{"id":"16e2c835-9ca6-4c4b-8a60-4420afe167ac","arxiv_id":"2411.19129","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"Even though GW170817 favors scalar-tensor gravity only weakly, the inferred neutron star radius and high-density equation of state depend on which gravity theory is assumed, so current EOS constraints may carry a theory bias.","lead":"Using gravitational-wave data from GW170817, the authors ask whether a modified theory of gravity could change what we infer about neutron star sizes and the nuclear equation of state. They find that scalar-tensor gravity and general relativity fit the data equally well while predicting different neutron star radii, so current EOS measurements could be biased if this theory is right.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Neglected scalar dipole radiation is the load-bearing gap: for beta=-6, the omitted -1PN dipole phase can dominate the tidal signal, so the claim that ST deviations go undetected is not yet established.","rationale":"The reader identified the same weakest assumption, and I concur that it is load-bearing. My emphasis is on the post-Newtonian ordering: compared to the 5PN tidal phase, dipole radiation enters at -1PN and can be many cycles for a scalarized neutron star, making the omission potentially larger than the effect under study. The paper is transparent about this simplification, which is why I do not recommend rejection; however, the abstract's general phrasing overstates the breadth of the conclusion. Since the reader's verdict is already CONDITIONAL and my concern aligns with the reader's, I recommend keeping the verdict unchanged rather than escalating it. The proposed concrete test would settle whether the omission changes the conclusion quantitatively for GW170817.","tokens_in":11246,"tokens_out":7569,"duration_ms":87615,"concrete_test":"Re-analyze GW170817 with the dipole-radiation phase included: compute the scalar charges alpha_A and alpha_B for the same sampled EOSs with beta=-6 and jinf=1e-3, add the leading-order -1PN dipole phase correction (e.g., from Bernard 2020) to the IMRPhenomPv2_NRTidal waveform, and rerun the DYNESTY sampling with identical priors. If the 90% credible intervals for component masses or tidal deformabilities shift by more than their statistical width, or if log10 B_ST/GR changes by more than about 1 relative to the reported +0.54, then the neglected scalar dipole radiation is material to the central claim.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The headline claim requires the ST waveform to be a faithful ST prediction, but the analysis changes only the tidal deformabilities in the GR waveform model IMRPhenomPv2_NRTidal while keeping the point-particle GR phasing unchanged. In the DEF model with beta=-6 and jinf=1e-3, spontaneous scalarization endows neutron stars with scalar charges, which source scalar dipole radiation at -1PN order. This is much earlier than the 5PN tidal phase, so the omitted dipole contribution can in principle shift the inferred masses, radii, and tidal deformabilities, and may make the full ST model strongly disfavored by GW170817. The paper explicitly acknowledges this omission in the final paragraph of Section 6, but does not estimate its magnitude. Consequently, the conclusion that 'deviations from GR could go undetected when analyzing current binary neutron star mergers' is only demonstrated for a restricted tidal-only model, not for the full ST theory.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper investigates whether deviations from general relativity in scalar-tensor (ST) theory could be hidden in current binary neutron star gravitational-wave data while simultaneously biasing inferences of the nuclear equation of state. Using the Damour-Esposito-Farèse model with β = −6 and φ∞ = 10⁻³, the authors compute ST ℓ = 2 tidal deformabilities for a 2500-member EOS ensemble and perform Bayesian parameter estimation on GW170817 with the IMRPhenomPv2_NRTidal waveform. They report that GR and ST posteriors are consistent in component masses and tidal deformabilities, that the inferred radius of the heavier component and the high-density EOS (around 3 n_sat) differ between the two theories, and that the Bayes factor slightly favors ST (log₁₀ B = 0.54 ± 0.03). They conclude that EOS measurements may be biased if non-GR effects are ignored and that such deviations could go undetected in current observations.","tokens_in":11469,"tokens_out":9333,"duration_ms":88788,"significance":"If established, the result would be important: it would show that a well-motivated ST theory can produce a GW170817-like signal that is statistically compatible with GR while shifting the inferred nuclear EOS, thereby coupling modified-gravity tests with dense-matter inference. The paper has clear strengths: it uses public LIGO/Virgo data, a standard nested-sampling pipeline, a transparent EOS parameterization, and it is explicit about many modeling simplifications. The main weakness is that the ST waveform is not a complete ST waveform: only the tidal deformabilities are modified, while the point-particle phasing remains the GR one. Because scalar dipole radiation in the DEF model enters at −1PN order, the paper's