REVIEW 2 major objections 5 minor 2 cited by
Gaussian Process Estimation of Transition Redshift
T0 review · 2 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read Reconstructing the deceleration parameter from Hubble and supernova data puts the transition redshift at 0.59–0.68 without assuming a cosmological model.
desk verdict The SNe Ia half is built on an algebraic error in Eq. (28), producing a spurious transition redshift; the H(z) half is fine. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The engine is Gaussian Process regression, a non-parametric Bayesian method that places a distribution over functions consistent with the data and can differentiate that distribution to yield covariances for $f'$, $f''$, etc. The paper uses three covariance kernels (a squared-exponential kernel and two smoother alternatives) to test kernel dependence, and propagates uncertainties through the derived expressions for $q(z)$: $q(z) = (1+z)H'(z)/H(z) - 1$ for the Hubble data and $q(z) = (1+z)^2 D''_L(z)/(D_L(z) - (1+z)D'_L(z)) + 1$ for the supernova distances. The zero-crossing condition $q(z_t) = 0$ defines the transition redshift.
What would settle it
Generate mock $H(z)$ and Type Ia supernova catalogs from a known flat Friedmann cosmology with a known transition redshift, apply the same Gaussian Process reconstruction, and check whether the recovered $z_t$ and its quoted uncertainty contain the input value; if the derivative reconstruction is biased at low redshift, the quoted model-independent $z_t$ would be systematically off.
Extended reading notes
Core claim
The central claim is that the transition redshift can be estimated directly from data. Reconstructing $H(z)$ and its first derivative from the 51-point H(z) compilation yields $q(z)$ through $q(z) = (1+z)H'(z)/H(z) - 1$, with a zero crossing at $z_t = 0.59^{+0.12}_{-0.11}$. Reconstructing the luminosity distance and its first two derivatives from 1048 supernovae, using $q(z) = (1+z)^2 D''_L(z)/(D_L(z) - (1+z)D'_L(z)) + 1$, gives $z_t = 0.683^{+0.11}_{-0.082}$ when spatial flatness is assumed. Both results are stable under the three covariance kernels, and the two probes agree within 1σ.
Load-bearing premise
The zero crossing of $q(z)$ is reliable only if the Gaussian Process derivative reconstructions $H'(z)$ and $D''_L(z)$ are unbiased, and the paper does not test this against simulated data; second derivatives from sparse supernova distances are especially sensitive to the kernel and the optimized length scale.
Editorial extensions
If this is right
- The transition redshift is determined without reference to any dark-energy model, so a value near $z_t \simeq 0.6$ becomes a direct empirical target that any theory of cosmic acceleration must reproduce.
- The agreement between the H(z) and supernova results across three covariance kernels indicates that the estimate is not sensitive to the choice of kernel.
- The supernova-based estimate inherits the spatial-flatness assumption; within a flat universe, the two probes together constrain when cosmic acceleration began.
- The same Gaussian Process machinery extends to reconstruct other derived functions of $H(z)$ and $D_L(z)$, such as the dark-energy equation of state, in the same model-independent way.
Reading between the lines
- The quoted uncertainties are posterior errors from a single optimized kernel; systematic error from kernel choice and from the Gaussian Process prior mean is not included, so the full error budget is probably larger than reported.
- The supernova reconstruction assumes spatial flatness. A joint Gaussian Process reconstruction of expansion and distance data could in principle relax that assumption and constrain curvature and $z_t$ simultaneously.
- The consistency between the two probes could be sharpened with the distance-duality relation $D_L = (1+z)^2 D_A$: a future mismatch between H(z)-based and supernova-based $z_t$ would then point to new physics or systematics rather than merely different data.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper uses Gaussian Process (GP) reconstructions, implemented with the GaPP package, to determine the transition redshift z_t where cosmic expansion changes from deceleration to acceleration. Two data sets are used: 51 H(z) measurements and the Pantheon SNe Ia sample. From the H(z) reconstruction, the authors obtain z_t = 0.59^{+0.12}_{-0.11}; from the SNe Ia luminosity-distance reconstruction, assuming spatial flatness, they obtain z_t = 0.683^{+0.11}_{-0.082}. These results are reported for a squared-exponential kernel and are stated to be consistent with the Matern(5/2) and Matern(7/2) kernels. The paper claims that the GP method makes the analysis model-independent, apart from the flatness assumption for the SNe Ia part.
