pith. sign in

arxiv: 2504.17047 · v3 · submitted 2025-04-23 · ✦ hep-ph · astro-ph.CO

Generalized neutrino isocurvature

Pith reviewed 2026-05-22 17:56 UTC · model grok-4.3

classification ✦ hep-ph astro-ph.CO
keywords neutrino isocurvaturematter isocurvatureisocurvature perturbationsmixing anglePlanck CMB dataearly universecosmological constraintsdensity fluctuations
0
0 comments X

The pith

Realistic early-universe scenarios generate both neutrino and matter isocurvature perturbations whose ratio is captured by a single mixing angle now bounded by Planck data.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

Searches for neutrino isocurvature have typically targeted one specific combination of initial density fluctuations. In contrast, this paper examines concrete cosmological models that naturally produce both neutrino and matter isocurvature at the same time. The relative size of these two types of fluctuations is described by one new constant parameter, the mixing angle. Using Planck cosmic microwave background observations, the authors derive the first numerical limits on the allowed range for this angle. The result opens a route to using future precision measurements to distinguish among different mechanisms that could have operated in the very early universe.

Core claim

In general, both neutrino and matter isocurvature perturbations are generated, whose ratio we parameterize by a newly introduced mixing angle. We obtain the first limits on this new mixing angle from PLANCK data, and discuss novel insights into the early Universe that could be provided by future measurements.

What carries the argument

A mixing angle that parameterizes the constant ratio between the amplitudes of correlated neutrino isocurvature and matter isocurvature perturbations.

If this is right

  • Existing upper limits on pure neutrino isocurvature must be reinterpreted once the possible accompanying matter isocurvature is taken into account.
  • The mixing angle supplies an additional observable that can discriminate among different early-universe scenarios capable of generating isocurvature.
  • Future CMB experiments can place tighter bounds on the mixing angle and potentially measure a nonzero value.
  • The parameterization reduces the number of free parameters needed to describe general isocurvature initial conditions in cosmological analyses.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The mixing angle may map onto specific parameters in concrete models such as multi-field inflation or curvaton scenarios, allowing direct comparison with microphysical predictions.
  • The same correlation could leave detectable signatures in large-scale structure surveys or 21-cm observations that probe different combinations of density perturbations.
  • Extending the approach to time- or scale-dependent mixing angles could be tested once next-generation data reduce statistical errors.
  • Incorporating the mixing angle into forecasts for experiments such as CMB-S4 would quantify the improvement in early-universe constraints.

Load-bearing premise

The ratio between neutrino and matter isocurvature perturbations stays constant across relevant scales and can be described by one mixing angle without introducing large degeneracies or unmodeled effects in the Planck data analysis.

What would settle it

A future CMB measurement that finds isocurvature power with a neutrino-to-matter ratio that cannot be reproduced by any single fixed value of the mixing angle.

Figures

Figures reproduced from arXiv: 2504.17047 by Christopher Gerlach, Pedro Schwaller, Wolfram Ratzinger.

Figure 1
Figure 1. Figure 1: FIG. 1. Cosmic histories in comparison: The lines correspond [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Comparison of temperature correlations in the CMB [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Comparison of the linear matter power spectrum [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Same as Fig [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Inferred values from the fit of the isocurvature frac [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗
read the original abstract

Searches for neutrino isocurvature usually constrain a specific linear combination of isocurvature perturbations. In this work, we discuss realistic cosmological scenarios giving rise to neutrino isocurvature. We show that in general both, neutrino and matter isocurvature perturbations are generated, whose ratio we parameterize by a newly introduced mixing angle. We obtain the first limits on this new mixing angle from PLANCK data, and discuss novel insights into the early Universe that could be provided by future measurements.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 0 minor

Summary. The paper argues that realistic cosmological scenarios generically produce both neutrino and matter isocurvature perturbations whose ratio can be captured by a single new constant mixing angle. It then derives the first constraints on this mixing angle from Planck CMB data and discusses implications for early-Universe physics.

