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Dark Radiation Matter Decoupling (DRMD)

Updated 7 July 2026
  • DRMD is a class of dark-sector models where dark matter transitions from a tightly coupled phase with dark radiation to a free-streaming state, imprinting features like dark acoustic oscillations.
  • Various approaches—including perturbative decoupling, effective DM–DR drag, DM-to-DR conversion, and kinematic decay—offer frameworks to understand its impact on the cosmic microwave background and matter power spectrum.
  • Observational constraints, such as ΔNeff limits (<0.29) and DAO scales (~60 Mpc/h), provide measurable tests for DRMD scenarios and their potential to address the Hubble and S8 tensions.

Searching arXiv for the cited DRMD-related papers to ground the article in recent and relevant literature. arxiv_search.query({"17search_query17 OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17"," OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17,"17sort_by17 arxiv_search.query({"17search_query17 OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17"," arxiv_search.query({"17search_query17 OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17sort_order17"dark radiation matter decoupling\" OR 17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17sort_order17"dark acoustic oscillations\"","17max_results17 OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17,"17sort_by17 Dark Radiation Matter Decoupling (DRMD) denotes a family of dark-sector cosmologies in which the relation between a non-relativistic dark-matter component and a relativistic dark-radiation component changes with time. In the narrow sense, it refers to an interacting DM–DR subsystem that is tightly coupled in the early universe and later decouples, thereby imprinting dark acoustic oscillations, collisional damping, and scale-dependent changes in the CMB and matter power spectrum. In broader phenomenological usage, closely related constructions also include post-BBN conversion of dark matter into free-streaming dark radiation, entropy transfer from matter into radiation after sector decoupling, and late decays that create a daughter species that is radiation-like at production but matter-like after redshifting. Taken together, these realizations make DRMD a unifying label for time-dependent drag, transfer, or relativistic-to-nonrelativistic transition in the dark sector (&&&17max_results17&&&, &&&17search_query17&&&, &&&17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17&&&).

The modern literature does not reduce DRMD to a single microscopic mechanism. One branch treats the problem as a perturbative transition in the radiation sector itself: dark radiation can be free-streaming, fluid, decoupling, instantaneous decoupling, or recoupling, with the distinction encoded directly in the Boltzmann hierarchy rather than in a specific UV completion (&&&17sort_order17&&&). In that setting, the key issue is whether the DR multipoles above the dipole are collisionally damped or freely propagated, and whether the transition occurs gradually or abruptly.

A second branch studies true DM–DR drag. In these models only a subcomponent of dark matter is interacting, while the dominant remainder stays collisionless. The interacting subcomponent and the dark radiation behave as a pressure-supported dark fluid before decoupling, and the rate and sharpness of the transition determine the amplitude and phase structure of dark acoustic oscillations (&&&17max_results17&&&). A closely related ETHOS formulation parameterizes the DR–DM scattering rate directly by a temperature power law and uses that effective drag history to classify the decoupling phenomenology (&&&17submittedDate17&&&).

A third branch is not a literal scattering-decoupling calculation but an effective transfer model in which cold dark matter is converted into a non-interacting relativistic component. In this usage, DRMD is better read as a post-decoupling DMPRESERVED_PLACEHOLDER_17search_query17DR conversion history than as a coupled-fluid kinetic decoupling problem. Both the general post-BBN conversion model of Bringmann et al. and the late-universe DMDR model of Pandey et al. fall into this category (&&&17search_query17&&&, &&&17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17&&&).

A fourth branch replaces interaction-driven decoupling by kinematics. In late-decay models, a heavy parent freezes out, decays after neutrino decoupling, and injects a stable daughter that behaves either as dark radiation or as warm dark matter depending on the mass ratio and lifetime. This is a radiation-to-matter transition caused by cosmological redshifting rather than by a scattering rate crossing the Hubble scale (&&&17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17&&&).

