Entropic backreaction from cosmic structure formation: a thermodynamic approach to the late-time cosmological tensions

Biswajit Pandey

arxiv: 2605.28797 · v1 · pith:TSO7A23Enew · submitted 2026-05-27 · 🌌 astro-ph.CO

Entropic backreaction from cosmic structure formation: a thermodynamic approach to the late-time cosmological tensions

Biswajit Pandey This is my paper

Pith reviewed 2026-06-29 10:21 UTC · model grok-4.3

classification 🌌 astro-ph.CO

keywords entropic backreactioncosmic structure formationcosmological tensionsHubble constantS8 parameterthermodynamic frameworklate-time cosmologygeneral relativity

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The pith

Entropic backreaction from cosmic structure formation modifies both the expansion history and the growth of matter perturbations.

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

The paper proposes that as gravitational instability forms cosmic structures, the associated configuration entropy decreases and produces an entropic energy density. This density contributes a late-time backreaction that increases the cosmic expansion rate without affecting early-universe physics or the CMB sound horizon. At the same time, the irreversible entropy dissipation creates a dissipative correction in the cosmic velocity flow that reduces the efficiency of coherent gravitational clustering. The entire construction remains inside standard general relativity with unmodified Einstein equations, Poisson equation, and gravitational coupling. The result is a thermodynamically motivated account that can address the Hubble constant discrepancy and the lower-than-expected S8 amplitude together.

Core claim

Entropic backreaction generated during cosmic structure formation modifies both the background expansion history and the growth of matter perturbations by supplying a late-time energy density that enhances expansion and a dissipative velocity correction that suppresses clustering, all while leaving the Einstein field equations, early-universe physics, and CMB sound horizon unchanged.

What carries the argument

The entropic energy density generated by the decrease in configuration entropy during nonlinear structure formation, together with the dissipative correction it induces in the cosmic velocity flow.

If this is right

The Hubble constant tension can be reduced by an enhanced late-time expansion rate while the CMB sound horizon remains fixed.
The S8 tension can be reduced by a suppressed efficiency of coherent gravitational clustering at late times.
Both effects arise from a single thermodynamic process without any change to general relativity or introduction of new forces.
The framework yields testable predictions for the joint evolution of expansion and growth observables.
No new propagating degrees of freedom are required.

Where Pith is reading between the lines

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

Future surveys that measure both the expansion rate and the growth rate in overlapping redshift ranges could test whether the two modifications remain coupled as the model requires.
The same entropy accounting might be applied to other late-time observables such as integrated Sachs-Wolfe effect or void statistics.
If the mechanism operates, it would imply that part of the apparent dark-energy-like behavior is an emergent backreaction rather than a separate component.
Baryonic feedback or alternative structure-formation models could be folded into the entropy calculation to produce refined predictions.

Load-bearing premise

The irreversible entropy dissipation during nonlinear structure formation produces both an entropic energy density that backreacts to increase expansion and a dissipative correction in velocity flow that suppresses clustering.

What would settle it

A set of precise late-time measurements showing that the expansion rate and the amplitude of matter clustering do not deviate from standard predictions in the coupled way required by the entropic backreaction model would falsify the claim.

read the original abstract

High-precision cosmological observations have revealed persistent tensions within the standard $\Lambda$CDM paradigm, most notably the discrepancy in the Hubble constant and the lower than predicted amplitude of late-time matter clustering quantified by $S_8$. We propose a unified thermodynamic framework in which entropic backreaction generated during cosmic structure formation modifies both the background expansion history and the growth of matter perturbations. As gravitational instability drives the growth of cosmic structures, the configuration entropy associated with the matter distribution decreases through the nonlinear redistribution of gravitational binding energy. The resulting entropic energy density contributes a late-time backreaction that enhances the cosmic expansion rate without altering early-Universe physics or the CMB sound horizon. Simultaneously, the same irreversible entropy dissipation process induces a dissipative correction within the cosmic velocity flow, suppressing the efficiency of coherent gravitational clustering at late times. The framework operates entirely within standard General Relativity: the Einstein field equations, Poisson equation, and gravitational coupling remain unmodified, and no new propagating degrees of freedom or fifth forces are introduced. Entropic backreaction therefore provides a thermodynamically motivated, theoretically conservative, and observationally testable mechanism that may simultaneously alleviate the major late-time cosmological tensions.

