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Theory-agnostic searches for non-gravitational modes in black hole ringdown
by Francesco Crescimbeni, Xisco Jimenez Forteza, Swetha Bhagwat, Julian Westerweck, Paolo Pani
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Submission summary
| Authors (as registered SciPost users): | Francesco Crescimbeni · Paolo Pani |
| Submission information | |
|---|---|
| Preprint Link: | scipost_202507_00026v2 (pdf) |
| Date submitted: | Oct. 23, 2025, 3:35 p.m. |
| Submitted by: | Francesco Crescimbeni |
| Submitted to: | SciPost Physics |
| Ontological classification | |
|---|---|
| Academic field: | Physics |
| Specialties: |
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| Approaches: | Theoretical, Phenomenological, Observational |
Abstract
In any extension of General Relativity (GR), extra fundamental degrees of freedom couple to gravity. Besides deforming GR forecasts in a theory-dependent way, this coupling generically introduces extra modes in the gravitational-wave signal. We propose a novel theory-agnostic test of gravity to search for these nongravitational modes in black hole merger ringdown signals. To leading order in the GR deviations, their frequencies and damping times match those of a test scalar or vector field in a Kerr background, with only amplitudes and phases as free parameters. By applying this test to GW150914, GW190521, and GW200129, we find no strong evidence for an extra mode; however, its inclusion modifies the inferred distribution of the remnant spin. This test will be applicable for future detectors, which will achieve signal-to-noise ratios higher than 100 (and as high as 1000 for space-based detectors such as LISA). Such sensitivity will allow measurement of these modes with amplitude ratios as low as 0.02 for ground-based detectors (and as low as 0.003 for LISA), relative to the fundamental mode, enabling stringent agnostic constraints or detection of scalar/vector modes.
Author indications on fulfilling journal expectations
- Provide a novel and synergetic link between different research areas.
- Open a new pathway in an existing or a new research direction, with clear potential for multi-pronged follow-up work
- Detail a groundbreaking theoretical/experimental/computational discovery
- Present a breakthrough on a previously-identified and long-standing research stumbling block
Author comments upon resubmission
We thank the Referee for the time dedicated to reviewing our work and for their constructive and
insightful comments. We address their points below; the corresponding revisions in the manuscript are
highlighted in red.
As a general remark, we note that since our original submission to the arXiv (more than one year
ago), our results have been confirmed and extended in a recent study by other authors [1], as well as in
a follow-up work by some of us [2], further highlighting the relevance of our original findings.
We believe that the main contribution of our work was to clarify that additional modes in the GW
signal are at least as significant as shifts in the QNM frequencies—an effect largely overlooked in most
previous analyses, including the LIGO–Virgo–KAGRA ringdown tests of gravity. As detailed below, we
do not claim that other effects (such as overtones, nonlinearities, or higher harmonics) are negligible;
rather, their relative importance depends on the specific modified-gravity theory and, crucially, on the
nature of the source.
Sincerely,
The Authors
[1] Jacopo Lestingi, Giovanni D’Addario, and Thomas P. Sotiriou. Frequency contamination from new
fundamental fields in black hole ringdowns. Phys. Rev. D, 112(6):064070, 2025.
[2] Francesco Crescimbeni, Xisco Jimenez-Forteza, and Paolo Pani. Black Hole Ringdown Amplitudescopy. 10 2025.
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