17max_results17. Effective transfer formalisms

The most conservative transfer description begins by relaxing the assumption that the cold-dark-matter density is covariantly conserved. Bringmann et al. parameterize the DM background as

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17^

where PRESERVED_PLACEHOLDER_17max_results17^ controls the total reduction in comoving DM density, PRESERVED_PLACEHOLDER_17sort_by17^ sets the transition epoch, and PRESERVED_PLACEHOLDER_17submittedDate17^ controls the transition width. In this parameterization the comoving DM density decreases overall by a factor PRESERVED_PLACEHOLDER_17sort_order17; PRESERVED_PLACEHOLDER_17descending17^ approximately reproduces decaying-DM-like behavior, while PRESERVED_PLACEHOLDER_17search_query17^ maps well to Sommerfeld-enhanced annihilation (&&&17search_query17&&&).

Energy conservation is imposed through

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018)17^

with the produced radiation taken to be non-interacting and free-streaming, “as for an additional neutrino species.” The resulting effective neutrino equivalent,

PRESERVED_PLACEHOLDER_17max_results17^

is generically time dependent rather than constant, which is one of the central differences between DMPRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17DR conversion and the usual constant-PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17^ parameterization (&&&17search_query17&&&).

Pandey et al. adopt the same general logic for a late-universe DMDR model, but reduce the background freedom to two parameters by imposing PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17max_results17, so that the conversion accelerates toward the present epoch. Their dark-matter density is

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17sort_by17^

Here PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17submittedDate17^ is “the total amount of dark matter that has already converted into dark radiation, divided by the amount of dark matter at the current time,” while PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17sort_order17^ characterizes the conversion rate. As in the Bringmann et al. framework, the source term is defined by

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17descending17^

and the perturbative prescription is fixed by the minimal choice PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17^ (&&&17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17&&&).

These effective-transfer descriptions share a specific physical assumption: the created DR is a separate dark relativistic component, not an extra massless neutrino species sharing the neutrino temperature or entropy bath. That distinction matters once massive neutrinos are included and once one asks whether the product species is free-streaming, fluid-like, or still coupled to a dark bath (&&&17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17&&&).

17sort_by17. Interaction-driven decoupling and dark acoustic oscillations

In true DRMD models, the primary object is not PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018)17^ but a momentum-exchange rate. A representative formulation splits the dark matter into a dominant non-interacting sector and an interacting subcomponent of fractional abundance

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17max_results17^

The interacting DM and dark radiation obey coupled Euler equations with drag terms,

PRESERVED_PLACEHOLDER_17max_results17search_query17^

PRESERVED_PLACEHOLDER_17max_results17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17^

with

PRESERVED_PLACEHOLDER_17max_results17max_results17^

The drag history is parameterized by

PRESERVED_PLACEHOLDER_17max_results17sort_by17^

with benchmark cases PRESERVED_PLACEHOLDER_17max_results17submittedDate17, PRESERVED_PLACEHOLDER_17max_results17sort_order17, and an PRESERVED_PLACEHOLDER_17max_results17descending17^ case supplemented by an exponential shutoff, PRESERVED_PLACEHOLDER_17max_results17search_query17. The preferred decoupling epoch is close to matter-radiation equality, PRESERVED_PLACEHOLDER_17max_results17id:(Bringmann et al., 2018)17^ (&&&17max_results17&&&).

When PRESERVED_PLACEHOLDER_17max_results17max_results17, the interacting DM and DR behave as a single tightly coupled fluid. Pressure support from the DR component suppresses the growth of iDM perturbations and generates dark acoustic oscillations. The sharpness of the DAO pattern is controlled by the sharpness of decoupling: fast decoupling freezes in a sharply defined oscillation phase, while slow decoupling smears the feature (&&&17max_results17&&&). In the ETHOS language, the relevant drag amplitude is organized by PRESERVED_PLACEHOLDER_17sort_by17search_query17, where PRESERVED_PLACEHOLDER_17sort_by17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17, and the comoving interaction rate is written as

PRESERVED_PLACEHOLDER_17sort_by17max_results17^

with

PRESERVED_PLACEHOLDER_17sort_by17sort_by17^

For PRESERVED_PLACEHOLDER_17sort_by17submittedDate17^ and PRESERVED_PLACEHOLDER_17sort_by17sort_order17, this yields sharp small-scale suppression plus DAO; for PRESERVED_PLACEHOLDER_17sort_by17descending17, the suppression is smoother because the proper-time momentum-transfer rate scales as PRESERVED_PLACEHOLDER_17sort_by17search_query17^ and remains comparatively important near recombination (&&&17submittedDate17&&&).

