We study the phenomenology of the ‘companion-axion model’ consisting of two coupled QCD axions. The second axion is required to rescue the Peccei–Quinn solution to the strong-CP problem from the effects of colored gravitational instantons. We investigate here the combined phenomenology of axion–axion and axion–photon interactions, recasting present and future single-axion bounds onto the companion-axion parameter space. Most remarkably, we predict that future axion searches with haloscopes and helioscopes may well discover two QCD axions, perhaps even within the same experiment.

Australia

Zhe Chen

Archil Kobakhidze

Ciaran O’Hare

Zachary Picker

Giovanni Pierobon

Phenomenology of the companion-axion model: photon couplings

We discuss the validity of the Thakurta metric to describe cosmological black holes by analysing the nature of its horizon. By adopting the preferred foliation of the Thakurta spacetime associated with the Kodama time, we demonstrate that the Thakurta horizon is indeed a future outer trapping horizon. Therefore, the respective observers see it as a cosmological black hole, contrary to some claims in the literature.

Apparent horizons of the Thakurta spacetime and the description of cosmological black holes

In this paper, we study gravitational waves generated by binary systems within an extension of General Relativity which is described by the addition of quadratic in curvature tensor terms to the Einstein–Hilbert action. Treating quadratic gravity as an effective theory valid in the low energy/curvature regime, we argue that reliable calculations can be performed in the early inspiral phase, and furthermore, no flux of additional massive waves can be detected. We then compute the Yukawa suppressed dipole-like corrections to the post-Newtonian (PN) expansion of the standard waveform. By confronting these theoretical calculations with available experimental data, we constrain both unknown parameters of quadratic gravity to be  $$0 \le \gamma \, \lesssim 5.9\times 10^{76}$$  <math> <mrow> <mn>0</mn> <mo>≤</mo> <mi>γ</mi> <mspace width="0.166667em"></mspace> <mo>≲</mo> <mn>5.9</mn> <mo>×</mo> <msup> <mn>10</mn> <mn>76</mn> </msup> </mrow> </math>  , and  $$-\frac{\gamma }{4} \le \beta \, \lesssim 9.8\times 10^{75} - \frac{\gamma }{4}$$  <math> <mrow> <mo>-</mo> <mfrac> <mi>γ</mi> <mn>4</mn> </mfrac> <mo>≤</mo> <mi>β</mi> <mspace width="0.166667em"></mspace> <mo>≲</mo> <mn>9.8</mn> <mo>×</mo> <msup> <mn>10</mn> <mn>75</mn> </msup> <mo>-</mo> <mfrac> <mi>γ</mi> <mn>4</mn> </mfrac> </mrow> </math>  .