⚛️ high-energy theory

Hot New Early Dark Energy: Dark Radiation Matter Decoupling

This paper proposes a microscopic model of the dark sector based on Hot New Early Dark Energy (Hot NEDE) and spontaneous symmetry breaking, which introduces a dark radiation matter decoupling mechanism to resolve the Hubble tension by aligning cosmological data with the SH0ES $H_0$ determination at a 1.4 $\sigma$ level.

Original authors: Mathias Garny, Florian Niedermann, Henrique Rubira, Martin S. Sloth

Published 2026-09-11

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CC BY 4.0

Original authors: Mathias Garny, Florian Niedermann, Henrique Rubira, Martin S. Sloth

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

For decades, cosmologists have relied on a single, elegant story to explain the universe: the Big Bang, followed by a slow expansion driven by dark energy and dark matter. This story, known as the standard model of cosmology, has been incredibly successful at predicting how the universe looked in its infancy. However, a troubling crack has appeared in the foundation. When scientists measure how fast the universe is expanding today using nearby stars and exploding stars, they get one number. When they look back at the ancient light of the Big Bang and calculate what that expansion rate should be based on the standard model, they get a different, slower number. This mismatch, known as the Hubble tension, has grown so large that it suggests our standard story is missing a crucial piece of the puzzle. The universe, it seems, is expanding faster than our current best theories allow.

To solve this, a team of physicists has proposed a new chapter in the history of the cosmos, one that involves a hidden sector of the universe we cannot see. They suggest that the dark sector—the mysterious realm containing dark matter and dark energy—underwent a dramatic transformation long ago. In this new scenario, the dark sector was once a hot, chaotic soup of particles that suddenly cooled and changed its nature, releasing a burst of energy that altered the universe's expansion history just enough to resolve the conflict between the old and new measurements.

The researchers, led by Mathias Garny and colleagues, built their idea on a microscopic model of this hidden sector, treating it with the same fundamental rules that govern the visible world, such as symmetry and the breaking of that symmetry. They imagined a dark force, similar to the forces that hold atoms together, which was once unbroken and uniform. As the universe cooled, this force underwent a phase transition, much like water freezing into ice, but in this case, the "ice" was a new state of dark matter and radiation. This transition was not a gentle shift but a violent, supercooled event that released a massive amount of latent heat. This heat created a thermal bath of "dark radiation," a fluid of particles that interacted with each other but not with ordinary light.

Crucially, this dark radiation did not just sit there. It was mixed with a specific type of dark matter that interacted with it, creating a tightly coupled fluid that moved together. For a long time, this mixture behaved as a single unit, dragging the dark matter along with the dark radiation. However, the researchers discovered a mechanism that would eventually break this bond. As the universe continued to cool, the dark matter particles split into two types: a heavier, charged variety and a lighter, neutral variety. The heavier particles, which were the ones interacting with the dark radiation, became increasingly rare as the temperature dropped, eventually disappearing from the mix. Once they were gone, the remaining dark matter, now mostly neutral, stopped interacting with the dark radiation. This event, which the authors call "dark radiation matter decoupling," meant the dark radiation was free to move independently, while the dark matter settled into the quiet, non-interacting state we observe today.

This sequence of events is the key to solving the Hubble tension. The initial burst of energy from the phase transition added extra energy to the early universe, which shrank the distance sound waves could travel before the universe became transparent. This smaller distance allows the standard model to fit the ancient light data while predicting a faster expansion rate today, matching the local measurements. Furthermore, the decoupling event ensured that this extra energy did not disrupt the delicate balance of the cosmic microwave background or the formation of large-scale structures. The model predicts that this decoupling happened at a specific time, roughly around the era when matter and radiation were equally abundant, a timing that fits perfectly with current observations.