A related effective description focuses on the dark-radiation perturbation sector rather than on explicit DM drag. In that language, the collision term damps the quadrupole and higher moments,

PRESERVED_PLACEHOLDER_17sort_by17id:(Bringmann et al., 2018)17^

PRESERVED_PLACEHOLDER_17sort_by17max_results17^

This makes the fluid-to-free-streaming transition explicit and supports free-streaming DR, fluid DR, decoupling DR, instantaneous decoupling DR, and recoupling DR within a common relaxation-time formalism (&&&17sort_order17&&&).

The acoustic scale associated with DRMD is the dark drag horizon,

PRESERVED_PLACEHOLDER_17submittedDate17search_query17^

where the tightly coupled dark fluid has effective sound speed PRESERVED_PLACEHOLDER_17submittedDate17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17. Using only Planck and SH17search_query17ES-calibrated supernovae, one finds evidence for a DAO signal with

PRESERVED_PLACEHOLDER_17submittedDate17max_results17^

at PRESERVED_PLACEHOLDER_17submittedDate17sort_by17^ posterior interval, with the DAO scale naturally near half the BAO scale because the preferred decoupling occurs around equality rather than baryon drag (&&&17sort_by17&&&).

17submittedDate17. Microphysical realizations

Several explicit particle models realize DRMD or closely adjacent phenomena. One realization embeds late kinetic decoupling inside a Dirac-fermion dark sector with a light vector mediator,

PRESERVED_PLACEHOLDER_17submittedDate17submittedDate17^

After kinetic decoupling, the DM velocity redshifts as PRESERVED_PLACEHOLDER_17submittedDate17sort_order17^ instead of PRESERVED_PLACEHOLDER_17submittedDate17descending17, so near a Sommerfeld resonance with PRESERVED_PLACEHOLDER_17submittedDate17search_query17, the annihilation rate per particle becomes increasingly important at late times and produces a second annihilation epoch long after chemical freeze-out. In that case, late kinetic decoupling is not itself the conversion; it is the mechanism that enables delayed annihilation into dark radiation (&&&17search_query17&&&).

A more direct kinetic-decoupling model uses a global PRESERVED_PLACEHOLDER_17submittedDate17id:(Bringmann et al., 2018)17^ dark sector with a pseudo-Dirac fermion PRESERVED_PLACEHOLDER_17submittedDate17max_results17^ as DM and a massless Goldstone boson PRESERVED_PLACEHOLDER_17sort_order17search_query17^ as DR. The DM–DR scattering cross section is

PRESERVED_PLACEHOLDER_17sort_order17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17^

which leads to the kinetic-decoupling condition

PRESERVED_PLACEHOLDER_17sort_order17max_results17^

and to the estimate

PRESERVED_PLACEHOLDER_17sort_order17sort_by17^

The associated acoustic cutoff scale is

PRESERVED_PLACEHOLDER_17sort_order17submittedDate17^

and the preferred DM mass range is PRESERVED_PLACEHOLDER_17sort_order17sort_order17^ (&&&17descending17&&&).

A non-Abelian realization starts from a hidden PRESERVED_PLACEHOLDER_17sort_order17descending17^ gauge theory spontaneously broken to a residual PRESERVED_PLACEHOLDER_17sort_order17search_query17. The broken generators become massive vector dark matter, while the unbroken PRESERVED_PLACEHOLDER_17sort_order17id:(Bringmann et al., 2018)17^ gauge bosons remain massless and constitute dark radiation. Because the residual symmetry is non-Abelian, the DM–DR scattering scales as

PRESERVED_PLACEHOLDER_17sort_order17max_results17^

rather than the Thomson-like PRESERVED_PLACEHOLDER_17descending17search_query17^ behavior of an Abelian residual subgroup. This temperature dependence makes the drag cosmologically relevant deep into the radiation era and numerically suppresses PRESERVED_PLACEHOLDER_17descending17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17^ relative to PRESERVED_PLACEHOLDER_17descending17max_results17CDM (&&&17search_query17&&&).