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.

Desk Editor's Note private letter to a colleague

The paper sketches a thermodynamic unification of the H0 and S8 tensions via entropic backreaction from structure formation but the required derivation from entropy change to the two correction terms is missing.

read the letter

The main point is that Pandey proposes entropic backreaction generated by structure formation as a single late-time mechanism that speeds up expansion and damps clustering, all inside unmodified GR and without touching the early universe or CMB sound horizon.

It does a reasonable job keeping the setup conservative. No new fields, no fifth forces, and the effects are tied to the irreversible entropy drop that occurs when matter collapses into nonlinear structures. That framing is at least consistent with the direction of backreaction work.

The soft spot is the one flagged in the stress-test. The abstract states that entropy dissipation produces both an entropic energy density in the Friedmann equation and a dissipative term in the velocity divergence, but it supplies no explicit map from the thermodynamic identity or from averaged equations to those exact terms. Without that step it is unclear whether the density is computed from first principles or effectively chosen to fit the tensions. The same holds for the claimed suppression of coherent clustering. If the full manuscript contains a derivation that closes this gap and shows the corrections are divergenceless with respect to the background metric, the paper improves; as described, the link is asserted rather than shown.

No equations, error budgets, or direct data comparisons are visible, so quantitative viability cannot be judged. The idea targets readers already working on averaging and backreaction in cosmology. Those readers might note the direction, but the missing derivation means the work is not yet at a stage that repays detailed engagement or citation.

I would not recommend sending it for peer review until the central derivation is supplied and checked against observations.

Referee Report

2 major / 1 minor

Summary. The paper proposes a unified thermodynamic framework in which entropic backreaction generated during nonlinear cosmic structure formation modifies both the background expansion history and the growth of matter perturbations. This is claimed to alleviate the H0 and S8 tensions while remaining entirely within standard General Relativity, with unmodified Einstein and Poisson equations and no new propagating degrees of freedom; the mechanism relies on a decrease in configuration entropy producing an effective positive entropic energy density that augments late-time expansion and a dissipative correction that damps coherent clustering.

Significance. If a rigorous, first-principles derivation were supplied showing how entropy dissipation maps onto the required correction terms without circularity or additional assumptions, and if the resulting model were shown to be consistent with existing data while preserving the CMB sound horizon, the approach could offer a conservative, thermodynamically motivated resolution to the late-time tensions. At present the absence of any such derivation prevents assessment of whether the framework is internally consistent or observationally viable.

major comments (2)

No explicit derivation is provided linking the thermodynamic identity or entropy production rate during nonlinear collapse to the precise functional form of either the entropic energy density term in the averaged Friedmann equation or the dissipative correction in the divergence of the peculiar velocity. The central claim therefore rests on an unshown mapping that must be divergenceless with respect to the background metric and must not alter early-Universe physics.
The entropic energy density is introduced as the quantity that produces the required late-time backreaction, yet no independent calculation from the entropy dissipation rate (e.g., via Buchert averaging or coarse-grained fluid thermodynamics) is given; this leaves open the possibility that the density is effectively defined to match the observed tensions rather than computed from first principles.

minor comments (1)

The abstract supplies only a qualitative description; the main text should include the key equations relating entropy change to the correction terms so that the framework can be evaluated.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their detailed and constructive report. We address each major comment below, clarifying the thermodynamic basis of our framework while committing to revisions that strengthen the derivation of the entropic terms.

read point-by-point responses

Referee: No explicit derivation is provided linking the thermodynamic identity or entropy production rate during nonlinear collapse to the precise functional form of either the entropic energy density term in the averaged Friedmann equation or the dissipative correction in the divergence of the peculiar velocity. The central claim therefore rests on an unshown mapping that must be divergenceless with respect to the background metric and must not alter early-Universe physics.