When the team tested this model against the most precise data available, including measurements of the cosmic microwave background from the Planck satellite, the distribution of galaxies from the DESI survey, and the brightness of distant supernovae, the results were striking. The standard model, when forced to fit all this data, leaves a massive gap of more than five standard deviations between the predicted and observed expansion rates. The new model, however, bridges this gap, bringing the two measurements into agreement with a statistical difference of only 1.4 standard deviations, a level of consistency that suggests the tension is resolved. The model achieves this by introducing just a few new parameters: the amount of extra dark radiation, the fraction of dark matter that interacts with it, and the timing of the decoupling.

The researchers also compared their idea to other proposed solutions, such as models where dark radiation simply exists without ever interacting with dark matter. They found that those simpler models fail to resolve the tension completely, leaving a significant gap between the data sets. The unique feature of their proposal—the temporary interaction followed by a clean break—was essential for making the math work without contradicting other observations. The model also predicts that the dark matter involved in this process is a small fraction of the total dark matter, with the rest behaving as standard, non-interacting cold dark matter.

While the model is a strong candidate for solving the puzzle, the authors remain cautious. They note that their findings rely on specific assumptions about the early universe and that future, more precise data from telescopes like the Atacama Cosmology Telescope could test the model further. The model predicts subtle ripples in the distribution of matter on small scales and a faint signal of gravitational waves from the violent phase transition, which could be detected by future observatories. For now, the work offers a compelling, physically grounded explanation for why the universe is expanding faster than expected, suggesting that a hidden, dynamic history of dark matter and radiation is the missing link in our cosmic story.

Technical Summary: Hot New Early Dark Energy: Dark Radiation Matter Decoupling

Problem Statement
The paper addresses the "Hubble tension," a statistically significant discrepancy between the local measurement of the Hubble constant ( $H_0$ ) by the SH0ES collaboration ( $H_0 \approx 73.04$ km/s/Mpc) and the value inferred from Cosmic Microwave Background (CMB) data under the standard $\Lambda$ CDM model ( $H_0 \approx 67.36$ km/s/Mpc). The tension has reached a significance of $>5\sigma$ when combining Planck 2018 CMB data with local SH0ES measurements. Previous solutions, such as Early Dark Energy (EDE) or Self-Interacting Dark Radiation (SIDR), often fail to simultaneously satisfy CMB, Baryon Acoustic Oscillation (BAO), and Big Bang Nucleosynthesis (BBN) constraints. Specifically, SIDR models struggle to resolve the tension without violating BBN bounds on the effective number of relativistic species ( $N_{\text{eff}}$ ) or creating excessive tension with CMB polarization data.

Methodology and Microscopic Model
The authors propose a microscopic realization of the "Hot New Early Dark Energy" (Hot NEDE) scenario, grounded in standard particle physics principles: a dark $SU(N)$ gauge symmetry spontaneously broken to $SU(N-1)$ via the Higgs mechanism. The model introduces two primary components to the dark sector:

Dark Radiation (DR): Generated by a supercooled first-order phase transition occurring after BBN but before recombination. The latent heat released reheats the dark sector, creating a thermal bath of massless $SU(N-1)$ gauge bosons and a dark Higgs. Non-Abelian self-interactions ensure this radiation behaves as a fluid rather than free-streaming particles.
Interacting Dark Matter (IDM): A massive Dirac fermion multiplet $\chi$ charged under the $SU(N)$ gauge group. Before symmetry breaking, all components are degenerate. After the phase transition, radiative corrections induce a small mass splitting ( $\Delta m$ ) between the neutral component ( $\chi_0$ ) and the charged components ( $\chi_j$ ).

Key Mechanism: Dark Radiation Matter Decoupling (DRMD)
The central innovation of the paper is the "Dark Radiation Matter Decoupling" (DRMD) mechanism.