headline conclusion that deviations from GR could go undetected is not yet demonstrated for the full ST theory.","major_comments":[{"comment":"The ST analysis uses the GR waveform model IMRPhenomPv2_NRTidal and inserts ST tidal deformabilities while leaving the point-particle phasing unchanged. In the DEF theory with β = −6 and φ∞ = 10⁻³, neutron stars acquire scalar charges that source dipolar radiation at −1PN order, which enters the gravitational-wave phase well before the 5PN tidal term. The authors acknowledge neglecting this effect but do not estimate its magnitude or show that it is negligible for the masses relevant to GW170817. Without including the −1PN dipole phase, or at least providing a quantitative bound based on the scalar charges, the abstract's claim that deviations from GR could go undetected in current binary neutron star mergers is not established for the full ST theory; a complete ST waveform could, in principle, be strongly disfavored by the data. Please either include a dipole-phasing term in the waveform model or restrict the conclusion to the tidal-only model and revise the abstract and Section 6 accordingly.","section":"Section 5 and Section 6, final paragraph"},{"comment":"The paper uses the word \"bias\" to describe differences between the GR and ST posteriors, but the analysis is performed on real data and does not include an injection-recovery study. The ST posterior for c_s²(3 n_sat) is reported as showing essentially no change from its prior, whereas the GR posterior is updated; this difference could be driven by the different prior support in the mass-radius plane rather than by a systematic mis-recovery of a true ST signal. To substantiate the claim that EOS measurements are biased when non-GR effects are ignored, the authors should perform injection tests: inject a GW170817-like signal generated with the ST model (ideally including dipole radiation) and show that recovery with the GR model shifts the EOS parameters relative to their injected values. If such a study is outside the scope of the paper, the \"bias\" language should be softened to \"model-dependent differences.\"","section":"Section 6, Figures 1 and 2"}],"minor_comments":[{"comment":"The sentence \"icos is taken to be uniform over [−1, 1]\" appears to be a typo for \"cos ι is taken to be uniform over [−1, 1]\"; please correct it.","section":"Section 5"},{"comment":"The expansion variable x is defined in the sentence after Eqs. (5)–(6) but is already used in those equations; please define x before its first use.","section":"Section 4, Eqs. (5)–(6)"},{"comment":"Please state explicitly whether the M_TOV > 1.9 M_sun and the 2.18 M_sun pulsar cutoffs are enforced using the GR or the ST mass-radius relation, since the theory-dependent filtering changes the EOS prior entering the parameter estimation.","section":"Section 4"},{"comment":"The caption says that the mass and radius priors are shown as dotted lines in the 1D marginal plots, but it does not clarify whether these priors are identical for the GR and ST runs; please state this explicitly.","section":"Section 6, Figure 1 caption"},{"comment":"Please clarify whether the quoted Bayes factor includes the EOS prior and the pulsar constraints in the evidence calculation, and state whether the quoted uncertainty (0.03 in log₁₀) is a nested-sampling sampling error.","section":"Section 6, Eq. (7) and following text"}],"recommendation":"major_revision","confidential_remarks":null},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: this is a careful proof-of-principle that ST tidal effects can shift NS radius/EOS posteriors while leaving masses and tidal deformabilities consistent. The new step is applying the ST ℓ=2 Love numbers (Brown 2023; Creci et al. 2023) to real GW170817 parameter estimation with EOS inference. The machinery is solid: public data, standard sampler, explicit priors, Bayes factor. The result that the heavier star's radius and the c_s^2(3n_sat) posterior shift between GR and ST is a useful warning for the dense-matter community.\n\nThe soft spot is exactly what the stress test flags. The waveform model is IMRPhenomPv2_NRTidal with only the tidal deformabilities swapped; the point-particle phasing is GR. In DEF theory at β=-6, scalar dipole radiation enters at -1PN, five orders earlier than the 5PN tidal phase. The paper's final paragraph says this is neglected, but the abstract's 'deviations from GR could go undetected' is not qualified to the tidal-only sector. Without a magnitude estimate for the dipole contribution, the claim that full ST is indistinguishable from GR on GW170817 is not established. This is a real gap, though the paper is transparent about it.\n\nTwo smaller points. First, the ST sound-speed posterior at 3n_sat appears prior-dominated, so the 'bias' in EOS inference rests mainly on the radius shift, not a tight measurement. Second, the Bayes factor of 3.5 is weak; neither theory is favored, so the paper's language about 'both consistent' is fair, but it shouldn't be read as evidence for ST.