Significance. If correct, the paper would provide useful, model-independent cross-checks of the transition redshift from two independent cosmological probes, with the strength of using the full Pantheon covariance matrix and testing three kernel choices. The H(z) result is broadly consistent with earlier estimates in the literature. However, the SNe Ia half of the central claim rests on an algebraic relation for q(z) that is incorrect, and the lack of validation of the GP derivative reconstructions leaves the quoted uncertainties unsupported. The paper is therefore not acceptable in its present form, but the issues are local and fixable by re-deriving the formula and re-running the analysis.
major comments (2)
- [III, Eqs. (23), (24), (28)] The derivation of q(z) from the luminosity distance is algebraically incorrect. Starting from D_L=(1+z)D_C, E=1/D_C', and q=-(1+z)D_C''/D_C'-1 (Eq. 22), the correct relation is q = 1 - (1+z)^2 D_L'' / [(1+z)D_L' - D_L]. The printed Eq. (28), q = (1+z)^2 D_L''/D_L - (1+z)D_L' + 1, has the wrong denominator and an incorrect sign; the same slip appears in Eqs. (23) and (24). For a flat LambdaCDM model at z=0.5, Eq. (28) gives q approximately +0.3 while the true deceleration parameter is approximately -0.1, so the zero crossing of Eq. (28) is not a physical deceleration-to-acceleration transition. Since Section V.B states that the q(z) reconstruction is performed "according to Eq. (28), the reported SNe Ia transition redshift in Table I is not a valid measurement as presented.
- [V.A and V.B] The paper asserts that Monte Carlo sampling and analytic error propagation give "negligible difference" for the q(z) uncertainties, but it does not show the comparison, and the GP derivative reconstructions are not validated against simulations. Because z_t is read off as the zero crossing of q(z), a biased GP derivative H'(z) or D_L''(z) shifts z_t systematically, and the optimized length scale and sparse high-redshift data are known to affect GP derivatives. The authors should add a mock-data test (e.g., reconstructing q(z) from a flat LambdaCDM model with the same redshift sampling) to demonstrate that the reconstructed q(z) and its zero crossing are unbiased, and they should show the Monte Carlo versus error-propagation comparison for the H(z) analysis. This is necessary to justify the quoted 1-sigma confidence intervals.
minor comments (5)
- [V.A] The sentence "we found negligible difference with the error propagation (29)" refers to Eq. (29), which is the SNe Ia uncertainty formula, not the H(z) uncertainty expression; the correct reference for the H(z) case is Eq. (14).
- [III, Eq. (27)] The Jacobian matrix in Eq. (27) is written as J = diag(sigma^2_DL), which is dimensionally inconsistent; the Jacobian should be J = diag(alpha D_Li). The final propagated variance formula is correct, but the notation should be fixed.
- [Table I] For the Matern(5/2) DL row, only one pair of upper/lower uncertainties is printed (0.83^{+0.25}_{-0.50}), whereas other rows list both 1-sigma and 2-sigma values; if 2-sigma values were intended, they should be provided.
- [Conclusion] There are small grammatical errors: "decelerator parameter" should be "deceleration parameter," and "module distance function" should be "luminosity distance function."
- [Throughout] Expressions such as "51 H(z) data" should read "51 H(z) data points" for clarity.
Circularity Check
No significant circularity: z_t is read off as a zero-crossing of GP-reconstructed q(z), not fitted; the SNe branch's Eq. (28) error is a correctness flaw, not a circular reduction.