Another class of constructions modifies the thermal initial conditions rather than the late-time interaction law. In asymmetric-reheating models, a long-lived modulus reheats the visible and dark sectors unequally, so the dark radiation bath is cold from the outset. Achieving PRESERVED_PLACEHOLDER_17descending17sort_by17^ requires

PRESERVED_PLACEHOLDER_17descending17submittedDate17^

which permits an interacting DM–DR sector while evading the overproduction of dark radiation that would follow from an earlier thermalized visible–dark history (&&&17id:(Bringmann et al., 2018)17&&&).

17sort_order17. Observational constraints and tension relief

The conversion class is already tightly bounded. Bringmann et al. showed that CMB data strongly constrain any significant DMPRESERVED_PLACEHOLDER_17descending17sort_order17DR conversion after BBN. For very early transitions, the model reduces observationally to a nearly constant-PRESERVED_PLACEHOLDER_17descending17descending17^ cosmology, with a frequentist bound roughly

PRESERVED_PLACEHOLDER_17descending17search_query17^

for PRESERVED_PLACEHOLDER_17descending17id:(Bringmann et al., 2018)17^ and PRESERVED_PLACEHOLDER_17descending17max_results17. For late conversion, CMB-only data require that the DM density not be reduced by more than a few percent after matter-radiation equality, while adding low-redshift data can allow up to around PRESERVED_PLACEHOLDER_17search_query17search_query17^ percent of DM to be converted during matter domination and gives only a mild frequentist preference, around PRESERVED_PLACEHOLDER_17search_query17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17, for late conversion (&&&17search_query17&&&).

The later DMDR analysis of DES-Y17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17, Planck-17max_results17search_query17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018)17, Pantheon, and BAO found no evidence for nonzero conversion. The converted fraction PRESERVED_PLACEHOLDER_17search_query17max_results17^ is constrained at PRESERVED_PLACEHOLDER_17search_query17sort_by17^ confidence level to be

PRESERVED_PLACEHOLDER_17search_query17submittedDate17^

PRESERVED_PLACEHOLDER_17search_query17sort_order17^

PRESERVED_PLACEHOLDER_17search_query17descending17^

The tension in PRESERVED_PLACEHOLDER_17search_query17search_query17^ is only slightly reduced, from PRESERVED_PLACEHOLDER_17search_query17id:(Bringmann et al., 2018)17^ to PRESERVED_PLACEHOLDER_17search_query17max_results17, the Hubble tension remains PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018)17search_query17, and the evidence ratio disfavors the DMDR extension relative to PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018)17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17CDM for the external and combined datasets (&&&17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17&&&).

For true DM–DR drag, the strongest scale-dependent limits come from combining CMB, BAO, and high-resolution Lyman-PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018)17max_results17^ data. In the ETHOS parameterization, robust PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018)17sort_by17^ limits are

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018)17submittedDate17^

for PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018)17sort_order17,

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018)17descending17^

for PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018)17search_query17, and

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018)17id:(Bringmann et al., 2018)17^

for PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018)17max_results17, with the caveat that for PRESERVED_PLACEHOLDER_17max_results17search_query17^ the effective-rate bound is largely CMB-driven and the Lyman-PRESERVED_PLACEHOLDER_17max_results17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17^ applicability is more limited (&&&17submittedDate17&&&). These bounds still leave room, especially for PRESERVED_PLACEHOLDER_17max_results17max_results17, for a cutoff in the halo mass function relevant to the missing-satellite problem.

The most aggressive Hubble-tension alleviation presently occurs in models with a small interacting DM fraction and decoupling near equality. Without ACT DR17descending17^ data, all three benchmark decoupling histories studied in that framework reduce the Hubble tension relative to PRESERVED_PLACEHOLDER_17max_results17sort_by17CDM, with the abrupt PRESERVED_PLACEHOLDER_17max_results17submittedDate17^ case performing best; with SH17search_query17ES included, that case reaches

PRESERVED_PLACEHOLDER_17max_results17sort_order17^

PRESERVED_PLACEHOLDER_17max_results17descending17^

PRESERVED_PLACEHOLDER_17max_results17search_query17^

and PRESERVED_PLACEHOLDER_17max_results17id:(Bringmann et al., 2018)17. Once ACT DR17descending17^ is added, however, the fit is significantly worsened, and the largest PRESERVED_PLACEHOLDER_17max_results17max_results17^ value at PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17search_query17^ confidence drops to PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17^ km/s/Mpc without SH17search_query17ES and rises only to PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17max_results17^ km/s/Mpc with SH17search_query17ES (&&&17max_results17&&&).