\n\nOverall, this is a legitimate proof-of-principle. It deserves a serious referee; the right request would be a quantitative estimate of the dipole phase contribution or a revised abstract that confines the claim to the tidal channel. I'd cite it as a caution when interpreting EOS constraints. I'd bring it to reading group as a useful example of theory-systematic in BNS PE.","headline":"First real-event application of ST tidal Love numbers; the core caution about EOS bias is plausible, but the neglected scalar dipole radiation makes the 'undetected deviations' claim broader than the analysis supports.","tokens_in":12013,"tokens_out":2149,"would_cite":true,"duration_ms":23403,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Reanalyzing GW170817 with scalar-tensor gravity changes neutron-star radii and the inferred equation of state without violating current limits, so deviations from general relativity could go undetected.","keywords":["gravitational waves","neutron stars","scalar-tensor gravity","tidal deformability","equation of state","GW170817","spontaneous scalarization","nuclear saturation density"],"falsifier":"A concrete check is to inject a full scalar-tensor waveform, including scalar dipole radiation, into the same GW170817 analysis: if the resulting posteriors separate from the general-relativistic fit, or if the Bayes factor moves decisively, the claimed degeneracy fails. With current data, comparing a fit that includes dipole radiation against the published scalar-tensor fit would already show whether the neglected term changes the inferred masses and tidal deformabilities.","tokens_in":11043,"feed_emoji":"🌌","tokens_out":7293,"duration_ms":60560,"temperature":0.7,"pith_summary":"This paper tests whether the unknown nuclear equation of state can hide a modified theory of gravity in today's neutron-star merger data. The authors re-analyze the GW170817 signal under a scalar-tensor theory with the strongest scalarization coupling that is still allowed, computing the tidal deformabilities in that theory while leaving the nuclear equation of state free. They find that the component masses and tidal deformabilities come out nearly the same as in general relativity, while the inferred neutron-star radii and the high-density equation of state shift. Neither shift violates current observational bounds. Their conclusion is that deviations from general relativity could go undetected in current binary neutron star analyses and that equation-of-state measurements near three times nuclear saturation density could be biased.","feed_headline":"Tidal effects can hide deviations from general relativity","feed_subtitle":"A scalar-tensor reanalysis of GW170817 shifts radii and EOS posteriors while staying consistent with the GR fit.","key_machinery":"The load-bearing object is the DEF scalar-tensor model, defined by the conformal coupling A(φ) = exp(β φ²/2) with β = −6, which causes spontaneous scalarization inside neutron stars when β is more negative than −4.35. For this theory the paper uses the recently derived ℓ = 2 electric tidal Love numbers, so that the scalar-tensor gravity changes the waveform only through the tidal deformabilities fed into the IMRPhenomPv2_NRTidal model. Around that tidal sector, the analysis varies a 2500-member equation-of-state ensemble built from a nuclear metamodel below twice nuclear saturation density and a sound-speed parameterization above it, and it compares the two gravity theories by nested-sampling Bayesian parameter estimation of GW170817.","core_discovery":"The paper claims that a binary neutron star merger analyzed today can mistake scalar-tensor gravity for general relativity when the equation of state is allowed to vary. Using GW170817, the authors compare a general-relativistic fit with a fit in the DEF spontaneous-scalarization model (β = −6 and scalar field at infinity equal to 10⁻³), where only the stars' tidal deformabilities carry the modified-gravity imprint. The two fits agree on component masses and tidal deformabilities, the Bayes factor is only weakly in favor of scalar-tensor theory (3.5 ± 0.2), and the differences show up instead in the radius of the heavier star and in the speed of sound at three times nuclear saturation density, where the scalar-tensor analysis prefers a softer high-density equation of state. The authors state that a measurement of the speed of sound at three times nuclear saturation density may therefore be the discriminator between general relativity and scalar-tensor theory.","pith_inferences":["I infer that the general-relativity/scalar-tensor degeneracy is likely to persist for other modified-gravity theories whose main effect is to rescale the tidal deformability at fixed mass, because the equation-of-state prior absorbs the shift.","I infer that including scalar dipole radiation, which the paper omits, would strengthen the scalar-tensor waveform's difference in the inspiral and could make the two theories distinguishable with existing events.","I infer that a population-level analysis of many binary neutron star mergers, rather than a single loud event, could reveal the systematic offset in radii that a single event hides.","I infer that rerunning the same analysis pipeline with coupling values between −4.35 and −6 would map how the bias grows with the strength of scalarization."],"forward_implications":["If ignored scalar-tensor deviations are