Torsion in string-inspired cosmologies and the universe dark sector

Abstract

I review several aspects of torsion in string-inspired cosmologies. In particular, I discuss its connection with fundamental, string-model independent, axion fields associated with the massless gravitational multiplet of the string. I argue in favour of the role of primordial gravitational anomalies coupled to such axions in inducing inflation of a type encountered in the "running-vacuum-model (RVM)" cosmological framework, without fundamental inflaton fields. The gravitational-anomaly terms owe their existence to the Green-Schwarz mechanism for the (extra-dimensional) anomaly cancellation, and may be non-trivial in such theories in the presence of (primordial) gravitational waves at early stages of the four-dimensional string Universe (after compactification). I also discuss how the torsion-induced stringy axions can acquire a mass in the post inflationary era, due to non perturbative effects, thus having the potential to play the rôle of (a component of) dark matter in such models. Finally, I describe briefly the current-era phenomenology of this model, with the emphasis placed on the possibility of alleviating tensions observed in the current-era cosmological data. A brief phenomenological comparison with other cosmological models in contorted geometries is also made.

37 pages revtex, 2 pdf figures incorporated.References and Subsection IV.C added, on potential primordial black hole effects on gravitational waves during inflation. No effects on conclusions. Version to be published in special issue of Universe "Beyond Riemannian Geometry in Classical and Quantum Gravity" (Guest eds. Marco Danilo Claudio Torri, Christian Pfeifer and Nicoleta Voicu)

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• Renormalizing the vacuum energy in cosmological spacetime: implications for the cosmological constant problem
• The Cosmological Constant Problem and Running Vacuum in the Expanding Universe
• Big Bang Nucleosynthesis constraints on higher-order modified gravities
• The effective field theory approach to the strong coupling issue in $f(T)$ gravity
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• Torsion at different scales: from materials to the Universe
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• Quantum-Ordering Ambiguities in Weak Chern-Simons 4D Gravity and Metastability of the Condensate-Induced Inflation
• Time-loops to spot torsion on bidimensional Dirac materials with dislocations