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Baryonic feedback and intrinsic alignments in profile: a frequentist view of DES Y3 cosmic shear

Cosmology and Nongalactic Astrophysics · Submitted 2 Nov 2026 · 83 references

Pith's verdict: First profile-likelihood treatment of DES Y3 cosmic shear shows the S8 upper limit tracks the baryonic feedback prior; the qualitative result holds, but the quoted 1σ numbers need calibration.

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arXiv ID2611.03537 (version 1)
Pith IDBS47LKG6, Pith's permanent record of this paper and its review
CitationAnoma Ganguly, Elisabeth Krause, Tanvi Karwal, Vivian Miranda. "Baryonic feedback and intrinsic alignments in profile: a frequentist view of DES Y3 cosmic shear." arXiv:2611.03537 (2026). https://pith.science/paper/BS47LKG6
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The paper

The paper claims

For DES Y3 cosmic shear alone, the upper 1σ limit on S8 is set by the baryonic feedback prior range, not the data; CMB information on ωb, ns, and h breaks the degeneracy and reconciles profile likelihood and posterior.

The reading

This paper asks whether the standard error bars on cosmic shear measurements are actually earned by the data or inherited from the prior volume of weakly constrained nuisance parameters. Reanalyzing the DES Y3 + KiDS-1000 'Hybrid' model with a profile likelihood — maximizing over all other parameters at each value of the target parameter rather than marginalizing — the authors find that cosmic shear alone leaves a near-flat degeneracy between $S_8$ and baryonic feedback strength $\log_{10}(T_{AGN}/K)$: the upper $1\sigma$ limit on $S_8$ moves with the assumed feedback range instead of being set by the sky. Adding a compressed Planck likelihood on $\omega_b$, $n_s$, and $h$ — parameters cosmic shear cannot see but that fix the shape of the matter power spectrum — breaks the degeneracy, and the frequentist profiles and Bayesian posteriors come into agreement. The same comparison shows that the extra parameters of the TATT intrinsic-alignment model widen the $S_8$ profile relative to NLA, and that adding DES Y3 shear-ratio data closes the gap between the two statistics. If the paper is right, cosmic-shear-only $S_8$ constraints, including part of the reported $S_8$ tension, should not be billed as fully data-driven.

What carries the argument

The load-bearing object is the profile likelihood, $L_p(\Theta) \equiv \max_n L(d|\Theta,n)$, which replaces Bayesian marginalization over nuisance parameters with a maximization at every value of the parameter of interest, with intervals read off from $\Delta\chi^2 = -2\ln(L_p/L_{\max}) \le 1$. The paper connects the two statistics through $P(\Theta) \propto L_p(\Theta)\,V(\Theta)$, where $V(\Theta)$ is the prior volume of nuisance space consistent with the data at fixed $\Theta$: when the best-fit nuisance parameters sit at the edge of their prior range, that volume shrinks the marginal posterior relative to the profile, which is exactly what happens for cosmic shear alone. Computationally, the profiles are carried by a covariance-adapting evolution strategy optimizer coupled to a fast cosmic shear likelihood that uses an emulator for the HMCode2020 matter power spectrum. The physics doing the real work is the near-degeneracy between $S_8$ and the baryonic feedback parameter $\log_{10}(T_{AGN}/K)$: both push the small-scale lensing signal in the same direction, so a flat likelihood ridge in the high-$S_8$ direction is the natural state of cosmic-shear-only data.

Extended reading notes

Core claim

Under the Hybrid model, the profile likelihood for DES Y3 cosmic shear alone peaks at $S_8 = 0.809^{+0.043}_{-0.022}$, in agreement with the marginal posterior mean $0.813 \pm 0.020$, but the profile is much broader toward high $S_8$: stronger baryonic feedback suppresses the small-scale matter power spectrum and can compensate for a higher $S_8$, so the likelihood is nearly flat in $S_8$ above the peak. The paper makes the prior-dependence explicit by extending the upper bound on $\log_{10}(T_{AGN}/K)$ from 8.0 to 8.3: the global best fit does not move, but the upper $1\sigma$ limit of the profile is pushed to higher $S_8$, and the marginal posterior shifts with it. Adding a compressed Planck 2018 Gaussian likelihood on $\omega_b$, $n_s$, and $h$ removes the degeneracy, so that profile and posterior agree ($S_8 = 0.808 \pm 0.019$ versus $0.805 \pm 0.017$) and the $S_8$ constraint becomes insensitive to the feedback prior. With the CMB included, small-scale data down to 2.5 arcmin close the feedback contour and prefer moderate feedback, $\log_{10}(T_{AGN}/K) = 7.80^{+0.23}_{-0.30}$, consistent with hydrodynamical simulations calibrated to observed gas fractions. In the intrinsic-alignment direction, the TATT model's extra free parameters broaden and shift the $S_8$ profile relative to NLA, and adding the DES Y3 shear-ratio likelihood removes the unphysical $a_1 < 0$ branch and brings profiles and posteriors into close agreement for both models.