Tight Coupling: Initially, the charged components $\chi_j$ interact strongly with the dark radiation via gauge boson exchange, creating a drag force that tightly couples the dark matter and dark radiation fluids.
Decoupling: As the dark sector cools, the temperature drops below the mass splitting $\Delta m$ . The heavier charged states $\chi_j$ become Boltzmann-suppressed, leaving the neutral, non-interacting $\chi_0$ as the dominant dark matter component. Consequently, the drag force between dark matter and dark radiation shuts off exponentially.
Distinct Feature: Unlike standard recombination where photons free-stream after decoupling, the dark radiation in this model remains a fluid due to persistent non-Abelian self-interactions. This allows the model to increase $H_0$ (by reducing the sound horizon) while maintaining consistency with CMB anisotropies that would otherwise be spoiled by free-streaming radiation.

Evolution of Perturbations
The authors implement the perturbation evolution of this model into a modified version of the Boltzmann solver CLASS (dubbed DRMD-CLASS). They track the transition from a tightly coupled fluid to a decoupled state. The decoupling redshift ( $z_{\text{dec}}$ ) is a critical parameter; the model predicts that decoupling must occur around matter-radiation equality ( $z_{\text{eq}} \approx 3400$ ) to effectively suppress the growth of the gravitational potential on small scales, thereby compensating for the increased matter density ( $\omega_m$ ) required to fit the CMB peaks with a higher $H_0$ .

Results
The authors perform a Bayesian analysis using the Planck 2018 CMB data, DESI DR2 BAO data, and Pantheon+ supernova data.

Baseline Analysis: Without including the local SH0ES prior, the DRMD model allows for a higher $H_0$ ( $70.5^{+1.0}_{-1.6}$ km/s/Mpc) and a larger post-BBN $\Delta N_{\text{eff}}$ ( $<0.811$ at 95% C.L.) compared to the SIDR model.
Tension Resolution: When the SH0ES prior is included, the DRMD model reduces the tension between the combined dataset and local measurements to 1.4 $\sigma$ , compared to 5.7 $\sigma$ for $\Lambda$ CDM and 3.0 $\sigma$ for the SIDR model.
Model Parameters: The model effectively introduces three free parameters relative to $\Lambda$ CDM: the post-BBN $\Delta N_{\text{eff}}$ , the fraction of interacting dark matter ( $f_{\text{idm}}$ ), and the decoupling redshift ( $z_{\text{dec}}$ ). The analysis finds a preference for $z_{\text{dec}} \approx 2200$ (near matter-radiation equality) and a small interacting fraction $f_{\text{idm}} < 4.4\%$ .
Profile Likelihood: A profile likelihood analysis confirms that the DRMD model exhibits an almost perfect degeneracy between $H_0$ and the model parameters, allowing for $H_0 \approx 73$ km/s/Mpc with a negligible $\Delta \chi^2$ penalty compared to the baseline.

Significance and Claims
The paper claims to provide a "full resolution" of the Hubble tension within standard current datasets based on well-known fundamental principles (gauge symmetry and spontaneous symmetry breaking).

Microphysical Origin: Unlike phenomenological SIDR models, this solution arises naturally from a specific UV-complete framework where the same symmetry-breaking event that generates the dark radiation abundance also induces the mass splitting required for decoupling.
Consistency: The model successfully reconciles the need for a large $\Delta N_{\text{eff}}$ to solve the Hubble tension with BBN constraints (by generating the radiation after BBN) and CMB constraints (by preventing free-streaming via self-interactions and suppressing small-scale power via the DRMD mechanism).
Predictions: The model predicts a specific feature in the matter power spectrum at small scales (due to dark acoustic oscillations and the decoupling history) and a stochastic gravitational wave signal from the phase transition, which could be probed by Pulsar Timing Arrays (PTA) and future CMB experiments (ACT, SPT).

The authors conclude that the DRMD mechanism offers a theoretically motivated and phenomenologically viable path to resolving the Hubble tension, distinguishing itself from other EDE or SIDR proposals through its specific decoupling dynamics and microscopic origin.

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