Significance. A well-justified constant-ratio parameterization would allow cleaner separation of neutrino isocurvature from matter isocurvature and could yield new early-Universe diagnostics once future data tighten the limits. The present Planck bounds constitute an initial exploration whose ultimate utility hinges on the absence of large degeneracies and scale dependence.

major comments (1)
  1. The central claim that a single constant mixing angle fully captures the correlated neutrino+matter isocurvature generated in realistic scenarios requires explicit demonstration that the ratio remains time- and scale-independent across the modes that source the Planck CMB spectra; any k- or time-dependence would render the reported single-angle limits inapplicable without additional modeling.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their thorough review and valuable feedback on our paper. We address the major comment in detail below and are prepared to revise the manuscript accordingly to strengthen our claims.

read point-by-point responses
  1. Referee: The central claim that a single constant mixing angle fully captures the correlated neutrino+matter isocurvature generated in realistic scenarios requires explicit demonstration that the ratio remains time- and scale-independent across the modes that source the Planck CMB spectra; any k- or time-dependence would render the reported single-angle limits inapplicable without additional modeling.

    Authors: We agree that an explicit demonstration is necessary to support the applicability of our parameterization. In the realistic cosmological scenarios discussed in the paper, the isocurvature perturbations are generated at very early times when all relevant modes are super-horizon. In this regime, the evolution equations for the neutrino and matter isocurvature perturbations lead to a constant ratio that is preserved until horizon entry. Since the CMB spectra are primarily sourced by modes that enter the horizon around or after recombination, and our parameterization is applied at the initial time, the ratio remains effectively constant for the purposes of Planck constraints. Nevertheless, to make this explicit and address potential concerns about scale dependence, we will include additional analysis in the revised version, such as plots of the ratio as a function of time and wavenumber for the modes contributing to the CMB power spectra. This will confirm the validity of the constant mixing angle approximation in the context of our limits. revision: yes

Circularity Check

0 steps flagged

No circularity: new mixing angle parameterization constrained by external Planck data

full rationale

The paper introduces a mixing angle to capture the ratio of generated neutrino and matter isocurvature perturbations in realistic cosmological scenarios, then reports the first constraints on this angle using Planck CMB data. No quoted equations or steps reduce the reported limits or the existence of the mixing angle to a quantity fitted from the same dataset or to a self-citation chain that bears the central claim. The parameterization is presented as a derived feature of the scenarios, and the limits rely on independent external observations rather than internal redefinition or renaming of inputs.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

Central claim rests on the existence of realistic scenarios that simultaneously generate both neutrino and matter isocurvature and on the assumption that their ratio is captured by one new parameter constrained by existing CMB data.

free parameters (1)
  • mixing angle
    Newly introduced parameter that sets the relative amplitude of neutrino versus matter isocurvature; its value is constrained rather than derived from first principles.
axioms (1)
  • domain assumption Standard cosmological perturbation theory applies to the mixed isocurvature modes
    Invoked when stating that both perturbation types are generated and can be parameterized by a single angle.

pith-pipeline@v0.9.0 · 5599 in / 1143 out tokens · 51325 ms · 2026-05-22T17:56:31.098141+00:00 · methodology

discussion (0)

Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

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.

Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. Isocurvature Induced Gravitational Waves at Pulsar Timing Arrays

    astro-ph.CO 2025-12 unverdicted novelty 7.0

    The work shows that free-streaming dark radiation isocurvature produces a qualitatively different gravitational wave spectrum than cold dark matter isocurvature and derives constraints on isocurvature power spectra ar...

Reference graph

Works this paper leans on

52 extracted references · 52 canonical work pages · cited by 1 Pith paper · 29 internal anchors

  1. [1]

    J. M. Bardeen, Gauge Invariant Cosmological Perturba- tions, Phys. Rev. D 22, 1882 (1980)

  2. [2]

    Must Cosmological Perturbations Remain Non-Adiabatic After Multi-Field Inflation?

    S. Weinberg, Must cosmological perturbations remain non-adiabatic after multi-field inflation?, Phys. Rev. D 70, 083522 (2004), arXiv:astro-ph/0405397

  3. [3]

    Can Non-Adiabatic Perturbations Arise After Single-Field Inflation?