full rationale
The H(z) branch is self-contained: Eq. (13), q = (1+z)H'/H - 1, is the standard kinematical definition, and z_t is not a fitted parameter but the root of the reconstructed q(z). The GP hyperparameters are optimized on the same data, but the target quantity is a feature of the reconstructed curve rather than a parameter renamed as a prediction, and the kernel dependence is explicitly tested with three kernels. The SNe branch is structurally similar: DL(z), D'_L(z) and D''_L(z) are reconstructed from Pantheon data and z_t is read off from q(zt)=0. However, Eq. (28) is not algebraically equivalent to the paper's own definitions in Eqs. (18)-(22): the correct reduction is q = 1 - (1+z)^2 D''_L / [(1+z)D'_L - D_L], while the printed Eq. (28) has a different denominator and an incorrect extra term, and the same slip appears in the unused Eqs. (23)-(24). This means the SNe z_t values are not a valid derived prediction, but this is an algebraic correctness flaw rather than a circular step: the output is not forced by construction to equal the input. No load-bearing self-citation or imported uniqueness theorem is used; Refs. [25,27] appear only for context and comparison. Therefore the circularity score is 0.
Assumptions & free parameters
free parameters (3)
- GP hyperparameters σ_f and l (Squared Exponential kernel) =
not reported
- GP hyperparameters σ_f and l (Matern 5/2 kernel) =
not reported
- GP hyperparameters σ_f and l (Matern 7/2 kernel) =
not reported
assumptions (5)
- domain assumption The functions H(z), D_L(z) and their derivatives are draws from a Gaussian Process with the chosen stationary kernel.
- standard math The derivative of a Gaussian Process is also a Gaussian Process with covariance given by differentiating the kernel.
- domain assumption Spatial flatness of the FLRW metric for the SNe Ia analysis.
- domain assumption SNe Ia are standardizable candles with a constant absolute magnitude after light-curve corrections, and the Pantheon covariance matrix fully characterizes the errors.
- domain assumption The 51 H(z) data points can be treated as independent with diagonal errors.
Cite this review
Pith. "Pith review of Gaussian Process Estimation of Transition Redshift." pith.science (2026). https://pith.science/paper/JGWGYTRU
@misc{pith2026190900090,
author = {Pith},
title = {Pith review of: Gaussian Process Estimation of Transition Redshift},
year = {2026},
howpublished = {\url{https://pith.science/paper/JGWGYTRU}},
note = {Machine review of arXiv:1909.00090}
}
abstract
This paper aims to put constraints on the transition redshift $z_t$, which determines the onset of cosmic acceleration, in cosmological-model independent frameworks. In order to do that, we use the non-parametric Gaussian Process method with $H(z)$ and SNe Ia data. The deceleration parameter reconstruction from $H(z)$ data yields $z_t=0.59^{+0.12}_{-0.11}$. The reconstruction from SNe Ia data assumes spatial flatness and yields $z_t=0.683^{+0.11}_{-0.082}$. These results were found with a Gaussian kernel and we show that they are consistent with two other kernel choices.
Figures
Forward citations
Cited by 2 Pith papers
-
A Joint Analysis of Strong Lensing and Type Ia Supernovae to Determine the Hubble Constant
A model-independent combination of strong lensing and supernova data gives H0 = 70.55 ± 7.44 km/s/Mpc, consistent with both Planck and SH0ES within 1sigma.
-
Is $\omega_0 \omega_a$CDM a good model for the clumpy Universe?
Against a Gaussian-process reconstruction of 15 sigma8(z) measurements, the DESI w0waCDM model fits slightly better than LambdaCDM, but the difference is tiny and the comparison metric is biased.