Load-bearing premise

The quoted intervals assume that the optimizer finds the true best fit at every value of $S_8$ and that the $\Delta\chi^2 \le 1$ rule delivers 68% coverage, even though the best fits sit at the edge of the parameter range where that rule is least reliable — a limitation the authors themselves acknowledge when they note that more noisy mock realizations are needed to calibrate the true coverage of the profile likelihood intervals.

Editorial extensions

If this is right

  • Cosmic-shear-only $S_8$ upper limits are prior-controlled: two analyses with identical data but different assumed ranges for $\log_{10}(T_{AGN}/K)$ will report different upper $1\sigma$ limits on $S_8$.
  • A compressed CMB likelihood on $\omega_b$, $n_s$, and $h$ is sufficient to break the $S_8$–feedback degeneracy; with it, profile likelihoods and marginal posteriors agree and the $S_8$ constraint no longer depends on the feedback prior.
  • Small-scale cosmic shear combined with CMB shape information constrains baryonic feedback to moderate strengths, $\log_{10}(T_{AGN}/K) = 7.80^{+0.23}_{-0.30}$, consistent with hydrodynamical simulation calibrations based on observed gas fractions.
  • Adding the DES Y3 shear-ratio likelihood removes the $a_1 < 0$ branch of the NLA profile and brings the NLA and TATT models into agreement with each other and with the Bayesian posteriors.
  • At Stage-IV precision, the forecast indicates that most parameters become well constrained within their priors, so profile likelihoods and marginal posteriors deliver comparable constraining power.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the feedback–$S_8$ degeneracy is as strong as the paper shows, the reported differences in $S_8$ among stage-III surveys — and part of the apparent $S_8$ tension with the CMB — could reflect where each analysis placed its baryon-prior boundary rather than what the sky says; re-running published cosmic-shear-only analyses with one common feedback range would test this directly.
  • The same prior-volume mechanism should afflict any nuisance parameter with a flat likelihood direction, not just $T_{AGN}$: flexible intrinsic-alignment extensions or a free neutrino mass will tend to drag marginal posteriors relative to the profile, so a profile-versus-posterior audit could be a cheap standard check for every cosmic shear release.
  • The noiseless Roman-style forecast is optimistic by construction; the claimed convergence of profile and posterior at Stage-IV precision should be re-examined on noisy realizations with realistic covariance, since the interval coverage that the DES Y3 analysis could not calibrate is precisely what would matter for quoted Stage-IV errors.
Original abstract

Current cosmic shear surveys primarily constrain $S_8$ and $\Omega_\mathrm{m}$, while most other model parameters remain dominated by priors. Consequently, marginal posteriors are vulnerable to prior-volume effects, particularly through degeneracies among $S_8$, baryonic feedback, and intrinsic alignment (IA) model parameters. To isolate data-driven constraints from prior volume effects, we compare profile likelihoods with Bayesian posteriors for DES Y3 cosmic shear under the DES Y3+KiDS-1000 Hybrid model. For cosmic shear alone, the profile global best fit and the marginal posterior mean agree in $S_8$, but the profile likelihood reveals a strong degeneracy between feedback strength and $S_8$, leaving the upper $1\sigma$ limit on $S_8$ determined by the assumed feedback range. External CMB information on parameters other than $\Omega_\mathrm{m}$ and $S_8$ breaks this degeneracy, suppressing prior-volume effects and reconciling the profiles with the posteriors. With CMB data included, small-scale cosmic shear data prefers moderate baryonic feedback strengths consistent with hydrodynamical simulations calibrated to observed gas fractions. Furthermore, additional unconstrained parameters in the tidal alignment and tidal torquing (TATT) model result in weaker constraints, shifting and broadening the $S_8$ profile relative to the nonlinear linear alignment (NLA) model. Adding DES Y3 shear ratios mitigates this degeneracy, bringing profile likelihood and marginal posteriors into close agreement.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-11-04: 1 request, 6,521 input tokens.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.69 · human participants 0.01 · controlled comparison 0.25 · randomized assignment 0.01 · states quantitative result 0.29 · compares with prior results 0.90 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