    S. Weinberg, Can non-adiabatic perturbations arise after single-field inflation?, Phys. Rev. D 70, 043541 (2004), arXiv:astro-ph/0401313

  4. [4]

    The General Primordial Cosmic Perturbation

    M. Bucher, K. Moodley, and N. Turok, The General pri- mordial cosmic perturbation, Phys. Rev. D 62, 083508 (2000), arXiv:astro-ph/9904231

  5. [5]

    WMAP data was analyzed in terms of up to two isocurva- ture initial conditions with arbitrary correlations [49–51]

  6. [6]

    D. Grin, O. Dore, and M. Kamionkowski, Compen- sated Isocurvature Perturbations and the Cosmic Mi- crowave Background, Phys. Rev. D 84, 123003 (2011), arXiv:1107.5047 [astro-ph.CO]

  7. [7]

    D. Grin, O. Dore, and M. Kamionkowski, Do baryons trace dark matter in the early universe?, Phys. Rev. Lett. 107, 261301 (2011), arXiv:1107.1716 [astro-ph.CO]

  8. [8]

    Planck 2018 results. X. Constraints on inflation

    Y. Akrami et al. (Planck), Planck 2018 results. X. Con- straints on inflation, Astron. Astrophys.641, A10 (2020), arXiv:1807.06211 [astro-ph.CO]

  9. [9]

    Lee and Y

    N. Lee and Y. Ali-Ha¨ ımoud, Probing small-scale baryon and dark matter isocurvature perturbations with cosmic microwave background anisotropies, Phys. Rev. D 104, 103509 (2021), arXiv:2108.07798 [astro-ph.CO]

  10. [10]

    Barreira, Constraints on compensated isocurvature perturbations from BOSS DR12 galaxy data, JCAP 08, 051, arXiv:2302.01927 [astro-ph.CO]

    A. Barreira, Constraints on compensated isocurvature perturbations from BOSS DR12 galaxy data, JCAP 08, 051, arXiv:2302.01927 [astro-ph.CO]

  11. [11]

    Isocurvature perturbations in extra radiation

    M. Kawasaki, K. Miyamoto, K. Nakayama, and T. Sekiguchi, Isocurvature perturbations in extra radi- ation, JCAP 02, 022, arXiv:1107.4962 [astro-ph.CO]

  12. [12]

    Future constraints on neutrino isocurvature perturbations in the curvaton scenario

    E. Di Valentino, M. Lattanzi, G. Mangano, A. Melchiorri, and P. Serpico, Future constraints on neutrino isocurva- ture perturbations in the curvaton scenario, Phys. Rev. D 85, 043511 (2012), arXiv:1111.3810 [astro-ph.CO]

  13. [13]

    Non-Gaussian isocurvature perturbations in dark radiation

    E. Kawakami, M. Kawasaki, K. Miyamoto, K. Nakayama, and T. Sekiguchi, Non-Gaussian isocur- vature perturbations in dark radiation, JCAP 07, 037, arXiv:1202.4890 [astro-ph.CO]

  14. [14]

    Adshead, G

    P. Adshead, G. Holder, and P. Ralegankar, BBN con- straints on dark radiation isocurvature, JCAP 09, 016, arXiv:2006.01165 [astro-ph.CO]

  15. [15]

    Ghosh, S

    S. Ghosh, S. Kumar, and Y. Tsai, Free-streaming and coupled dark radiation isocurvature perturbations: con- straints and application to the Hubble tension, JCAP 05 (05), 014, arXiv:2107.09076 [astro-ph.CO]

  16. [16]

    Preskill, M

    J. Preskill, M. B. Wise, and F. Wilczek, Cosmology of the Invisible Axion, Phys. Lett. B 120, 127 (1983)

  17. [17]

    L. F. Abbott and P. Sikivie, A Cosmological Bound on the Invisible Axion, Phys. Lett. B 120, 133 (1983)

  18. [18]

    Dine and W

    M. Dine and W. Fischler, The Not So Harmless Axion, Phys. Lett. B 120, 137 (1983)

  19. [19]

    A. D. Linde, GENERATION OF ISOTHERMAL DEN- SITY PERTURBATIONS IN THE INFLATIONARY UNIVERSE, JETP Lett. 40, 1333 (1984)

  20. [20]

    Isocurvature forecast in the anthropic axion window

    J. Hamann, S. Hannestad, G. G. Raffelt, and Y. Y. Y. Wong, Isocurvature forecast in the anthropic axion win- dow, JCAP 06, 022, arXiv:0904.0647 [hep-ph]

  21. [21]

