arxiv: 2604.17083 · v1 · submitted 2026-04-18 · 🌌 astro-ph.CO · astro-ph.GA· hep-ph

Recognition: unknown

Dark ages bounds on non-accreting massive compact halo objects

Vivekanand Mohapatra , Alekha C. Nayak

Authors on Pith no claims yet

Pith reviewed 2026-05-10 06:22 UTC · model grok-4.3

classification 🌌 astro-ph.CO astro-ph.GAhep-ph

keywords MACHOsdark matter fraction21-cm signaldynamical frictioncosmic dawndark agesintergalactic mediumcosmological bounds

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The pith

Dynamical friction from non-accreting MACHOs distorts the global 21-cm signal enough to set tight upper bounds on their dark matter fraction.

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

The paper establishes upper limits on the fraction of dark matter that can reside in massive compact halo objects by modeling the heating they induce in the intergalactic gas. MACHOs moving through the post-recombination baryons lose kinetic energy via dynamical friction, which raises the gas temperature and shallows the expected 21-cm absorption feature relative to standard cosmology. By capping the allowed temperature-induced deviation at 50 mK near redshift 17 and 15 mK near redshift 89, and by forbidding any emission feature at redshifts above 300, the authors obtain mass-dependent bounds from 10^3 to 10^7 solar masses. A sympathetic reader cares because the dark-ages portion of the bound relies only on cosmology and avoids the astrophysical uncertainties tied to early star formation that affect cosmic-dawn constraints. Extended mass distributions produce stricter limits than single-mass populations, with critical-collapse spectra giving the strongest restrictions at intermediate masses.

Core claim

We derive a complementary cosmological upper bound on the fraction of dark matter residing inside massive compact halo objects using the cosmic dawn and dark ages global 21-cm signal. MACHOs of masses above 10^3 solar masses moving through the post-recombination baryonic fluid transfer kinetic energy via dynamical friction, raising the gas temperature and distorting the 21-cm signal from the Lambda CDM prediction. Imposing that the deviation Delta T_21 does not exceed 50 mK at z approximately 17 or 15 mK at z approximately 89, and that no emission signal appears at z greater than or equal to 300, yields upper bounds on the MACHO fraction f_M across 10^3 to 10^7 solar masses for both a single

What carries the argument

Dynamical friction drag exerted by non-accreting MACHOs on the surrounding baryonic fluid, which deposits kinetic energy as heat and thereby reduces the depth of the 21-cm absorption trough.

If this is right

The dark-ages bound at z approximately 89 is both tighter and free of astrophysical uncertainties associated with star formation.
Extended MACHO mass distributions produce generally more stringent constraints than monochromatic populations.
Critical-collapse mass functions yield the strongest limits at intermediate masses.
The conditions also forbid any 21-cm emission feature at redshifts greater than or equal to 300 for the allowed MACHO fractions.

Where Pith is reading between the lines

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

Future instruments that measure the 21-cm signal with smaller error bars at multiple high redshifts could translate directly into proportionally tighter MACHO fraction limits.
The method isolates the effect of non-accreting compact objects and therefore supplies a test that is orthogonal to accretion or lensing searches for the same objects.
If independent evidence later shows a significant MACHO fraction, the absence of the predicted heating would indicate either efficient cooling channels or a breakdown of the dynamical-friction calculation at those redshifts.

Load-bearing premise

The only cause of deviation in the global 21-cm signal from the Lambda CDM prediction is dynamical friction heating by non-accreting MACHOs, with no other astrophysical or cosmological effects contributing at the chosen deviation thresholds.

What would settle it

A measurement of the global 21-cm absorption amplitude at redshift approximately 89 that deviates from the standard prediction by more than 15 mK after all other heating sources have been independently ruled out would require the MACHO fraction to exceed the derived upper limit.

Figures

Figures reproduced from arXiv: 2604.17083 by Alekha C. Nayak, Vivekanand Mohapatra.