Reference graph
Works this paper leans on
-
[1]
A. G. Riess et al. [Supernova Search Team], Astron. J. 116 (1998) 1009 [astro-ph/9805201]
arXiv 1998
-
[2]
S. Perlmutter et al. [Supernova Cosmology Project Collaboration], Astrophys. J. 517 (1999) 565 [astro-ph/9812133]
arXiv 1999
-
[3]
P. Astier et al. [SNLS Collaboration], Astron. Astrophys. 447 (2006) 31 [astro-ph/0510447]
arXiv 2006
-
[4]
A. G. Riess et al. , Astrophys. J. 659 (2007) 98 [astro-ph/0611572]
arXiv 2007
-
[5]
T. M. Davis et al. , Astrophys. J. 666 (2007) 716 [astro-ph/0701510]
work page Pith review arXiv 2007
-
[6]
M. Kowalski et al. [Supernova Cosmology Project Collaboration], Astrophys. J. 686 (2008) 749 [arXiv:0804.4142 [astro-ph]]
arXiv 2008
-
[7]
R. Amanullah et al. , Astrophys. J. 716 (2010) 712 [arXiv:1004.1711 [astro-ph.CO]]
arXiv 2010
-
[8]
N. Suzuki et al. , Astrophys. J. 746 (2012) 85 [arXiv:1105.3470 [astro-ph.CO]]
arXiv 2012
Show all 62 references
-
[9]
Komatsu et al
E. Komatsu et al. [WMAP Collaboration], Astrophys. J. Suppl. 192 (2011) 18 [arXiv:1001.4538 [astro-ph.CO]]
2011 arXiv
-
[10]
Larson et al
D. Larson et al. , Astrophys. J. Suppl. 192 (2011) 16 [arXiv:1001.4635 [astro-ph.CO]]
2011 arXiv
-
[11]
P. A. R. Ade et al. [Planck Collaboration], Astron. Astrophys. 571 (2014) A16 [arXiv:1303.5076 [astro-ph.CO]]
2014 arXiv
-
[12]
D. J. Eisenstein et al. [SDSS Collaboration], Astrophys. J. 633 (2005) 560 [astro-ph/0501171]
2005 arXiv
-
[13]
W. J. Percival, S. Cole, D. J. Eisenstein, R. C. Nichol, J. A. Peacock, A. C. Pope and A. S. Sza- lay, Mon. Not. Roy. Astron. Soc. 381 (2007) 1053 [arXiv:0705.3323 [astro-ph]]
2007 arXiv
-
[14]
Schlegel et al
D. Schlegel et al. [with input from the SDSS-III Collaboration], arXiv:0902.4680 [astro-ph.CO]
-
[15]
D. J. Eisenstein et al. [SDSS Collaboration], Astron. J. 142 (2011) 72 [arXiv:1101.1529 [astro- ph.IM]]
2011 arXiv
-
[16]
K. S. Dawson et al. [BOSS Collaboration], Astron. J. 145 (2013) 10 [arXiv:1208.0022 [astro- ph.CO]]
2013 arXiv
-
[17]
Farooq, D
O. Farooq, D. Mania and B. Ratra, Astrophys. J. 764 (2013) 138 [arXiv:1211.4253 [astro- ph.CO]]
2013 arXiv
-
[18]
Farooq and B
O. Farooq and B. Ratra, Astrophys. J. 766 (2013) L7 [arXiv:1301.5243 [astro-ph.CO]]. 16
2013 arXiv
-
[19]
Farooq, F
O. Farooq, F. R. Madiyar, S. Crandall and B. Ratra, Astrophys. J. 835 (2017) no.1, 26 [arXiv:1607.03537 [astro-ph.CO]]
2017 arXiv
-
[20]
G. S. Sharov and E. G. Vorontsova, JCAP 1410 (2014) 10, 057 [arXiv:1407.5405 [gr-qc]]
2014 arXiv
-
[21]
Shapiro and M
C. Shapiro and M. S. Turner, Astrophys. J. 649 (2006) 563 [astro-ph/0512586]
2006 arXiv
-
[22]
J. V. Cunha and J. A. S. Lima, Mon. Not. Roy. Astron. Soc. 390 (2008) 210 [arXiv:0805.1261 [astro-ph]]
2008 arXiv
-
[23]
A. C. C. Guimaraes, J. V. Cunha and J. A. S. Lima, JCAP 0910 (2009) 010 [arXiv:0904.3550 [astro-ph.CO]]
2009 arXiv
-
[24]
N. Rani, D. Jain, S. Mahajan, A. Mukherjee and N. Pires, JCAP 1512 (2015) no.12, 045 [arXiv:1503.08543 [gr-qc]]
2015 arXiv
-
[25]
J. A. S. Lima, J. F. Jesus, R. C. Santos and M. S. S. Gill, arXiv:1205.4688 [astro-ph.CO]
-
[27]
for DC(z),H(z) and q(z) parametrizations, respectively
yieldedzt = 0.806±0.094, 0.870±0.063 and 0.973±0.058 at 1σ c.l. for DC(z),H(z) and q(z) parametrizations, respectively. It is a result compatible with our DL(z) Mat´ ern(5/2) and Mat´ ern(7/2) reconstructions. They are incompatible with all ourH(z) reconstructions 13 0.0 0.5 1...
