform rotation curve), would show only for things in/through the spin region. In case of "dark matter", the effects should show irrespective of whether the phenomena is taking place inside or outside the cloud.

For example, yes, the uniform rotation curve of a spiral galaxy does require some form of dark entity. Do the attraction between two spiral galaxies also require same dark entity? If not, then there is no dark "matter", it got to be something else. If yes, then it likely is matter.

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  • $\begingroup$ "the uniform rotation curve of a spiral galaxy does require some form of dark entity", the Tully-Fisher law (flat rotation curve) is explained nicely by the MOND (modified Newtonian dynamics) hypothesis. But MOND failed miserably at inter-galactic scale and has nothing to say about cosmological scale dark matter (there are some stilted attempts of relativistic MOND extension). So "dark spin current" may come to the rescue. $\endgroup$ – MadMax Oct 26 '18 at 14:02

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