The impact of our peculiar motion on primordial non-Gaussianity measurements using the LIGER4GAL framework
Pith reviewed 2026-05-22 11:31 UTC · model grok-4.3
The pith
Omitting the observer's peculiar velocity effect in galaxy power spectra biases f_nl measurements by more than one sigma in 40 percent of cosmic realizations.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
By implementing all linear-order relativistic RSDs at the tracer level in high-resolution N-body simulations, LIGER4GAL produces mock galaxy samples whose power spectrum multipoles show that the finger-of-the-observer effect must be included in the modeling; otherwise the inferred f_nl is biased by more than 1 sigma_fnl in 40 percent of realizations and by more than 0.25 sigma_fnl in 80 percent of realizations when fitting scales down to k_min = 0.0015 h/Mpc.
What carries the argument
LIGER4GAL, the updated implementation that adds every linear-order relativistic redshift-space distortion at the individual tracer level inside N-body simulations while preserving non-linear clustering on smaller scales.
If this is right
- Power-spectrum analyses of upcoming surveys must include the observer's peculiar velocity term to keep f_nl constraints unbiased at the target precision.
- The reported bias fractions imply that the effect is common rather than a rare outlier, so it cannot be ignored in standard modeling pipelines.
- The same framework can be applied to test whether other relativistic RSD contributions produce comparable shifts in f_nl or in other large-scale parameters.
- Surveys that reach k_min values around 0.0015 h/Mpc will need this level of modeling; shallower k_min cuts reduce but do not eliminate the potential bias.
Where Pith is reading between the lines
- Similar observer-motion corrections may be needed for other ultra-large-scale observables such as the integrated Sachs-Wolfe cross-correlation or the scale-dependent bias induced by PNG itself.
- Real-data pipelines could adopt the LIGER4GAL approach to forward-model the full set of linear relativistic effects rather than applying analytic corrections after the fact.
- The 40 percent figure suggests that a single survey realization has a substantial chance of being affected, which strengthens the case for including the term as a default rather than an optional check.
Load-bearing premise
Linear-order relativistic RSDs placed at the tracer level are enough to produce the reported bias percentages on f_nl, and higher-order or non-linear corrections would not change those fractions.
What would settle it
Generate many independent realizations of the same DESI-like sample with and without the finger-of-the-observer term, fit each for f_nl down to k_min = 0.0015 h/Mpc, and check whether the fraction of realizations showing a bias larger than 1 sigma_fnl is close to 40 percent.
Figures
read the original abstract
Current and forthcoming galaxy surveys will map the observable Universe with unprecedented depth, sky coverage, and precision. These maps are affected by relativistic redshift-space distortions (RSDs), which become increasingly relevant on ultra-large scales. Accurate modelling of these relativistic RSDs is essential to avoid systematic biases in key cosmological measurements, such as primordial non-Gaussianity (PNG). To address this, we introduce an updated implementation of the LIGER method, LIGER4GAL, which incorporates all linear-order relativistic RSDs directly at the tracer level of high-resolution N-body simulations. We demonstrate that LIGER4GAL improves upon previous iterations of the LIGER method by reproducing the expected non-linear clustering while maintaining accuracy for relativistic RSDs on large scales. We use the updated code to generate a DESI-like sample of luminous red galaxies from the Huge MultiDark Planck simulation. By measuring the power spectrum multipoles of this sample with and without the imprint of relativistic RSDs, we assess the impact of relativistic effects on measurements of the local PNG signal ($f_\mathrm{nl}$). We find that the omission of the''finger-of-the-observer'' (sourced by the peculiar velocity of the observer) effect in the power spectrum modelling can bias measurements of $f_{\rm nl}$ by more than $1$ ($0.25$) $ \sigma_{f_{\rm nl}}$ in 40% (80%) of the possible realizations of the universe if scales down to $k_\mathrm{min} = 0.0015\,h/\mathrm{Mpc}$ are included.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces LIGER4GAL, an updated implementation of the LIGER method that incorporates all linear-order relativistic redshift-space distortions directly at the tracer level in high-resolution N-body simulations. Using a DESI-like sample of luminous red galaxies drawn from the Huge MultiDark Planck simulation, the authors measure power spectrum multipoles with and without the imprint of these relativistic RSDs. They quantify the resulting bias on local primordial non-Gaussianity parameter f_nl, reporting that omission of the finger-of-the-observer effect (sourced by the observer's peculiar velocity) can bias f_nl measurements by more than 1 sigma_fnl in 40% of realizations (and more than 0.25 sigma_fnl in 80%) when scales down to k_min = 0.0015 h/Mpc are included.
