🔭 astrophysics

Is dark matter decaying ?

This paper argues that while dark matter decay could be detected by comparing matter density between the recombination era and the local Universe, current systematic uncertainties in local baryonic and dark matter inventories prevent a definitive conclusion, though next-generation observational technologies are expected to resolve these gaps and enable a conclusive test.

Original authors: Jeremy Mould

Published 2026-06-19

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Jeremy Mould

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Question: Is the Universe Losing Weight?

Imagine the Universe as a giant, invisible bank account. For a long time, astronomers have known exactly how much "matter" (stuff like stars, gas, and invisible dark matter) was in this account when the Universe was a baby, about 13.8 billion years ago. We know this because we can look at the "baby photos" of the Universe—the Cosmic Microwave Background (CMB)—which act like a precise receipt showing the initial deposit was roughly 31.5% of the total energy budget of the cosmos.

This paper asks a simple but tricky question: Has that account balance changed? specifically, has the amount of matter decreased as the Universe aged to its current state (today)?

If dark matter is unstable, it might be slowly "decaying" or turning into other particles (like invisible ghosts or light) over time. If this is happening, the Universe today should have less matter than it did back then.

The Detective Work: Counting the Stuff Today

To answer this, the author tries to do a "current audit" of the Universe. He looks at nearby galaxies and tries to weigh everything:

Visible stuff: Stars and gas.
Invisible stuff: Dark matter halos surrounding galaxies.
The "Lost" stuff: Gas floating between galaxies (the Intergalactic Medium).

The Problem: It's like trying to weigh a messy room where some items are hidden under piles of clothes, some are floating in the air, and some are just hard to see.

The author finds that current measurements of local matter are messy and uncertain.
When he adds up the stars, the gas, and the estimated dark matter, he gets a total of about 22.8%.
This is lower than the 31.5% we saw in the early Universe.

The Analogy: Imagine you baked a cake and weighed it perfectly before putting it in the oven (the early Universe). Now, you take it out, and it weighs less. You suspect it might be evaporating. But, you also realize you might have missed some crumbs on the floor or forgotten to weigh the steam rising from the cake. Because of these "missing crumbs," we can't be 100% sure the cake actually shrank yet.

The Suspects: How Could Dark Matter Decay?

The paper explores three main ways dark matter might be disappearing:

The "Leaky Bucket" (Neutrinos): Imagine a heavy, invisible particle that slowly breaks apart into a neutrino (a tiny, ghostly particle) and something else invisible. This would reduce the amount of "clumping" dark matter we see today.
The "Tiny Black Holes" (Primordial Black Holes): Imagine the dark matter is made of billions of tiny black holes. These black holes slowly evaporate (like ice cubes in a warm room). The paper checks if this evaporation would have messed up the formation of the first stars. The math suggests that if they were too small, they would have evaporated too fast and ruined the early Universe. If they were big enough to survive, they haven't lost much mass yet.
The "Energy Dump" (Particle Decay): Imagine a dark matter particle simply vanishing and turning into pure energy (light or radiation). This is tricky because if it turns into energy, it usually heats up the gas in the Universe, which we can detect. The paper notes that we don't see enough of this heating to prove this is happening, unless the particles have a very specific way of hiding their energy.

The Evidence from Supernovae

The author also looks at exploding stars (supernovae) to see if the expansion of the Universe matches the idea of losing matter.

Standard View: The data fits a model where matter stays constant (about 33%).
Alternative View: The data could also fit a model where matter is much lower (about 10%), but this requires the Universe to be shaped differently.
The Verdict: The data is "equivocal." It's not clear enough to say "Yes, matter is decaying" or "No, it isn't." The uncertainties in our measurements are still too big to make a final call.

The Future: Better Scales for a Messier Room

The paper concludes that while the idea of decaying dark matter is interesting, we currently lack the tools to prove it. Our "scales" (telescopes and surveys) aren't precise enough to distinguish between:

"The dark matter is actually decaying."
"We just haven't found all the hidden gas and dark matter yet."

The Hope: The author is optimistic about the next decade. New tools like the Square Kilometre Array (a massive radio telescope), FRB maps (using cosmic radio bursts to count gas), and the Vera Rubin Observatory will act like a high-resolution scanner. They will help us find those "missing crumbs" and weigh the Universe with much greater precision.

Summary

The paper is a careful check of the math. It says: "We see a gap between the matter we saw in the baby Universe and the matter we see today. It could be because dark matter is decaying, but it could also just be because we haven't finished counting everything in the local Universe yet. We need better telescopes to solve the mystery."

Technical Summary: Is Dark Matter Decaying?

Problem Statement
The central problem addressed is the potential discrepancy in the matter density parameter ( $\Omega_m$ ) between the recombination era ( $z \approx 1000$ ) and the local Universe ( $z = 0$ ). While Cosmic Microwave Background (CMB) measurements (e.g., Planck) provide a precise determination of the initial matter budget ( $\Omega_m \approx 0.315$ ), local measurements at $z=0$ are less consistent and often yield lower values. If $\Omega_m$ has decreased over cosmic time, it would serve as an enduring signature of unstable dark matter constituents decaying into other particles. However, current local inventories of baryonic and dark matter are plagued by systematic uncertainties, particularly regarding the warm-hot intergalactic medium (WHIM), diffuse intra-cluster media, and the density profiles of low-mass dark matter halos.