present, published equation-of-state constraints near three times nuclear saturation density from binary neutron star mergers could be biased.","A measurement of the squared speed of sound at three times nuclear saturation density is a promising discriminator between general relativity and scalar-tensor theory.","The disagreement between two X-ray pulsar radius analyses falls within the bias introduced by alternate theories of gravity, so radius measurements alone cannot yet settle the dispute.","Third-generation gravitational-wave detectors, with order-of-magnitude improvements in event numbers and signal-to-noise, should be able to pin down the equation of state at these densities and break the degeneracy."],"supporting_citations":[{"why":"Supplies the ℓ = 2 electric tidal Love numbers in scalar-tensor theory that are used for the scalar-tensor fit.","marker":"S. M. Brown 2023"},{"why":"Defines the DEF spontaneous-scalarization model and its conformal coupling, the theory compared against general relativity.","marker":"T. Damour & G. Esposito-Farèse 1993"},{"why":"Provides the IMRPhenomPv2_NRTidal waveform model used for both general-relativistic and scalar-tensor parameter estimation.","marker":"T. Dietrich et al. 2019a, 2019b"},{"why":"Supplies the Bayesian inference methodology and nested-sampling setup for the GW170817 analysis.","marker":"C. D. Capano et al. 2020"},{"why":"Provides the nuclear empirical parameter metamodel used for the low-density equation of state below twice nuclear saturation density.","marker":"J. Margueron et al. 2018a, 2018b"},{"why":"Gives the sound-speed parameterization used to extend the equation of state above twice nuclear saturation density.","marker":"I. Tews et al. 2018"},{"why":"Supplies the fixed density grid and sound-speed extension used for the high-density equation-of-state parameters.","marker":"R. Somasundaram et al. 2023"},{"why":"The reference cited for the scalar dipole radiation effects that the paper neglects in the binary dynamics.","marker":"L. Bernard 2020"}],"fun_headline_variants":["GW170817 leaves GR and scalar-tensor gravity indistinguishable","Equation of state uncertainty masks modified gravity in neutron star mergers","Scalar-tensor gravity fits GW170817 with softer high-density equation of state","Speed of sound could reveal whether gravity deviates from general relativity","Tidal deformability alone can't expose deviations from general relativity"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The analysis assumes that scalar-tensor gravity changes only the stars' tidal stretching and not the orbital inspiral itself, so the same general-relativistic waveform is used and scalar dipole radiation is ignored.","fun_headline_variants_meta":{"raw":{"variants":["GW170817 leaves GR and scalar-tensor gravity indistinguishable","Equation of state uncertainty masks modified gravity in neutron star mergers","Scalar-tensor gravity fits GW170817 with softer high-density equation of state","Speed of sound could reveal whether gravity deviates from general relativity","Tidal deformability alone can't expose deviations from general relativity"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00076,"raw_usage":{"total_tokens":3399,"prompt_tokens":993,"completion_tokens":2406,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":609,"completion_tokens_details":{"reasoning_tokens":2318}},"tokens_in":609,"tokens_out":2406,"duration_ms":15218,"temperature":1.0,"reasoning_tokens":2318,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T10:30:10.734327+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A concrete check is to inject a full scalar-tensor waveform, including scalar dipole radiation, into the same GW170817 analysis: if the resulting posteriors separate from the general-relativistic fit, or if the Bayes factor moves decisively, the claimed degeneracy fails. With current data, comparing a fit that includes dipole radiation against the published scalar-tensor fit would already show whether the neglected term changes the inferred masses and tidal deformabilities.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the ℓ = 2 electric tidal Love numbers in scalar-tensor theory that are used for the scalar-tensor fit."},{"cited_title":"1993, PhRvL, 70, 2220","cited_arxiv_id":null,"evidence_quote":"Defines the DEF spontaneous-scalarization model and its conformal coupling, the theory compared against general relativity."},{"cited_title":"D., Tews, I., Brown, S","cited_arxiv_id":null,"evidence_quote":"Supplies the Bayesian inference methodology and nested-sampling setup for the GW170817 analysis."},{"cited_title":"2018, ApJ, 860, 149","cited_arxiv_id":null,"evidence_quote":"Gives the sound-speed parameterization used to extend the equation of state above twice nuclear saturation density."},{"cited_title":"2023, PhRvC, 107, 025801","cited_arxiv_id":null,"evidence_quote":"Supplies the fixed density grid and sound-speed extension used for the high-density equation-of-state parameters."},{"cited_title":"2020, PhRvD, 101, 021501","cited_arxiv_id":null,"evidence_quote":"The reference cited for the scalar dipole radiation effects that the paper neglects in the binary dynamics."}],"review_version":1}