**Figure 2.** Figure 2: FIG. 2 [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗

**Figure 3.** Figure 3: FIG. 3: Presents the upper bounds on the fraction of [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗

read the original abstract

We derive a complementary cosmological upper bound on the fraction of dark matter residing inside massive compact halo objects (MACHOs) using the cosmic dawn and dark ages global 21-cm signal $(T_{21})$. MACHOs of masses $M\gtrsim 10^3~M_\odot$ moving through the post-recombination baryonic fluid transfer kinetic energy to the intergalactic medium via dynamical friction, raising the gas temperature and distorting the 21-cm signal from the $\Lambda$CDM prediction. We consider both a monochromatic and two extended MACHO mass distributions: log normal and critical collapse. Imposing the conditions that the deviation in the global 21-cm signal $\Delta T_{21}$ does not exceed $50~\rm mK$ at $z\sim 17$ or $15~\rm mK$ at $z\sim 89$, and that no emission signal appears at $z \gtrsim 300$, we derive upper bounds on the MACHO fraction $f_M$ across the mass range $10^3 \lesssim M_c/M_\odot \lesssim 10^7$. The dark ages criterion yields constraints that are both tighter and free from astrophysical uncertainties associated with star formation, providing a complementary cosmological window. Extended distributions produce bounds that are generally more stringent than their monochromatic counterpart, with the critical collapse models yielding the strongest constraints at intermediate masses.

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

This paper sets new upper limits on MACHO dark matter fraction from dynamical friction heating in the dark ages 21-cm signal, with stronger constraints for extended mass functions than monochromatic ones.

read the letter

The main takeaway is that the authors calculate how non-accreting MACHOs above 10^3 solar masses heat the intergalactic gas via dynamical friction and then translate that into upper bounds on the MACHO fraction f_M so the global 21-cm signal stays within 50 mK of the standard prediction at z~17 and 15 mK at z~89, plus no emission at z>300. They do this for monochromatic, log-normal, and critical-collapse distributions and find the extended cases often tighten the limits at intermediate masses. The dark-ages focus is the clean part: it avoids any modeling of stars or galaxies because the heating happens before first light. The heating rate follows the usual Chandrasekhar formula adapted to the expanding background, integrated over the mass function, which is standard and reproducible. That gives a genuinely independent cosmological window that can be compared later with microlensing or gravitational-wave searches. The soft spot is the deviation thresholds themselves. They are imposed by hand as conservative prospective cuts rather than coming from existing data or a self-consistent model of the 21-cm response. The bounds are therefore what future measurements could achieve, not what is already excluded. The assumption that dynamical friction is the only relevant heating source at those redshifts is reasonable in the dark ages but still needs explicit checks against other possible contributions. This work is aimed at people who track MACHO constraints or 21-cm cosmology. A reader who wants explicit numbers for different mass functions and a calculation free of star-formation uncertainties will get something concrete from it. The method is straightforward enough and the extension to extended distributions is new enough that it deserves a serious referee.

Referee Report

2 major / 2 minor

Summary. The manuscript claims to derive upper limits on the MACHO dark matter fraction f_M by computing the IGM heating from dynamical friction of non-accreting MACHOs with masses above 10^3 solar masses and mapping the resulting gas temperature increase to deviations in the global 21-cm signal. Specifically, it requires that Delta T_21 remains below 50 mK at z~17 and 15 mK at z~89, and that there is no emission at z>300, for both monochromatic and extended (log-normal, critical collapse) mass distributions. The dark ages bound is highlighted as being free from star-formation uncertainties.

Significance. This approach offers a new window on MACHOs using the dark ages 21-cm signal, which is less affected by astrophysical modeling than cosmic dawn signals. The consideration of different mass distributions adds robustness. If the dynamical friction heating calculation holds and the thresholds are accepted as prospective, it provides complementary constraints to existing bounds from microlensing and other probes.

major comments (2)

[Abstract] Abstract: The specific numerical thresholds for Delta T_21 (50 mK at z~17 and 15 mK at z~89) and the no-emission condition at z>300 are imposed without derivation from the underlying model or from existing data. These choices are load-bearing for the quantitative f_M bounds, and the manuscript should include an analysis of how the limits vary with different threshold values to demonstrate robustness.
[21-cm response derivation] The section deriving the 21-cm response: The mapping from the gas temperature rise (computed via the adapted Chandrasekhar dynamical friction formula) to Delta T_21 assumes that dynamical friction heating is the dominant or sole contributor at the level of the chosen thresholds. Potential contributions from other cosmological effects should be estimated to support this isolation.

minor comments (2)

[Mass distribution section] The notation and parameters for the extended mass distributions (log-normal and critical collapse) should be defined more explicitly, including any normalization or cutoff choices, to facilitate reproducibility of the f_M limits.
[Results figures] Any figures comparing bounds across mass distributions would benefit from including error bands or sensitivity to the heating rate assumptions.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive review and positive assessment of the novelty of using the dark ages 21-cm signal for MACHO constraints. We address each major comment below with specific revisions planned for the manuscript.

read point-by-point responses

Referee: [Abstract] Abstract: The specific numerical thresholds for Delta T_21 (50 mK at z~17 and 15 mK at z~89) and the no-emission condition at z>300 are imposed without derivation from the underlying model or from existing data. These choices are load-bearing for the quantitative f_M bounds, and the manuscript should include an analysis of how the limits vary with different threshold values to demonstrate robustness.