Significance. If the linear-order treatment proves sufficient, the work provides a concrete, simulation-based estimate of a previously under-appreciated systematic for ultra-large-scale PNG analyses. The direct with/without comparison within the same simulation suite is a strength, as it avoids circularity in the bias measurement and offers falsifiable percentages that can be tested against analytic expectations or higher-resolution runs. This is relevant for DESI and future surveys where relativistic effects on the largest scales must be controlled to reach the target precision on f_nl.
major comments (2)
- [Validation of LIGER4GAL and results on f_nl bias] The central percentages (40% and 80% of realizations) rest on the assumption that linear-order relativistic RSDs implemented at the tracer level fully capture the finger-of-the-observer contribution without higher-order relativistic or non-linear corrections altering the bias distribution on k_min = 0.0015 h/Mpc scales. The validation section demonstrates reproduction of non-linear clustering and large-scale RSD accuracy, but does not quantify the size of second-order terms or their cross-talk with PNG; this directly affects whether the reported fractions remain robust.
- [Power spectrum multipole measurements and f_nl impact assessment] The assessment of bias relative to sigma_fnl is obtained from the with/without power-spectrum comparison, yet the manuscript provides no error bars on the reported percentages, no convergence tests with respect to simulation volume or resolution, and no explicit comparison against analytic expectations for the relativistic correction. These omissions make it difficult to judge the statistical significance of the 40% and 80% figures.
minor comments (2)
- [Abstract] The abstract states that LIGER4GAL 'improves upon previous iterations of the LIGER method by reproducing the expected non-linear clustering while maintaining accuracy for relativistic RSDs'; a short quantitative statement (e.g., fractional difference in the quadrupole or hexadecapole on large scales) would make this claim more precise.
- [Introduction and method description] Notation for the observer's peculiar velocity contribution to the finger-of-the-observer effect is introduced without a brief equation or reference to the standard relativistic RSD expansion; adding this would aid readers new to the topic.
Simulated Author's Rebuttal
We thank the referee for their positive assessment of the work's significance and for the constructive major comments. We respond to each point below, indicating planned revisions to the manuscript.
read point-by-point responses
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Referee: [Validation of LIGER4GAL and results on f_nl bias] The central percentages (40% and 80% of realizations) rest on the assumption that linear-order relativistic RSDs implemented at the tracer level fully capture the finger-of-the-observer contribution without higher-order relativistic or non-linear corrections altering the bias distribution on k_min = 0.0015 h/Mpc scales. The validation section demonstrates reproduction of non-linear clustering and large-scale RSD accuracy, but does not quantify the size of second-order terms or their cross-talk with PNG; this directly affects whether the reported fractions remain robust.
Authors: We agree that higher-order terms could in principle modify the reported bias fractions. Our work targets the leading linear-order relativistic RSDs, which dominate the finger-of-the-observer effect on the ultra-large scales considered. The existing validation confirms consistency with linear theory for these effects. In the revised manuscript we will add a dedicated paragraph estimating the expected size of second-order relativistic corrections from the literature and arguing that they remain sub-dominant for the f_nl bias at the precision of our results; we will also flag a full non-linear treatment as future work. revision: partial
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Referee: [Power spectrum multipole measurements and f_nl impact assessment] The assessment of bias relative to sigma_fnl is obtained from the with/without power-spectrum comparison, yet the manuscript provides no error bars on the reported percentages, no convergence tests with respect to simulation volume or resolution, and no explicit comparison against analytic expectations for the relativistic correction. These omissions make it difficult to judge the statistical significance of the 40% and 80% figures.
