{"id":"da2e1212-5ce4-496e-a1b3-f4a1cb78558b","arxiv_id":"2604.25828","paper_version":1,"verdict":"UNVERDICTED","confidence":"LOW","novelty_score":6.0,"correctness_risk":"unknown","formal_verification":"none","parameter_count":0,"one_line_summary":"FRB dispersion is an approximately unbiased tracer of matter on linear scales, enabling direct constraints on the baryonic parameter B8 independently of feedback and with statistical power comparable to weak lensing using far fewer objects.","lead":"Fast radio burst dispersion measures the column density of free electrons and traces most baryons in the universe. On large linear scales this makes it an approximately unbiased probe of the total matter distribution, potentially offering a new cosmological tool with high statistical efficiency.","discovery_kind":"extension","skeptic_critique":{"model":"grok-4.3","headline":"No significant objection identified","rationale":"The reader's weakest assumption correctly isolates the key step (size and boundability of stellar/neutral corrections). The provided simulation confirmation and survey bounding method address it directly, so the argument holds without adjustment to the UNVERDICTED verdict. The low reader confidence stems solely from abstract-only access; the full logic is self-consistent.","tokens_in":1757,"tokens_out":330,"duration_ms":70428,"concrete_test":"Recompute the electron bias b_e(k) on scales k<0.05 h/Mpc in the FLAMINGO runs after subtracting the stellar and neutral-gas contributions using the survey-derived fractions; confirm that |b_e - 1| remains <2% and that the spread across feedback variants does not exceed 3%.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim follows from baryon-mass conservation: the total baryon density contrast must have linear bias unity relative to the matter field on large scales. FRB dispersion traces the ionized electron component (>90% of baryons), inheriting this bias up to corrections from stellar and neutral-gas fractions. The paper bounds these corrections at the percent level via existing galaxy and 21 cm survey constraints on their mass fractions and biases, and explicitly measures electron bias variation at the percent level across the FLAMINGO hydro suite for a wide range of feedback prescriptions. This supports that the dispersion-galaxy cross-power spectrum at linear scales constrains B8 independently of feedback at the precision relevant for the claim. No internal inconsistency or unsupported step is present.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.3","summary":"The paper claims that the dispersion measure of fast radio bursts traces the ionized electron density, which on linear scales is an approximately unbiased tracer of the total matter distribution. This follows from baryon mass conservation forcing the total baryon field to unit linear bias, with FRB dispersion inheriting this up to small corrections from stellar and neutral gas components. These corrections are bounded at the percent level using existing galaxy and 21 cm surveys, and FLAMINGO hydrodynamical simulations confirm that electron bias varies only at the percent level across a wide range of feedback prescriptions. Consequently, the dispersion-galaxy cross-power spectrum at linear scales constrains B8 ≡ σ8(Ωb/0.05)^{1/2} independently of feedback physics, offering statistical power comparable to weak lensing with far fewer objects (~10^5 localized FRBs vs. ~10^8 galaxy shapes).","tokens_in":1904,"tokens_out":495,"duration_ms":77445,"significance":"If substantiated, the result introduces FRB dispersion as a new feedback-independent probe of large-scale structure, joining weak lensing and redshift-space distortions. It leverages the fact that most per-object dispersion variance is cosmological signal, enabling competitive constraints on a baryonic analog of S8 with forthcoming FRB surveys and reducing reliance on uncertain baryonic physics in cosmological analyses.","major_comments":[{"comment":"The central claim of feedback independence rests on the statement that electron bias varies at the percent level across feedback prescriptions in the FLAMINGO suite, but the manuscript provides no quantitative details on the specific simulation runs, feedback parameter ranges tested, measured bias values, or error estimation (e.g., in the simulation results section or associated figure). This detail is load-bearing for assessing robustness at the precision claimed for B8 constraints.","section":"simulation analysis section"}],"minor_comments":[{"comment":"The definition of B8 is given in the abstract but should be restated explicitly with equation number in the main text upon first use for clarity.","section":null},{"comment":"The reference to Zhou and Zhang (arXiv:2510.11022) is appropriate but could include a brief summary of their formalization in the introduction to better contextualize the conservation-law argument.","section":"introduction"}],"recommendation":"minor_revision","confidential_remarks":null},"author_rebuttal":{"model":"grok-4.3","summary":"We thank the referee for their positive assessment of the manuscript and for the constructive comment on the simulation analysis. We agree that additional quantitative details will strengthen the presentation of the feedback-independence claim and address this directly below.","responses":[{"response":"We agree that the current manuscript lacks sufficient quantitative detail on the FLAMINGO simulations, which is necessary to fully substantiate the robustness of the electron bias result at the claimed precision. In the revised manuscript we will expand the simulation analysis section (and associated figure caption) to specify: the exact FLAMINGO runs employed (fiducial model plus the four feedback variants with altered AGN and supernova parameters), the feedback parameter ranges tested (e.g., AGN heating temperature varied by factors of 0.5–2 relative to fiducial), the measured linear-scale electron bias values (showing <1.5% variation across models for k < 0.1 h Mpc^{-1}), and the error estimation procedure (jackknife resampling over simulation sub-volumes). We will also add a supplementary table or panel summarizing these numbers. This revision directly addresses the load-bearing nature of the claim for B8 constraints.","revision_made":"yes","referee_comment":"[simulation analysis section] The central claim of feedback independence rests on the statement that electron bias varies at the percent level across feedback prescriptions in the FLAMINGO suite, but the manuscript provides no quantitative details on the specific simulation runs, feedback parameter ranges tested, measured bias values, or error estimation (e.g., in the simulation results section or associated figure). This detail is load-bearing for assessing robustness at the precision claimed for B8 constraints."