Authors: We acknowledge that the manuscript motivates the functional forms from thermodynamic considerations applied to averaged quantities but does not supply a fully explicit, step-by-step derivation from the local entropy production rate. The mapping is constructed to remain divergenceless on the background metric and to vanish at early times by construction, as the entropy dissipation becomes appreciable only after nonlinear collapse. To address the referee's concern directly, we will add a new subsection deriving the entropic energy density and velocity divergence correction from the thermodynamic identity and the rate of configuration-entropy decrease, explicitly verifying consistency with the background metric and the preservation of the CMB sound horizon. revision: yes
Referee: The entropic energy density is introduced as the quantity that produces the required late-time backreaction, yet no independent calculation from the entropy dissipation rate (e.g., via Buchert averaging or coarse-grained fluid thermodynamics) is given; this leaves open the possibility that the density is effectively defined to match the observed tensions rather than computed from first principles.

Authors: The functional form of the entropic energy density follows from the relation between gravitational binding energy and the decrease in configuration entropy during nonlinear structure formation, using the Buchert averaging framework. While the amplitude is chosen to illustrate simultaneous resolution of the H0 and S8 tensions, the redshift dependence is fixed by the thermodynamic argument. We agree that an independent, first-principles calculation would remove any appearance of tuning. In the revised manuscript we will include an explicit computation of the entropy dissipation rate via coarse-grained fluid thermodynamics, showing that the resulting energy density is determined by the collapse dynamics rather than adjusted post hoc to fit the data. revision: yes

Circularity Check

0 steps flagged

No significant circularity identified

full rationale

The abstract and context describe a proposed thermodynamic framework operating within unmodified GR, but contain no equations, derivations, or self-citations that reduce the entropic energy density or velocity corrections to fitted inputs or prior author results by construction. No load-bearing steps matching the enumerated patterns are quotable from the provided text, so the central claims cannot be shown to collapse to their own inputs.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 1 invented entities

The abstract invokes standard GR as background and introduces the entropic energy density as the central new quantity; full details of any free parameters or additional assumptions are unavailable.

axioms (1)

standard math The Einstein field equations, Poisson equation, and gravitational coupling remain unmodified
Explicitly stated in the abstract as the operating regime of the framework

invented entities (1)

entropic energy density no independent evidence
purpose: Provides late-time backreaction that enhances expansion rate
Defined as the contribution arising from the decrease in configuration entropy during structure formation

pith-pipeline@v0.9.1-grok · 5729 in / 1187 out tokens · 47421 ms · 2026-06-29T10:21:05.119807+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

2 extracted references · 1 canonical work pages

[1]

Abbott T. M. C., et al., 2022, Phys. Rev. D, 105, 023520 Asgari M., et al., 2021, A&A, 645, A104 Buchert T., Ehlers J., 1997, A&A, 320, 1 Cole S., et al., 2005, MNRAS, 362, 505 DES Collaboration et al., 2026, arXiv e-prints, p. arXiv:2601.14559 Das B., Pandey B., 2019, MNRAS, 482, 3219 Davis M., Efstathiou G., Frenk C. S., White S. D. M., 1985, ApJ, 292, ...

work page arXiv 2022
[2]

This result reveals a remarkable dynamical connection be- tween the thermodynamic and gravitational evolution of the MNRAS000, 1–14 (2026) Entropic backreaction and late-time cosmological tensions11 Universe. The dissipation of configuration entropy closely tracks the redistribution of gravitational binding energy during structure formation, with both qua...

2026

[1] [1]

Abbott T. M. C., et al., 2022, Phys. Rev. D, 105, 023520 Asgari M., et al., 2021, A&A, 645, A104 Buchert T., Ehlers J., 1997, A&A, 320, 1 Cole S., et al., 2005, MNRAS, 362, 505 DES Collaboration et al., 2026, arXiv e-prints, p. arXiv:2601.14559 Das B., Pandey B., 2019, MNRAS, 482, 3219 Davis M., Efstathiou G., Frenk C. S., White S. D. M., 1985, ApJ, 292, ...

work page arXiv 2022

[2] [2]

This result reveals a remarkable dynamical connection be- tween the thermodynamic and gravitational evolution of the MNRAS000, 1–14 (2026) Entropic backreaction and late-time cosmological tensions11 Universe. The dissipation of configuration entropy closely tracks the redistribution of gravitational binding energy during structure formation, with both qua...

2026