Authors: We agree that explicit justification and robustness tests for the thresholds are warranted, as they directly set the scale of the reported f_M bounds. These values are chosen as conservative, prospective limits informed by the anticipated precision of global 21-cm measurements (drawing from EDGES results at cosmic dawn and the expected performance of proposed dark-ages experiments). The no-emission condition at z>300 enforces consistency with the standard absorption-only prediction in the absence of additional heating. In the revised manuscript we will expand the abstract slightly for context and add a new subsection (or appendix) with a sensitivity analysis: we will recompute the f_M limits for threshold variations of factors 0.5, 2, and 5, demonstrating that the order-of-magnitude constraints remain stable across this range. A supplementary figure will illustrate the scaling. revision: yes
Referee: [21-cm response derivation] The section deriving the 21-cm response: The mapping from the gas temperature rise (computed via the adapted Chandrasekhar dynamical friction formula) to Delta T_21 assumes that dynamical friction heating is the dominant or sole contributor at the level of the chosen thresholds. Potential contributions from other cosmological effects should be estimated to support this isolation.

Authors: We appreciate the referee's emphasis on isolating the dynamical-friction contribution. In the dark-ages regime (z ≳ 300) the IGM is expected to be free of astrophysical heating sources, so the only relevant processes are adiabatic cooling and the relative baryon-dark-matter velocity; we will add a short paragraph quantifying that the latter contributes negligibly below our thresholds. At cosmic dawn (z ~ 17) we acknowledge that early star formation could in principle add heating, but our bounds are presented as upper limits on the MACHO fraction under the assumption that any observed deviation is attributable to MACHOs. To address the comment directly, the revised manuscript will include an order-of-magnitude estimate of competing effects (e.g., variations in the baryon-dark-matter streaming velocity and minimal X-ray pre-heating from Population III stars) showing they remain sub-dominant at the 15–50 mK level for the f_M values of interest. This discussion will be placed in the 21-cm response section. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation is first-principles plus external thresholds

full rationale

The paper computes IGM heating via the standard Chandrasekhar dynamical friction formula in an expanding background, integrates over monochromatic/log-normal/critical-collapse mass functions, solves the resulting T_gas(z) evolution, and maps the temperature rise to Delta T_21 using the usual 21-cm brightness-temperature expression. Upper limits on f_M are then obtained by imposing externally chosen conservative thresholds (Delta T_21 <= 50 mK at z~17, <=15 mK at z~89, no emission at z>300). These thresholds are not derived from the model itself but are imposed as prospective observational limits; the heating calculation contains no fitted parameters that are later renamed as predictions, no self-citations that carry the central claim, and no self-definitional loops. The result is therefore independent of its own inputs.

Axiom & Free-Parameter Ledger

3 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard LCDM cosmology, the dynamical-friction heating formula, and the chosen deviation thresholds; no new particles or forces are introduced.

free parameters (3)

50 mK deviation threshold at z~17
Hand-chosen maximum allowed deviation from LCDM 21-cm signal.
15 mK deviation threshold at z~89
Hand-chosen maximum allowed deviation from LCDM 21-cm signal.
No-emission condition at z>300
Ad-hoc requirement that the signal remains in absorption.

axioms (2)

standard math Standard Lambda-CDM expansion history and baryon temperature evolution after recombination
Invoked to compute the baseline T21 without MACHOs.
domain assumption Dynamical friction is the dominant energy-transfer mechanism between MACHOs and baryonic gas
Assumed for non-accreting MACHOs in the post-recombination era.

pith-pipeline@v0.9.0 · 5558 in / 1491 out tokens · 45756 ms · 2026-05-10T06:22:29.310898+00:00 · methodology

discussion (0)

Reference graph

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