Authors: We acknowledge these omissions. In the revised manuscript we will add bootstrap-derived error bars on the 40% and 80% fractions. We will also report convergence tests using sub-volumes of the Huge MultiDark Planck simulation. Finally, we will include a direct comparison of the simulated power-spectrum multipoles (with and without the finger-of-the-observer term) against the corresponding analytic expressions, confirming consistency at linear order. revision: yes
Circularity Check
No significant circularity: bias fractions obtained via direct simulation comparison
full rationale
The central result (40% of realizations exceeding 1 sigma_fnl bias when omitting finger-of-the-observer) is obtained by explicit with/without measurement of power spectrum multipoles on the same DESI-like LRG sample generated from the Huge MultiDark Planck simulation using LIGER4GAL. This is a direct numerical comparison rather than a derivation, fit, or self-referential equation. No load-bearing step reduces to a fitted parameter renamed as prediction, self-citation chain, or ansatz smuggled via prior work. The linear-order RSD implementation is an input assumption whose adequacy is an external validity question, not a circularity issue. The paper remains self-contained against its own simulation benchmarks.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Linear-order relativistic RSDs dominate the observer-motion imprint on ultra-large scales
Lean theorems connected to this paper
-
IndisputableMonolith/Foundation/RealityFromDistinction.leanreality_from_one_distinction unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
We introduce an updated implementation of the LIGER method, LIGER4GAL, which incorporates all linear-order relativistic RSDs directly at the tracer level... omission of the 'finger-of-the-observer' effect... bias measurements of f_nl by more than 1 sigma_fnl in 40% of realizations
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IndisputableMonolith/Cost/FunctionalEquation.leanwashburn_uniqueness_aczel unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
power spectrum multipoles... full-shape fits of the power spectrum multipoles using the EFT model
What do these tags mean?
- matches
- The paper's claim is directly supported by a theorem in the formal canon.
- supports
- The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
- extends
- The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
- uses
- The paper appears to rely on the theorem as machinery.
- contradicts
- The paper's claim conflicts with a theorem or certificate in the canon.
- unclear
- Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.
Reference graph
Works this paper leans on
-
[1]
Angular clustering Following the approach adopted in Elkhashab et al. [45], we use the angular power spectrum,C ℓ, to validate the clus- tering properties of our mocks against the theoretical predic- tions, which we calculate using the CAMB [70] code. We measure theC ℓ for both the H-FS and BCH-FS halo samples in the redshift binz∈[0.5,0.6] , using the ps...
-
[2]
The FOTO signal The FOTO signal is an oscillatory imprint in the power spectrum sourced by the3 o term in Eq. (11). Its contribution to the monopole of the power spectrum for a full-sky survey is given by [45, 46] P0,FOTO(k)= 16π2 3 32 o H2 0 I2 1(k) 4π R r2 r1 r2 ¯n2g dr ,(19) where I1(k)= Z r2 r1 r¯ng α0 a H/H0 j1(kr) dr,(20) with the integration limits...
-
[3]
Local PNG In this section, we measure the bias in measurements of lo- cal PNG from the observed power spectrum due to relativistic RSDs. We characterize the deviation from Gaussianity of a random fieldϕ NG by the following parametrization [see e.g. 63]: ϕNG =ϕ+f NL(ϕ2 − ⟨ϕ⟩2),(23) whereϕis a Gaussian random field andf NL denotes the am- plitude of the dev...
-
[4]