    Caputo, M

    A. Caputo, M. Geller, and G. Rossi, New source for light dark matter isocurvature in low scale inflation, Phys. Rev. D 110, 055027 (2024), arXiv:2306.00056 [hep-ph]

  22. [22]

    Lesgourgues and S

    J. Lesgourgues and S. Pastor, Massive neutrinos and cosmology, Phys. Rept. 429, 307 (2006), arXiv:astro- ph/0603494

  23. [23]

    Status of neutrino properties and future prospects - Cosmological and astrophysical constraints

    M. Lattanzi and M. Gerbino, Status of neutrino prop- erties and future prospects - Cosmological and as- trophysical constraints, Front. in Phys. 5, 70 (2018), arXiv:1712.07109 [astro-ph.CO]

  24. [24]

    P. F. De Salas, S. Gariazzo, O. Mena, C. A. Ternes, and M. T´ ortola, Neutrino Mass Ordering from Oscillations and Beyond: 2018 Status and Future Prospects, Front. Astron. Space Sci. 5, 36 (2018), arXiv:1806.11051 [hep- ph]

  25. [25]

    Gerlach, W

    C. Gerlach, W. Ratzinger, and P. Schwaller, to appear,

  26. [26]

    A new approach to the evolution of cosmological perturbations on large scales

    D. Wands, K. A. Malik, D. H. Lyth, and A. R. Liddle, A New approach to the evolution of cosmological pertur- bations on large scales, Phys. Rev. D 62, 043527 (2000), arXiv:astro-ph/0003278

  27. [27]

    D. H. Lyth and D. Wands, Conserved cosmological perturbations, Physical Review D 68, 10.1103/phys- revd.68.103515 (2003)

  28. [28]

    D. H. Lyth and Y. Rodr´ ıguez, Inflationary prediction for primordial non-gaussianity, Physical Review Letters 95, 10.1103/physrevlett.95.121302 (2005)

  29. [29]

    Artigas, J

    D. Artigas, J. Grain, and V. Vennin, Hamiltonian for- malism for cosmological perturbations: the separate- universe approach, Journal of Cosmology and Astropar- ticle Physics 2022 (02), 001

  30. [30]

    Dark Matter

    M. Cirelli, A. Strumia, and J. Zupan, Dark Matter, (2024), arXiv:2406.01705 [hep-ph]

  31. [31]

    The origin of the matter-antimatter asymmetry

    M. Dine and A. Kusenko, The Origin of the matter - antimatter asymmetry, Rev. Mod. Phys. 76, 1 (2003), arXiv:hep-ph/0303065

  32. [32]

    D. H. Lyth, C. Ungarelli, and D. Wands, The Primordial density perturbation in the curvaton scenario, Phys. Rev. D 67, 023503 (2003), arXiv:astro-ph/0208055

  33. [33]

    D. H. Lyth and D. Wands, The CDM isocurvature pertur- bation in the curvaton scenario, Phys. Rev. D68, 103516 (2003), arXiv:astro-ph/0306500

  34. [34]

    The generation of the baryon asymmetry would need to be a much weaker residual ef- fect

    In scenarios where the SM plasma reaches temperatures well above the electroweak scale, sphaleron processes equilibrate the lepton and baryon asymmetry, favoring a tiny lepton asymmetry [52]. The generation of the baryon asymmetry would need to be a much weaker residual ef- fect

  35. [35]

    D. Blas, J. Lesgourgues, and T. Tram, The Cosmic Linear Anisotropy Solving System (CLASS) II: Approximation schemes, JCAP 07, 034, arXiv:1104.2933 [astro-ph.CO]

  36. [36]

    Planck 2018 results. VI. Cosmological parameters

    N. Aghanim et al. (Planck), Planck 2018 results. VI. Cosmological parameters, Astron. Astrophys. 641, A6 (2020), [Erratum: Astron.Astrophys. 652, C4 (2021)], arXiv:1807.06209 [astro-ph.CO]

  37. [37]

    Planck 2018 results. V. CMB power spectra and likelihoods

    N. Aghanim et al. (Planck), Planck 2018 results. V. CMB power spectra and likelihoods, Astron. Astrophys. 641, A5 (2020), arXiv:1907.12875 [astro-ph.CO]

  38. [38]