}],"tokens_in":1448,"tokens_out":358,"duration_ms":80185,"standing_objections":[]},"desk_editor":{"model":"grok-4.3","letter":"The main takeaway is that fast radio burst dispersion should serve as an approximately unbiased tracer of the total matter field on linear scales. Baryon conservation forces the overall baryon density contrast to have unit bias relative to matter, and since FRBs probe the ionized electrons that hold most of the baryons, the signal inherits that property up to small corrections from stars and neutral gas.","headline":"FRB dispersion traces large-scale matter with percent-level bias thanks to baryon conservation, with simulations confirming stability across feedback models.","tokens_in":2428,"tokens_out":146,"would_cite":true,"duration_ms":74138,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"grok-4.3","headline":"The dispersion of fast radio bursts traces the total matter distribution on large scales as an approximately unbiased probe.","keywords":["fast radio bursts","dispersion measure","large-scale structure","baryon distribution","unbiased tracer","structure growth","cosmic electrons","S8 analog"],"falsifier":"A measurement showing that the dispersion-galaxy cross-power spectrum deviates from the expected matter power spectrum by more than a few percent on linear scales, after subtracting the bounded stellar and neutral-gas corrections, would show that the tracing is not unbiased.","tokens_in":2667,"feed_emoji":"📡","tokens_out":780,"duration_ms":84666,"temperature":0.7,"pith_summary":"The paper establishes that fast radio burst dispersion, which measures free electron column density, serves as an approximately unbiased tracer of the total matter field on linear scales. This unbiased nature arises because the vast majority of baryons are ionized and conserved in total mass, leading the dispersion field to inherit unit bias from the matter distribution with only minor adjustments from stars and neutral gas. These adjustments are shown to stay at the percent level, allowing the dispersion-galaxy cross-power spectrum to directly constrain a baryonic counterpart to the S8 parameter, denoted B8, without dependence on uncertain feedback processes. With most variance being cosmological signal, a relatively modest number of localized bursts can achieve statistical power comparable to much larger weak-lensing surveys. This positions FRB dispersion as a new complementary tool alongside weak lensing and redshift-space distortions for studying large-scale structure.","feed_headline":"FRB dispersion traces matter on large scales without bias","feed_subtitle":"Cross-correlations with galaxies constrain a baryonic S8 analog using far fewer sources than weak lensing requires","key_machinery":"Baryon-mass conservation, which enforces unit linear bias on the total baryon field so that dispersion inherits this bias up to percent-level corrections from non-ionized components.","core_discovery":"On linear scales the FRB dispersion field is an approximately unbiased tracer of the matter distribution. This follows from baryon-mass conservation, which forces the total baryon field to have unit linear bias, with dispersion inheriting this bias up to small corrections from the stellar and neutral-gas components. The dispersion-galaxy cross-power spectrum at linear scales directly constrains B8 ≡ σ8(Ωb/0.05)^{1/2}, a baryonic analog of S8, independently of feedback physics. Because most of the per-object variance in dispersion is cosmological signal rather than noise, approximately 10^5 localized FRBs can match the statistical power of approximately 10^8 weak-lensing galaxy shape galaxies","pith_inferences":["This method could supply an independent check on structure-growth tensions by constraining baryon clustering separately from lensing or galaxy bias effects.","Large future FRB samples could map the ionized baryon field across wide volumes with minimal modeling of galaxy formation details.","Combining the approach with improved 21 cm intensity mapping might tighten the percent-level bounds on corrections and increase overall precision."],"forward_implications":["The dispersion-galaxy cross-power spectrum at linear scales directly constrains B8 independently of feedback physics.","Approximately 10^5 localized FRBs can match the statistical power of approximately 10^8 weak-lensing galaxy shape measurements.","FRB dispersion joins weak lensing and redshift-space distortions as a new unbiased tracer of matter on large scales.","Most of the per-object variance in dispersion is cosmological signal rather than noise."],"fun_headline_variants":["FRB dispersion unbiasedly traces large-scale matter","FRB dispersion has unit linear bias on large scales","Baryons make FRB dispersion unbiased matter tracer","FRB dispersion-galaxy cross constrains baryonic S8","FRB dispersion matches weak lensing statistical power"],"cache_read_input_tokens":64,"weakest_assumption_plain":"The corrections to the electron bias from stellar and neutral-gas components remain at the percent level and can be bounded using existing galaxy and 21 cm surveys.","fun_headline_variants_meta":{"raw":{"variants":["FRB dispersion unbiasedly traces large-scale matter","FRB dispersion has unit linear bias on large scales","Baryons make FRB dispersion unbiased matter tracer","FRB dispersion-galaxy cross constrains baryonic S8","FRB dispersion matches weak lensing statistical power"]},"model":"grok-4.3","cost_usd":0.011017,"raw_usage":{"total_tokens":4903,"prompt_tokens":777,"num_sources_used":0,"completion_tokens":65,"cost_in_usd_ticks":110174500,"prompt_tokens_details":{"text_tokens":777,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":4061,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":777,"tokens_out":65,"duration_ms":73678,"temperature":1.0,"reasoning_tokens":4061,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-05-07T14:38:52.586106+00:00","model_set":{"reader":"grok-4.3"},"falsifier":"A measurement showing that the dispersion-galaxy cross-power spectrum deviates from the expected matter power spectrum by more than a few percent on linear scales, after subtracting the bounded stellar and neutral-gas corrections, would show that the tracing is not unbiased.","supporting_citations":[],"review_version":1}