Relativistic effects impact on full-shape analysis In this subsection we examine the impact of relativistic RSDs on the cosmological parameters inferred from the full- shape analysis of the power spectrum. To that end, we infer the cosmological parameters from the LRG-DESI catalogue 15 TABLE IV: The prior distributions used for parameter infer- ence in th...
work page 2016
-
[5]
Adame, A. G. et al. 2025, JCAP, 07, 017
work page 2025
-
[6]
Adame, A. G. et al. 2025, JCAP, 09, 008
work page 2025
-
[7]
Ade, P. A. R. et al. 2014, Astron. Astrophys., 571, A16
work page 2014
-
[8]
Aghamousa, A. et al. 2016 [1611.00036]
work page internal anchor Pith review Pith/arXiv arXiv 2016
-
[9]
Aghanim, N. et al. 2020, Astron. Astrophys., 641, A1
work page 2020
-
[10]
Alvarez, M. et al. 2014 [1412.4671]
work page internal anchor Pith review Pith/arXiv arXiv 2014
- [11]
-
[12]
Bahr-Kalus, B., Bertacca, D., Verde, L., & Heavens, A. 2021, JCAP, 11, 027
work page 2021
-
[13]
Bardeen, J. M. 1980, Phys. Rev. D, 22, 1882
work page 1980
-
[14]
2020, Journal of Cosmology and Astroparticle Physics, 2020, 013
Barreira, A., Cabass, G., Schmidt, F., Pillepich, A., & Nelson, D. 2020, Journal of Cosmology and Astroparticle Physics, 2020, 013
work page 2020
-
[15]
2004, Physics Reports, 402, 103–266
Bartolo, N., Komatsu, E., Matarrese, S., & Riotto, A. 2004, Physics Reports, 402, 103–266
work page 2004
-
[16]
Behroozi, P. S., Wechsler, R. H., & Wu, H.-Y . 2013, Astro- phys. J., 762, 109
work page 2013
-
[17]
Berlind, A. A. & Weinberg, D. H. 2002, Astrophys. J., 575, 587
work page 2002
-
[18]
2015, Classical and Quantum Gravity, 32, 195011
Bertacca, D. 2015, Classical and Quantum Gravity, 32, 195011
work page 2015
-
[19]
2020, International Journal of Modern Physics D, 29, 2050085
Bertacca, D. 2020, International Journal of Modern Physics D, 29, 2050085
work page 2020
-
[20]
2014, Journal of Cosmology and Astroparticle Physics, 2014, 037–037
Bertacca, D., Maartens, R., & Clarkson, C. 2014, Journal of Cosmology and Astroparticle Physics, 2014, 037–037
work page 2014
-
[21]
Bertacca, D., Maartens, R., Raccanelli, A., & Clarkson, C. 2012, JCAP, 2012, 025
work page 2012
-
[22]
Beutler, F. & McDonald, P. 2021, Journal of Cosmology and Astroparticle Physics, 2021, 031
work page 2021
-
[23]
Beutler, F. et al. 2014, Mon. Not. Roy. Astron. Soc., 443, 1065
work page 2014
-
[24]
Bianchi, D., Gil-Mar ´ın, H., Ruggeri, R., & Percival, W. J. 2015, Mon. Not. Roy. Astron. Soc., 453, L11
work page 2015
-
[25]
B ¨ohme, L., Schwarz, D. J., Tiwari, P., et al. 2025 [2509.16732]
- [26]
- [27]
-
[28]
Breton, M.-A., Rasera, Y ., Taruya, A., Lacombe, O., & Saga, S. 2019, Monthly Notices of the Royal Astronomical Society, 483, 2671, arXiv:1803.04294 [astro-ph]
work page internal anchor Pith review Pith/arXiv arXiv 2019
-
[29]
Broadhurst, T. J., Taylor, A. N., & Peacock, J. A. 1995, The Astrophysical Journal, 438, 49
work page 1995
-
[30]
Disentangling non-Gaussianity, bias and GR effects in the galaxy distribution
Bruni, M., Crittenden, R., Koyama, K., et al. 2012, Physical Review D, 85, 041301, arXiv:1106.3999 [astro-ph]
work page internal anchor Pith review Pith/arXiv arXiv 2012
-
[31]
Bruni, M., Crittenden, R., Koyama, K., et al. 2012, Phys. Rev. D, 85, 041301
work page 2012
-
[32]
Bryan, G. L. & Norman, M. L. 1998, Astrophys. J., 495, 80
work page 1998
-
[33]
Camera, S., Maartens, R., & Santos, M. G. 2015, Monthly Notices of the Royal Astronomical Society: Letters, 451, L80, arXiv:1412.4781 [astro-ph]
work page internal anchor Pith review Pith/arXiv arXiv 2015
-
[34]
2023, Journal of Cosmology and Astroparticle Physics, 2023, 028
Carrilho, P., Moretti, C., & Pourtsidou, A. 2023, Journal of Cosmology and Astroparticle Physics, 2023, 028
work page 2023
- [35]
-
[36]
2019, Journal of Cosmology and Astroparticle Physics, 2019, 010
Castorina, E., Hand, N., Seljak, U., et al. 2019, Journal of Cosmology and Astroparticle Physics, 2019, 010
work page 2019
- [37]
-
[38]
Chaussidon, E., Y `eche, C., de Mattia, A., et al. 2024 [2411.17623]
- [39]
-
[40]
Dalal, N., Dor ´e, O., Huterer, D., & Shirokov, A. 2008, Physi- cal Review D, 77