    The clustering of galaxies in the completed SDSS-III Baryon Oscillation Spectroscopic Survey: cosmological analysis of the DR12 galaxy sample

    S. Alam et al. (BOSS), The clustering of galaxies in the completed SDSS-III Baryon Oscillation Spectroscopic Survey: cosmological analysis of the DR12 galaxy sam- ple, Mon. Not. Roy. Astron. Soc. 470, 2617 (2017), arXiv:1607.03155 [astro-ph.CO]. 10

  39. [39]

    A. J. Ross, L. Samushia, C. Howlett, W. J. Percival, A. Burden, and M. Manera, The clustering of the SDSS DR7 main Galaxy sample – I. A 4 per cent distance mea- sure at z = 0.15, Mon. Not. Roy. Astron. Soc. 449, 835 (2015), arXiv:1409.3242 [astro-ph.CO]

  40. [40]

    The 6dF Galaxy Survey: Baryon Acoustic Oscillations and the Local Hubble Constant

    F. Beutler, C. Blake, M. Colless, D. H. Jones, L. Staveley- Smith, L. Campbell, Q. Parker, W. Saunders, and F. Watson, The 6dF Galaxy Survey: Baryon Acous- tic Oscillations and the Local Hubble Constant, Mon. Not. Roy. Astron. Soc.416, 3017 (2011), arXiv:1106.3366 [astro-ph.CO]

  41. [41]

    Conservative Constraints on Early Cosmology: an illustration of the Monte Python cosmological parameter inference code

    B. Audren, J. Lesgourgues, K. Benabed, and S. Prunet, Conservative Constraints on Early Cosmology: an illus- tration of the Monte Python cosmological parameter in- ference code, JCAP 1302, 001, arXiv:1210.7183 [astro- ph.CO]

  42. [42]

    MontePython 3: boosted MCMC sampler and other features

    T. Brinckmann and J. Lesgourgues, MontePython 3: boosted MCMC sampler and other features, Phys. Dark Univ. 24, 100260 (2019), arXiv:1804.07261 [astro- ph.CO]

  43. [43]

    Due to the absence of sig- nificant evidence for isocurvature these runs did not con- verge

    We made some initial runs in which the spectral index niso was varied freely as well. Due to the absence of sig- nificant evidence for isocurvature these runs did not con- verge. Therefore we only present bounds on the simpler yet well motivated scenario niso = 1

  44. [44]

    Dodelson, Modern Cosmology (Academic Press, Ams- terdam, 2003)

    S. Dodelson, Modern Cosmology (Academic Press, Ams- terdam, 2003)

  45. [45]

    D. H. Lyth and A. R. Liddle, The Primordial Density Perturbation (2009)

  46. [46]

    Kodama and M

    H. Kodama and M. Sasaki, Evolution of Isocurvature Perturbations. 2. Radiation Dust Universe, Int. J. Mod. Phys. A 2, 491 (1987)

  47. [47]

    D. H. Lyth and A. Riotto, Particle physics models of in- flation and the cosmological density perturbation, Phys. Rept. 314, 1 (1999), arXiv:hep-ph/9807278

  48. [48]

    R. K. Sachs and A. M. Wolfe, Perturbations of a cos- mological model and angular variations of the microwave background, Astrophys. J. 147, 73 (1967)

  49. [49]

    Constraining Isocurvature Perturbations with CMB Polarization

    M. Bucher, K. Moodley, and N. Turok, Constraining isocurvature perturbations with CMB polarization, Phys. Rev. Lett. 87, 191301 (2001), arXiv:astro-ph/0012141

  50. [50]

    Constraints on isocurvature models from the WMAP first-year data

    K. Moodley, M. Bucher, J. Dunkley, P. G. Ferreira, and C. Skordis, Constraints on isocurvature models from the WMAP first-year data, Phys. Rev. D 70, 103520 (2004), arXiv:astro-ph/0407304

  51. [51]

    R. Bean, J. Dunkley, and E. Pierpaoli, Constraining Isocurvature Initial Conditions with WMAP 3-year data, Phys. Rev. D74, 063503 (2006), arXiv:astro-ph/0606685

  52. [52]

    J. A. Harvey and M. S. Turner, Cosmological baryon and lepton number in the presence of electroweak fermion number violation, Phys. Rev. D 42, 3344 (1990)