Axion coupling accelerates the Universe through <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi mathvariant="script">PT</mml:mi> </mml:mrow> </mml:math> -symmetric phases

The conjecture by two of the authors (N. E. M. and S. S.) that a <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mi mathvariant="script">PT</a:mi> </a:mrow> </a:math> -symmetric phase plays a role in understanding singular renormalization group (RG) flows for a Chern-Simons (CS) gauge theory of axions, has been reexamined and significantly improved. We have used the more complete Wetterich equation, which includes gravitational couplings in a systematic way from the start, to understand the emergence of this phase. The singular structure of the RG flows has persisted on including gravitational couplings, thereby offering further support to the conjecture that <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:mrow> <d:mi mathvariant="script">PT</d:mi> </d:mrow> </d:math> -symmetric phases of (repulsive) gravity characterize string-effective CS gravitational theories, where the axion is the massless string-model independent axion, which can also play a role of a totally antisymmetric torsion degree of freedom. This has suggested a novel interpretation of the currently observed acceleration of the expansion of the Universe in terms of such a phase at large (cosmological) scales.