Methodology
The paper employs a comparative audit of matter density estimates from two distinct epochs:

Local Universe ( $z \approx 0$ ) Inventory: The author constructs a local matter census using the Ohlson et al. (2024) catalog of nearby galaxies, extending out to a fiducial volume of 20.5 Mpc (based on $H_0 = 73$ $H_{0} = 73$ km s $^{-1}$ $^{- 1}$ Mpc $^{-1}$ $^{- 1}$ ).
- Mass Estimation: Total masses are derived from 21 cm velocity widths ( $w_{20}$ ), calibrated against stellar mass using the Behroozi et al. (2019) mass-to-light relation.
- Baryon Accounting: The analysis incorporates intergalactic medium (IGM) gas fractions derived from Fast Radio Burst (FRB) dispersion measures (Connor et al., 2025), assuming a pristine ratio of dark matter to baryons in the IGM, while acknowledging feedback effects from star formation and active galactic nuclei.
- Cluster Corrections: Masses for the Virgo and Fornax clusters are added separately to account for contents exceeding the sum of individual cataloged galaxies.
- Alternative Samples: Results are cross-referenced with the GAMA DR4 survey and stellar mass functions to validate the local density estimates.
Theoretical Decay Models: The paper evaluates three specific decay scenarios to see if they align with a potential reduction in $\Omega_m$ $Ω_{m}$ :
- Neutrino Decay Channels: Models where heavy dark matter particles ( $\chi$ ) decay into neutrinos and light invisible fermions ( $\chi \to \nu + \phi$ ), reducing clustering DM at late times.
- Primordial Black Holes (PBHs): Analysis of mass loss via Hawking radiation ( $dm/dt \propto -m^{-2}$ ) for PBHs in the asteroid mass window. The paper calculates ionization rates against recombination rates to constrain PBH lifetimes.
- Electromagnetic Decay: Models involving finite lifetimes where particles decay into electromagnetic channels, constrained by CMB anisotropies and the requirement that ionization rates do not exceed recombination rates at $z > 5$ .
Supernova Cosmology: The behavior of $\Omega_m$ over the last 5 Gyr is tested using Dark Energy Survey (DES) supernova data, comparing standard $\Lambda$ CDM fits against models with reduced matter density.

Key Results

Local Density Estimates: The local audit yields a total $\Omega_m \approx 0.228^{+0.072}_{-0.040}$ (Table 1, Row 4). This is lower than the Planck CMB value of $0.315 \pm 0.007$ , suggesting a potential difference ( $\delta\Omega_m$ ) of approximately $-0.1$, though the error bars allow for zero difference.
Systematic Uncertainties: The local measurement is highly sensitive to the treatment of the IGM and the "missing" baryons. Even with corrections for IGM gas and associated dark matter, the local value remains lower than the CMB value, but the uncertainties in accounting for the warm-hot intergalactic medium and low-mass halos prevent a definitive conclusion.
Decay Constraints:
- PBHs: For PBHs with masses $m_{PBH} < 10^{-13} M_\odot$ , the ionization rate from Hawking radiation would disrupt the recombination history (Lyman $\alpha$ clouds at $z \sim 5$ ), effectively ruling out these masses as the sole dark matter component. For $m_{PBH} > 10^{-12} M_\odot$ , mass loss over the age of the Universe is insignificant.
- Particle Decay: Models involving decay into electromagnetic channels are heavily constrained by the requirement that the global free electron fraction remains consistent with CMB observations. The lifetime of such particles must exceed proton lifetime estimates.
- Two-Body Decay: A model where cold dark matter decays into two identical products (conserving energy via kinetic energy) is noted as a possibility that evades ionization constraints, motivated by DESI measurements suggesting a $\sim 5\%$ lower late-time matter density.
Supernova Data: While a standard fit yields $\Omega_m = 0.330 \pm 0.015$ , the data can also accommodate a lower density model ( $\Omega_m = 0.1$ ) within systematic uncertainties, though this requires an open universe geometry.

Significance and Claims
The paper concludes that the evidence for a decrease in $\Omega_m$ since the CMB era is equivocal. Neither the current catalogs of nearby galaxies nor supernova standard candles provide a "clinching" argument against dark matter decay, primarily because the "lost mass" cannot be definitively accounted for in a particle or energy dump.

The author asserts that the current inability to pin down a definitive verdict is due to systematic uncertainties in local baryon and mass mapping rather than a confirmed detection of decay. The paper posits that the next generation of observational technology is required to resolve these gaps. Specifically, the Square Kilometre Array (SKA) for 21 cm mass measurements, all-sky FRB maps for IGM density, and the Vera Rubin Observatory for microlensing detection of dark matter (specifically PBHs) are identified as the necessary tools to transform the "late-time matter audit" into a cleaner, more definitive test of dark matter stability. Until these measurements are realized, the hypothesis of late-time dark matter decay remains plausible but unproven.