Gravitational wave detections from black hole mergers have opened a new window to test General Relativity (GR) in strong-field regimes. A key technique involves analyzing the "ringdown" phase, the final stage of coalescence where the remnant black hole settles into its stable state, emitting gravitational waves with characteristic frequencies known as quasinormal modes (QNMs). Traditionally, research has focused on searching for shifts in these QNM frequencies from those predicted by GR's Kerr metric.

However, recent work suggests that ringdown analysis might be more complex than anticipated. If new fields, beyond those described by GR, exist and couple non-minimally to gravity, their own quasinormal modes could "contaminate" the ringdown signal. This implies that observed deviations might not solely be due to shifts in the GR QNM frequencies, but also to the presence of additional QNMs associated with these new fields.

Researchers investigated this concept within the framework of the shift-symmetric Horndeski action, which describes interactions between a massless scalar field and gravity, leading to second-order equations. Using a perturbative analysis, expanding in the scalar charge per unit black hole mass (q), they demonstrated that, up to order q², the coupling between the scalar and the Gauss-Bonnet invariant is the only term contributing to both frequency shifts and contamination. Both effects appear at the same perturbative order. If the assumption about the scalar amplitude being suppressed by q is relaxed, contamination can appear at leading order in q, dominating over frequency shifts and receiving additional corrections from other couplings. This finding underscores the importance of considering the potential presence of scalar fields when interpreting black hole ringdown signals.