A complementary effective analysis that infers DAO from Planck plus SH17search_query17ES-calibrated supernovae finds

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17sort_by17^

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17submittedDate17^

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17sort_order17^

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17descending17^

and a residual Hubble tension of PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17search_query17, while simultaneously predicting a DAO feature at PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17id:(Bringmann et al., 2018)17^ with few-percent amplitude (&&&17sort_by17&&&).

A recurrent misconception is that every DRMD paper computes a literal DM–DR scattering-decoupling epoch. Several influential works do not. Light thermal relics with PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17max_results17^ MeV that freeze out after neutrino decoupling modify PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17^ and hence PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17, but the relevant event is chemical freeze-out plus entropy transfer between already separated radiation baths. In that case, the non-equilibrium treatment strengthens both the increase of PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17max_results17^ for neutrino heating and the decrease of PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17sort_by17^ for electromagnetic heating relative to the equilibrium approximation (&&&17max_results17&&&).

Likewise, the long-lived-decay scenario PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17submittedDate17^ realizes DRMD only in a kinematic sense. If

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17sort_order17^

the daughter PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17descending17^ stays relativistic to recombination and contributes to PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17. If instead

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018)17^

the daughter becomes non-relativistic early enough to be viable warm dark matter. The warm-DM branch predicts only

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17max_results17^

so the model yields either observable dark radiation or warm dark matter, but not both simultaneously at a significant level (&&&17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17search_query17&&&).

The background expansion history can also alter any DRMD sector without changing its microphysics. Freeze-out during an early matter-dominated era changes the Hubble scaling from PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17max_results17search_query17^ to PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17max_results17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17, and late decay of the matter-dominating species dilutes the relic according to

PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17max_results17max_results17^

This is directly relevant whenever a DRMD sector is embedded in a nonstandard pre-BBN background, because the decoupling or freeze-out calculation then depends on the bath temperature relevant to the dark sector rather than on the visible temperature alone (&&&17submittedDate17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17&&&).

Finally, a systematic analysis of interacting dark radiation alone shows that present CMB+BAO data do not yet impose statistically significant constraints on the DR couplings once one allows free-streaming, fluid, decoupling, instantaneous decoupling, and recoupling cases in a unified relaxation-time framework. There is, however, a slight preference for the fluid-like limit, and in the instantaneous-decoupling case that limit corresponds to a late transition redshift around recombination (&&&17sort_order17&&&). This result isolates an important limiting case of DRMD: even without explicit DM drag, a transition between fluid-like and free-streaming radiation can already leave observable, and still currently allowed, signatures in the perturbation sector.

In that broader sense, DRMD is best understood not as a single model but as a structured class of dark-sector histories. Its central variables are the drag or transfer rate, the dark-radiation abundance, the fraction of dark matter participating in the interaction, the sharpness and timing of decoupling or conversion, and the thermodynamic relation between visible and dark sectors. The distinguishing empirical consequences are a time-dependent PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17max_results17sort_by17-like radiation density, altered CMB acoustic structure, suppression or oscillation of the matter power spectrum, modified small-scale cutoff masses, and, in the most optimistic realizations, partial relief of the PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17max_results17submittedDate17^ and PRESERVED_PLACEHOLDER_17id:(Bringmann et al., 2018) OR id:(Chen et al., 2020) OR id:(Buen-Abad et al., 20 Nov 2025) OR id:(Garny et al., 27 Feb 2026) OR id:(Archidiacono et al., 2019) OR id:(Brinckmann et al., 2022) OR id:(Chu et al., 2014) OR id:(Ko et al., 2016) OR id:(Reece et al., 2015) OR id:(Heo et al., 2015) OR id:(Bari et al., 2013)17max_results17sort_order17^ tensions (&&&17submittedDate17&&&, &&&17max_results17&&&, &&&17sort_by17&&&).

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