-
[28]
L. Xu, W. Li and J. Lu, JCAP 0907 (2009) 031 [arXiv:0905.4552 [astro-ph.CO]]
2009 arXiv
-
[29]
Jesus, R.F.L
J.F. Jesus, R.F.L. Holanda and S.H. Pereira, JCAP 05 (2018) 073
2018
-
[30]
Seikel, C
M. Seikel, C. Clarkson and M. Smith, JCAP 06 (2012) 036
2012
-
[31]
Betoule et al
M. Betoule et al. [SDSS Collaboration], Astron. Astrophys. 568 (2014) A22 [arXiv:1401.4064 [astro-ph.CO]]
2014 arXiv
-
[32]
Holsclaw et al., Phys
T. Holsclaw et al., Phys. Rev. Lett. 105 (2010) 241302
2010
-
[33]
Rasmussen and C
C. Rasmussen and C. Williams, Gaussian Processes for Machine Learning , MIT Press, Cam- bridge U.S.A. (2006)
2006
-
[34]
Shafieloo, A
A. Shafieloo, A. G. Kim and E. V. Linder, Phys. Rev. D 85, (2012) 123530
2012
-
[35]
Holsclaw et al., Phys
T. Holsclaw et al., Phys. Rev. D 82 (2010) 103502
2010
-
[36]
M. K. Yennapureddy and F. Melia, Eur. Phys. J. C (2018) 78: 258
2018
- [37]
-
[38]
Melia and M
F. Melia and M. K. Yennapureddy, JCAP 02, 034 (2018)
2018
-
[39]
M. K. Yennapureddy and F. Melia, JCAP 11, 029 (2017)
2017
-
[40]
A. H. Guth, Phys. Rev. D 23 (1981) 347 [Adv. Ser. Astrophys. Cosmol. 3 (1987) 139]
1981
-
[41]
Melia and A
F. Melia and A. Shevchuk, MNRAS 419 2579
-
[42]
Magana, M
J. Magana, M. H. Amante, M. A. Garcia-Aspeitia and V. Motta, Mon. Not. Roy. Astron. Soc. 476 (2018) no.1, 1036 [arXiv:1706.09848 [astro-ph.CO]]
2018 arXiv
- [43]
-
[44]
Stern, R
D. Stern, R. Jimenez, L. Verde, M. Kamionkowski and S. A. Stanford, Cosmic chronometers: constraining the equation of state of dark energy. I: H(z) measurements, J. of Cosmology and Astropart. Phys. 02 (2010) 008 [arXiv:0907.3149]
2010 arXiv
-
[45]
Simon, L
J. Simon, L. Verde and R. Jimenez, Constraints on the redshift dependence of the dark energy potential, Phys. Rev. D 71 (2005) 123001 [astro-ph/0412269]
2005 arXiv
-
[46]
Zhang, H
C. Zhang, H. Zhang, S. Yuan, T. J. Zhang and Y. C. Sun, Res. Astron. Astrophys. 14, no. 10, 1221 (2014) [arXiv:1207.4541 [astro-ph.CO]]
2014 arXiv
-
[47]
Moresco et al., Improved constraints on the expansion rate of the Universe up to z 1.1 from the spectroscopic evolution of cosmic chronometers , J
M. Moresco et al., Improved constraints on the expansion rate of the Universe up to z 1.1 from the spectroscopic evolution of cosmic chronometers , J. of Cosmology and Astropart. Phys. 8 (2012) 006 [arXiv:1201.3609]
2012 arXiv
-
[48]
Moresco et al
M. Moresco et al. , JCAP 1605 (2016) no.05, 014 [arXiv:1601.01701 [astro-ph.CO]]
2016 arXiv
-
[49]
Moresco, Raising the bar: new constraints on the Hubble parameter with cosmic chronome- ters at z ≈ 2,, Mon
M. Moresco, Raising the bar: new constraints on the Hubble parameter with cosmic chronome- ters at z ≈ 2,, Mon. Not. Roy. Astron. Soc. 450 (2015) L16 [arXiv:1503.01116]
2015 arXiv
-
[50]
Blake et al
