REVIEW
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.
T0 · AI review
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Referee report
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
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
- 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.
The abstract, sentence by sentence
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
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| 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 was examined, and what was not
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Figures
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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