work page 2008
- [41]
-
[42]
Desjacques, V ., Crocce, M., Scoccimarro, R., & Sheth, R. K. 2010, Physical Review D, 82
work page 2010
- [43]
-
[44]
D., Durrer, R., Marozzi, G., & Montanari, F
Dio, E. D., Durrer, R., Marozzi, G., & Montanari, F. 2014, Journal of Cosmology and Astroparticle Physics, 2014, 017–017
work page 2014
-
[45]
Cosmology with the SPHEREX All-Sky Spectral Survey
Dor ´e, O., Bock, J., Ashby, M., et al. 2015 [1412.4872]
work page internal anchor Pith review Pith/arXiv arXiv 2015
-
[46]
2020, Journal of Cosmology and Astroparticle Physics, 2020, 005–005
d’Amico, G., Gleyzes, J., Kokron, N., et al. 2020, Journal of Cosmology and Astroparticle Physics, 2020, 005–005
work page 2020
-
[47]
2022, Monthly Notices of the Royal Astronomical Society, 519, 2962
Eggemeier, A., Camacho-Quevedo, B., Pezzotta, A., et al. 2022, Monthly Notices of the Royal Astronomical Society, 519, 2962
work page 2022
-
[48]
Y ., Bertacca, D., Porciani, C., et al
Elkhashab, M. Y ., Bertacca, D., Porciani, C., et al. 2025, As- tronomy & Astrophysics, 697, A85
work page 2025
-
[49]
Y ., Porciani, C., & Bertacca, D
Elkhashab, M. Y ., Porciani, C., & Bertacca, D. 2021, Mon. Not. Roy. Astron. Soc., 509, 1626
work page 2021
-
[50]
Y ., Porciani, C., & Bertacca, D
Elkhashab, M. Y ., Porciani, C., & Bertacca, D. 2025, JCAP, 03, 044
work page 2025
-
[51]
Elkhashab, M. Y . et al. 2025, Astron. Astrophys., 697, A85
work page 2025
-
[52]
Elsner, F., Leistedt, B., & Peiris, H. V . 2016, Mon. Not. Roy. Astron. Soc., 456, 2095
work page 2016
-
[53]
Euclid Collaboration Monaco, P., Elkhashab, M. Y ., Granett, B. R., et al. 2025, Euclid preparation. Controlling angular sys- tematics in the Euclid spectroscopic galaxy sample
work page 2025
-
[54]
Feldman, H. A., Kaiser, N., & Peacock, J. A. 1994, Astrophys. J., 426, 23
work page 1994
-
[55]
Foglieni, M., Pantiri, M., Di Dio, E., & Castorina, E. 2023, Phys. Rev. Lett., 131, 111201
work page 2023
- [56]
- [57]
-
[58]
G ´orski, K. M., Hivon, E., Banday, A. J., et al. 2005, Astro- phys. J., 622, 759
work page 2005
-
[59]
L., Jolicoeur, S., & Maartens, R
Guedezounme, S. L., Jolicoeur, S., & Maartens, R. 2025, JCAP, 07, 063
work page 2025
-
[60]
Hadzhiyska, B. et al. 2023, MNRAS, 525, 4367
work page 2023
-
[61]
Hearin, A. P. et al. 2017, Astron. J., 154, 190
work page 2017
-
[62]
Ishiyama, T. et al. 2021, Mon. Not. Roy. Astron. Soc., 506, 4210
work page 2021
- [63]
-
[64]
Jeong, D., Schmidt, F., & Hirata, C. M. 2012, Phys. Rev. D, 85, 023504
work page 2012
- [65]
-
[66]
Klypin, A., Yepes, G., Gottl¨ober, S., Prada, F., & Heß, S. 2016, MNRAS, 457, 4340
work page 2016
- [67]
-
[68]
Kravtsov, A. V ., Berlind, A. A., Wechsler, R. H., et al. 2004, Astrophys. J., 609, 35
work page 2004
-
[69]
Lange, J. U. 2023, Monthly Notices of the Royal Astronomical Society, 525, 3181
work page 2023
-
[70]
Leistedt, B. et al. 2016, Astrophys. J. Suppl., 226, 24
work page 2016
-
[71]
2025, Astronomy & Astrophysics, 694, A321
Lepori, F., Schulz, S., Tutusaus, I., et al. 2025, Astronomy & Astrophysics, 694, A321
work page 2025
-
[72]
Lepori, F., Tutusaus, I., Viglione, C., et al. 2022, A&A, 662, A93
work page 2022
- [73]
-
[74]
Lewis, A. & Challinor, A. 2011, CAMB: Code for Anisotropies in the Microwave Background, Astrophysics Source Code Library, record ascl:1102.026
work page 2011
- [75]
-
[76]
2000, The Astrophysical Journal, 537, L77–L80
Matsubara, T. 2000, The Astrophysical Journal, 537, L77–L80
work page 2000
-
[77]
Constraining the Cosmological Constant from Large-Scale Redshift-Space Clustering
Matsubara, T. & Szalay, A. S. 2001, The Astrophysical Jour- nal, 556, L67, arXiv:astro-ph/0105493
work page internal anchor Pith review Pith/arXiv arXiv 2001
- [78]
-
[79]
J., Brieden, S., Tr ¨oster, T., & Heymans, C
Mead, A. J., Brieden, S., Tr ¨oster, T., & Heymans, C. 2021, Monthly Notices of the Royal Astronomical Society, 502, 1401–1422
work page 2021
- [80]
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