C. Blake et al. , The WiggleZ Dark Energy Survey: Joint measurements of the expansion and growth history at z <1 , Mon. Not. Roy. Astron. Soc. 425(1) (2012) 405 [arXiv:1204.3674]
2012 arXiv
-
[51]
Gazta˜ naga, A
E. Gazta˜ naga, A. Cabre, L. Hui, Clustering of Luminous Red Galaxies IV: Baryon Acous- tic Peak in the Line-of-Sight Direction and a Direct Measurement of H(z), Mon. Not. Roy. Astron. Soc. 399(3) (2009) 1663 [arXiv:0807.3551]
2009 arXiv
-
[52]
Anderson et al., Mon
L. Anderson et al., Mon. Not. Roy. Astron. Soc. 439, no. 1, 83 (2014) [arXiv:1303.4666 [astro- ph.CO]]
2014 arXiv
-
[53]
N. G. Busca et al. , Baryon Acoustic Oscillations in the Ly α forest of BOSS quasars , Astron. and Astrop. 552 (2013) A96 [arXiv:1211.2616]
2013 arXiv
-
[54]
Delubac et al
T. Delubac et al. [BOSS Collaboration], Astron. Astrophys. 574 (2015) A59 [arXiv:1404.1801 [astro-ph.CO]]. 18
2015 arXiv
-
[55]
Font-Ribera et al
A. Font-Ribera et al. , Quasar-Lyman α Forest Cross-Correlation from BOSS DR11: Baryon Acoustic Oscillations , J. of Cosmology and Astroparticle Phys. 05 (2014) 027 [arXiv:1311.1767]
2014 arXiv
-
[56]
Oka et al., Simultaneous constraints on the growth of structure and cosmic expansion from the multipole power spectra of the SDSS DR7 LRG sample , Mon
A. Oka et al., Simultaneous constraints on the growth of structure and cosmic expansion from the multipole power spectra of the SDSS DR7 LRG sample , Mon. Not. Roy. Astron. Soc. 439(3) (2014) 2515 [arXiv:1310.2820]
2014 arXiv
-
[57]
Chuang and Y
C.H. Chuang and Y. Wang, Modeling the Anisotropic Two-Point Galaxy Correlation Function on Small Scales and Improved Measurements of H(z), DA(z), and f(z)σ8(z) from the Sloan Digital Sky Survey DR7 Luminous Red Galaxies , Mon. Not. Roy. Astron. Soc. 435(1) (2013) 255 [arXiv:1209.0210]
2013 arXiv
-
[58]
A. G. Riess, S. Casertano, W. Yuan, L. M. Macri and D. Scolnic, Astrophys. J. 876 (2019) no.1, 85 [arXiv:1903.07603 [astro-ph.CO]]
2019 arXiv
-
[59]
J. L. Bernal, L. Verde and A. G. Riess, JCAP 1610 (2016) no.10, 019 [arXiv:1607.05617 [astro-ph.CO]]
2016 arXiv
-
[60]
Riess, A.G. et al. , Astrophys. J. 730 (2011) 119 Erratum: [Astrophys. J. 732 (2011) 129] [arXiv:1103.2976 [astro-ph.CO]]
2011 arXiv
-
[61]
find 0.33<z t < 1 at 1σ c.l., compatible with our results. 14 VI. CONCLUSION The transition from decelerated to the current accelerated phase of expansion of the universe is an important question in modern cosmology. It is well known that the transition redshift zt is strongly ...
2017
-
[62]
D. M. Scolnic et al. , Astrophys. J. 859 (2018) no.2, 101 [arXiv:1710.00845 [astro-ph.CO]]
2018 arXiv
-
[63]
H. Yu, B. Ratra and F. Y. Wang, Astrophys. J. 856 (2018) no.1, 3 [arXiv:1711.03437 [astro- ph.CO]]. 19
2018 arXiv
Reviewed August 14, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.