Isotropic cosmic birefringence from string axion domain walls without cosmic strings and DESI results

Recently, results from the Atacama Cosmology Telescope (ACT) DR6 have shown a preference for isotropic cosmic birefringence, consistent with previous analyses based on Planck and WMAP data. Separately, the Dark Energy Spectroscopic Instrument (DESI) DR2 results suggest that dark energy evolves over cosmic history, pointing to new physics in the late-time universe. In this paper, we propose that domain walls associated with the string axion can naturally explain the isotropic cosmic birefringence, focusing on the case in which the axion starts near a hilltop. Interestingly, to avoid the domain wall problem, these walls must form well after recombination. The predicted rotation angle, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi>β</a:mi><a:mo>≈</a:mo><a:mn>0.21</a:mn><a:msub><a:mrow><a:mi>c</a:mi></a:mrow><a:mrow><a:mi>γ</a:mi></a:mrow></a:msub></a:mrow></a:math> degrees (with anomaly coefficient <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:msub><c:mrow><c:mi>c</c:mi></c:mrow><c:mrow><c:mi>γ</c:mi></c:mrow></c:msub><c:mo>≈</c:mo><c:mn>1</c:mn></c:mrow></c:math>), is in excellent agreement with observations. This scenario can be further tested by probing anisotropic birefringence of photons emitted long after recombination, as well as gravitational waves. Moreover, starting the axion oscillation from a hilltop naturally enhances its abundance via anharmonic effects, thus contributing to the dark energy component. We discuss how this hilltop axion scenario may connect with the DESI results.