Current cosmic shear surveys primarily constrain $S_8$ and $\Omega_\mathrm{m}$, while most other model parameters remain dominated by priors. Consequently, marginal posteriors are vulnerable to prior-volume effects, particularly through degeneracies among $S_8$, baryonic feedback, and intrinsic alignment (IA) model parameters. To isolate data-driven constraints from prior volume effects, we compare profile likelihoods with Bayesian posteriors for DES Y3 cosmic shear under the DES Y3+KiDS-1000 Hybrid model. For cosmic shear alone, the profile global best fit and the marginal posterior mean agree in $S_8$, but the profile likelihood reveals a strong degeneracy between feedback strength and $S_8$, leaving the upper $1\sigma$ limit on $S_8$ determined by the assumed feedback range. External CMB information on parameters other than $\Omega_\mathrm{m}$ and $S_8$ breaks this degeneracy, suppressing prior-volume effects and reconciling the profiles with the posteriors. With CMB data included, small-scale cosmic shear data prefers moderate baryonic feedback strengths consistent with hydrodynamical simulations calibrated to observed gas fractions. Furthermore, additional unconstrained parameters in the tidal alignment and tidal torquing (TATT) model result in weaker constraints, shifting and broadening the $S_8$ profile relative to the nonlinear linear alignment (NLA) model. Adding DES Y3 shear ratios mitigates this degeneracy, bringing profile likelihood and marginal posteriors into close agreement.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 Current cosmic shear surveys primarily constrain $S_8$ and $\Omega_\mathrm{m}$, while most other model parameters remain dominated by priors. background 0.98 0.07 0.04 asserted 1.00
s1 Consequently, marginal posteriors are vulnerable to prior-volume effects, particularly through degeneracies among $S_8$, baryonic feedback, and intrinsic alignment (IA) model parameters. background 0.80 0.13 0.06 asserted 0.88
s2 To isolate data-driven constraints from prior volume effects, we compare profile likelihoods with Bayesian posteriors for DES Y3 cosmic shear under the DES Y3+KiDS-1000 Hybrid model. method 0.68 0.93 0.07 asserted 0.94
s3 For cosmic shear alone, the profile global best fit and the marginal posterior mean agree in $S_8$, but the profile likelihood reveals a strong degeneracy between feedback strength and $S_8$, leaving the upper $1\sigma$ limit on $S_8$ determined by the assumed feedback range. result 0.99 0.92 0.24 asserted 1.00
s4 External CMB information on parameters other than $\Omega_\mathrm{m}$ and $S_8$ breaks this degeneracy, suppressing prior-volume effects and reconciling the profiles with the posteriors. result 0.83 0.81 0.25 asserted 1.00
s5 With CMB data included, small-scale cosmic shear data prefers moderate baryonic feedback strengths consistent with hydrodynamical simulations calibrated to observed gas fractions. result 0.96 0.83 0.17 asserted 0.79
s6 Furthermore, additional unconstrained parameters in the tidal alignment and tidal torquing (TATT) model result in weaker constraints, shifting and broadening the $S_8$ profile relative to the nonlinear linear alignment (NLA) model. result 1.00 0.85 0.06 asserted 1.00
s7 Adding DES Y3 shear ratios mitigates this degeneracy, bringing profile likelihood and marginal posteriors into close agreement. result 0.98 0.90 0.23 asserted 1.00

What kind of paper the classifier reports this is: empirical study 0.69 · evidence from analysis of existing data 0.99.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. FIG. 1. Comparison of Bayesian and frequentist approaches on one noiseless synthetic DES Y3-like cosmic shear data vector
Figure 2. FIG. 2. 2D constraints from Bayesian (dashed contours)
Figure 3. FIG. 3. Addition of compressed CMB Planck 2018 TT,TE,EE
Figure 4. FIG. 4. Profiles (solid) and posterior (dashed) comparison
Figure 6. FIG. 6. Profile comparison between NLA and TATT models
Figure 7. FIG. 7. Forecast for a wide Roman survey over the LSST Y10
Figure 8. FIG. 8. Effect of prior (parameter) boundaries on marginal
Figure 9. FIG. 9. CAMB and emulator profiles run using CMA-ES
Figure 11. FIG. 11. Validation on noiseless synthetic cosmic-shear data
Figure 12. FIG. 12. Validation on ten synthetic noisy realizations of
Figure 13. FIG. 13. Posterior comparison with public DES-KiDS (Σ
Figure 14. FIG. 14. Allowing Σ

Showing 12 of the paper's 14 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:2611.03537 by the authors.

References

Discussion

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